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                               CHAPTER 1

              SIMULA FOR DEC SYSTEM 10             TD, RTS

                      SIMULA 67 FOR DEC SYSTEM 10

                          THE RUN TIME SYSTEM

                         74-11-13 Lars Enderin
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-1

741111                                            Lars Enderin



III.1.1  Introduction

The Run Time System (RTS) is a collection  of  subroutines  designed  to
support  an  executing  SIMULA program.  The RTS consists of low segment
parts loaded on demand with the compiled program and high segment  parts
loaded  at  run  time.   The debugging subsystem SIMDDT may be loaded at
program start, via REENTER after a ^C interrupt, or after  program  exit
for a new execution.  SIMDDT is always loaded on run time errors.

III.1.2  RTS design goals and principles

The RTS is designed to take advantage of the hardware characteristics of
the  KI-10  CPU.   The data structures have been designed with a view to
efficient operation in a paged virtual memory environment.   The  number
of  storage  allocations  has  been reduced by allocating most subblocks
within already existing blocks.   Data  referencing  locality  has  been
improved  by  allocation  of  display  vectors  adjacent  to their block
instances.  Calling sequences and data representation have been designed
to  be  efficient.   The  most  commonly  occurring  sequences have been
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-2

741111                                            Lars Enderin

III.1.4  RTS coding and naming conventions

III.1.4.1  Naming conventions in the RTS

As in the compiler, a fairly rigid naming scheme has  been  observed  to
aid  in  deducing  the  meaning  of a name in the system and to simplify
consistency checks by visual inspection.   Normally,  the  first  letter
signifies the type of quantity as follows:

Q starts a compile-time constant.

S or SW signify a switch (Boolean variable) which is a 1-bit  or  36-bit
field in an accumulator or storage cell.

X stands for an accumulator.

Y is a global cell or a local cell  in  a  subroutine.   Also  used  for
offsets in the static low segment area, based on the value in .JBOPS.

YY in some  subroutine  specifications  is  used  to  signify  a  formal

XX is used in some routines when designating a formal  parameter  in  an

Z starts a record field designator, defined via the DF macro (see  I.6).
Record  names (used in DR macros and prefixed by Q to form a record type
code) are three letters  starting  with  Z.   Record  field  designators
consist  of a record name suffixed by 2 or 3 letters and/or digits.  The
first 5 characters of a field designator should be unique since  symbols
are derived from it by prefixing special characters (% and $).

Component names are two letters,  e  g  CP  (class  and  prefixed  block
handling).  Subroutine names are formed from component names followed by
two and sometimes more characters (two preferred).  The symbols L1,  L2,
...   , L10 are preferrably used as local labels (available within BEGIN
- ENDD or PROC - EPROC), otherwise the component name is used as prefix.
Global  symbols  (INTERN  or  ENTRY)  always have a dot "." as prefix to
avoid name clashes with SIMULA procedures.  OPDEF's have  been  used  in
some modules as a convenient means of coding procedure calls.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-3

741111                                            Lars Enderin

III.1.4.2  Register conventions

The accumulators (registers) have been given the basic mnemonics X0, X1,
..., X7, X10, ..., X17 (octal numbering).  These names are used in local
contexts only.

With very few exceptions, X0 is used only internally in the RTS, e g  to
hold the first word (with the flags) of a dynamic record.  One exception
is the mathematical subroutines, which return results in X0 and possibly
X1 (=XSAC).

X1 is called XSAC and  is  used  for  various  purposes,  e  g  to  pass
prototype  address  or  the  like  to  a  RTS  routine.  It is used as a
parameter register chiefly when no extra code has to be executed in  the
compiler, which normally compiles code using XWAC1 etc.  In other words,
XSAC is used when the parameter to be passed could not  be  compiled  to
some work ac more easily.

X2 is called XTAC and is used for example to pass the number of the  top
ac to standard functions.

X0 through X2 can normally be used in the RTS without  being  saved  and
restored (except for internal subroutine interface).

X3 is called XWAC1 and also XRAC.  This is the first ac allocated by the
compiler  and  also  the  result  ac  from  thunks,  procedures  and RTS
functions, together with XWAC2  (=XRAC1)  in  some  cases.   New  object
addresses are also computed to this accumulator.

X4-X14 (octal) are the rest of the work accumulators, also called XWAC2,
XWAC3,  ...,  XWAC7, XWAC10, XWAC11, XWAC12.  The upper limit is defined
by the compile-time constant QNAC which is the number of work ac's.

X15 is called XCB and should always point to the nearest (current) block
with  a  display  vector attached.  XCB is the root of the run-time data
structure.  From this all referenceable quantities can be found,  except
for some quantities pointed to by global low segment static locations at

SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-4

741111                                            Lars Enderin

X16 is called XIAC,XFP, or XLOW.  XIAC is used to point  to  some  block
other  than  XCB  when a variable is accessed.  The contents of XIAC are
regarded by the compiler as lost whenever a change of control can  occur
as  when  calling  a  procedure,  at an explicit or implicit label, when
intermediate results may have to be saved, and when calling mathematical
subroutines.  Inside the RTS, the ac can be used as XLOW to point to the
static area in the low segment.  The UNIVERSAL file  SIMRPA  contains  a
macro  LOWADR  which  loads  a specified register with this address, and
assigns the name XLOW to the register.  If no parameter  is  given,  the
default XLOW=XIAC is used.  The macro SETLOW(x) assigns the name XLOW to
x (default XIAC if no  parameter  is  given),  but  does  not  load  the
register.   Should  be used when it is known which register contains the
address.  The name XFP  is  used  on  X16  in  connection  with  FORTRAN
subroutines,  as  shown  in  the previous section.  The standard calling
sequence in FORTRAN-10 is:
        MOVEI   XFP,arg
        PUSHJ   XPDP,routine
where the argument list has the form:
        XWD     -number of arguments,0
arg:    Z       type code,address of first argument
        Z       type code,address of second argument
The result is returned in X0 and X1 if applicable.

X17 is only referred to as XPDP and used for all PUSHJ-POPJ  calls.   It
refers to the run-time stack whose bottom is at YOBJRT, so named because
the return address to SIMULA code is placed there  when  calling  a  RTS

III.1.4.3  Coding conventions

III.  Preserving registers within the RTS

Inside the RTS, all work registers can be  used  provided  any  possibly
relevant  registers are preserved.  When some storage allocation routine
is called, the calling RTS routine is  responsible  for  preserving  the
integrity  of  object pointers which may exist in registers temporarily.
These registers must be saved  in  locations  which  are  known  to  the
garbage  collector  as  possible object references.  The array YOBJAD in
the low segment is provided for  that  purpose.   Any  unused  cells  in
YOBJAD  must  be  zero when not in use.  Other global cells known to the
garbage collector are used for relocatable address esin  the  SIMULATION
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-5

741111                                            Lars Enderin

module (SU) and in SIMDDT.  For a temporary text variable, YTXZTV should
be used in the same fashion.  Registers  which  contain  non-relocatable
values will be saved and restored by the garbage collector.

III.  Addressing the low segment

The low segment is addressed through  the  right  half  of  .JBOPS.   As
explained  above, the macros LOWADR and SETLOW are used to establish the
base register, which is always called XLOW (renamed by the macros).  The
storage  allocating  routines  in  particular  always  use  the standard
XLOW=XIAC  to  prevent  confusion  when  calling  them  from  other  RTS
routines.   The use of global variables is avoided when the stack can be
used equally well.

III.  Saving intermediate results

The RTS procedures which are of a function  nature,  i  e  can  be  used
inside  an  expression,  warrant  special  consideration  because of the
requirement to save intermediate  results  over  the  execution  of  the
procedure.   If  the procedure involves storage allocation, an acs (ZAC)
object must be created for the intermediate results.  This  is  done  in
either of two ways.

1)      The entry point .CSSA  is  called  from  compiled  code  in  the
following way:
        PUSHJ   XPDP,CSSA       ;via transfer vector
        XWD     N,ADMAP
Only the accumulators below the top ac (as given  by  YTAC)  are  saved.
This  method  cannot  be  used  when the top accumulators are important.
.CSSA creates a ZAC object, saves  the  intermediate  results  and  sets
YCSZAC(XLOW)  to  point to the acs object.  YCSZAC is then used by a run
time setup routine (.CSSN, .CPNE) to set ZDRZAC of the  display  vector.
YCSZAC is cleared after use.

2)      The "XWD N,ADMAP" word is passed as an inline parameter to  some
RTS  routines  which  may  allocate  storage directly or indirectly (e g
through the actions of a thunk).  It is the  responsibility  of  such  a
routine  to  preserve  any quantities which will not be saved in the acs
object, in the special locations reserved for object addresses  or  text
variables,  then to call .CSSA.  (secondary entry point to .CSSA) with a
copy of the inline parameter in XSAC, provided it is non-zero.   A  zero
inline parameter signifies that no intermediate results exist.
When returning from an object generator, procedure, thunk evaluation  or
some  run  time function such as .TXCY, YCSZAC is recovered and .CSRA is
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-6

741111                                            Lars Enderin

entered if YCSZAC is non-zero.  The result of the function is in XRAC  &
XRAC+1 and is placed in the proper position on the stack by .CSRA before
restoring the intermediate quantities.  The result is thus always placed
in the top accumulators.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-7

741111    780302      6                           Lars Enderin

 III.1.5 RTS storage allocation

 II.1.5.1 Types of records allocated by the RTS

         Several types of dynamic blocks are allocated  by  the  RTS  in
         response  to  the  expressed  needs  of  a SIMULA program.  The
         different block types are:

         Record  Function                Allocated by
         ZBI     block instance          SAAB (via CSSB)
                 (unreduced subblock)
         ZBP     procedure block         SADB (via CSSN or CSSW
                                               or CSSW0)
         ZCL     class object            SADB (via CPNE)
         ZPB     prefixed block          SADB (via CPSP)
         ZTE     text record             SAAR (via TXBL, TXCY, IOIT)
         ZTT     temporary text variable SAAR (via TXDA)
         ZAR     array record            SAAR (via CSNA or CSCA)
         ZAC     accumulator stack rec.  CSSA
         ZER     eventnotice record      SAAR (via SANE)
         ZDR     display record          SADB (via CSSN, CSSW, CSSW0,
                                               CPNE, CPSP)
         ZYS     "system" record         SAAR
         ZXB     extended lookup block   SAAR (via IOCF)

         The block layouts are documented in TD I.5 and LH2 appendix B.

 III.1.5.2 Some notes on the various block types

         The ZDNTYP field in the first word of all  these  blocks  shows
         the  block  type.   The  numeric values of block type codes are
         determined by an expansion of the TYPZDN  macro  in  SIMMCR.MAC
         (III.5).   The  symbolic names are formed from the record names
         by prefixing the letter Q, e.g. QZCL is the  assembly  constant
         for the type code of a class object record.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-8

741111    780302      6                           Lars Enderin

 III. Subblocks (ZBI)

         ZBI (unreduced subblock  instance)  is  the  simplest  type  of
         independent  program  block.   The block types ZBP, ZCL and ZPB
         all contain a ZBI  part.   A  ZBI  block  may  contain  several
         reduced  subblocks corresponding to Algol blocks at inner block
         The ZBIBNM field in the block instance indicates the  innermost
         currently active reduced subblock.  The value in ZBIBNM is used
         by the garbage collector to find the correct block map(s)  (see
         I.5) and by SIMDDT to find the correct symbol table segment.
         Unreduced subblocks are only allocated in the following cases:
         * A subblock immediately enclosed by a  class  instance.   This
         saves   space  for  detached  class  objects  by  allowing  the
         subblocks to be garbage collected.
         * The outermost block in a connection statement.
         In all other cases, subblocks  are  reduced  into  the  nearest
         enclosing  instance of a procedure, prefixed block or unreduced
         subblock.  Reduction of subblocks  saves  storage  allocations,
         reduces  the  size  of display vectors (since several subblocks
         can be addressed from the  same  base  address)  an  speeds  up
         execution.   In  return,  some  run time procedures become more
         complicated, e.g. CSGO.  See III.1.6.

 III. Procedure blocks (ZBP)

         Procedure subblocks (ZBP) are set up by calls to CSSN, CSSW  or
         CSSW0.   They  have  an  attached  display  vector (ZDR).  SADB
         allocates both ZDR and ZBP at the same time as  one  contiguous
         storage block.
         SADB also fills in those fields which are common to  the  block
         types  handled  (ZDR  + ZBP, ZCL or ZPB), e.g. the reactivation
         point (ZDRZBI,ZDRARE).
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.1-9

741111    780302      6                           Lars Enderin

 III. Class objects (ZCL), prefixed blocks (ZPB)

         Class objects (ZCL) and prefixed blocks (ZPB) are allocated  in
         the  same fashion as procedure blocks by calls to CPNE or CPSP,
         which call SADB.

 III. Text records (ZTE)

         Text records (ZTE) are allocated by  SAAR.   They  contain  the
         ASCII representation of the text contents.

 III. Temporary text variables (ZTT)

         ZTT records are created in connection with parameter  handling.
         A  ZTT  record contains a text variable (ZTV instance) which is
         used as stand in for a text expression in certain  cases.   See
         parameter handling.

 III. Array records (ZAR)

         ZAR records (arrays) are created by array declarations or  when
         passing  array parameters by value.  CSNA is used to create the
         first array in an array segment.  CSCA is used  to  create  the
         other arrays in the segment by copying and also when passing an
         array parameter by value.
         Before the array record is allocated in CSNA, its size must  be
         computed  from  the subscript bounds.  The subscript bounds are
         in ac's (possibly also in pseudo ac's) and are saved in  a  ZAC
         record during evaluation.  Temporarily, the array thus requires
         its own space plus that needed by the ZAC record (4 + 2n words,
         where n is number of subscripts).
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-10

741111    780302      6                           Lars Enderin

 III. Accumulator stacks (ZAC)

         ZAC records (accumulator  stacks)  are  created  in  situations
         where  intermediate  results are in ac's and possibly in pseudo
         ac's, and the  garbage  collector  may  be  called  because  of
         storage allocation.
         Since space is always reserved at the top of the  storage  pool
         for  a  ZAC  record of maximal size, CSSA can create the record
         and copy the ac values  into  it  without  first  checking  the
         storage  limits.   If  sufficient  space for another ZAC is not
         left then, the garbage collector  is  called  to  provide  more
         space.  Relocation information for the saved values is provided
         in the accumulator map pointed to by the ZACZAM  field  of  the
         ZAC.   The  ac  map is a bit vector where a one in bit position
         n-1 signifies that saved result n (XWACn)  contains  an  object
         address  in its right half, e.g. a REF value, the first word of
         a TEXT variable, an array address, a procedure dynamic address,
         If pseudo ac's are used, relocation  bits  are  found  starting
         with bit 18 of the map word.
         The address of the ZAC object is placed by CSSA in YCSZAC(XLOW)
         whence  it  may be copied to another location (e.g. ZDRZAC of a
         display vector).  YCSZAC(XLOW) will be cleared when  it  is  no
         longer  needed.   ZAC records may be created when the following
         RTS routines are called:
         CSSA (before a call to CSSN or CPNE), CSSW, PHFA,  PHFV,  PHFM,
         The other procedures call CSSA at its entry point .CSSA. .
         Note that the current "top" ac's  are  NOT  saved  in  the  ZAC
         object.   Instead, they are saved by the invoked RTS routine in
         global locations provided for that  purpose,  if  pointers  are
         involved,  or  on  the  RTS  stack  for  quantities  which  are
         invariant under garbage collection.

 III. Display records (ZDR)

         ZDR (display)  records  exist  for  procedures,  non-terminated
         class  objects  and  for  prefixed  blocks  (including the main
         program block).  For classes with local classes as  attributes,
         the  display record is kept even when terminated because it may
         be needed to establish the environment of a procedure attribute
         of a local class.
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-11

741111    780302      6                           Lars Enderin

 III. Static display

         The ZDR record contains the traditional  static  display  known
         from  Algol  implementations.   Instead  of one central display
         vector, we have here one display for each  active  block  of  a
         type  described  above.   This  means  that  several  pieces of
         information  are  duplicated  in  the  system.   Because   most
         subblocks  are  reduced,  however, quite few display levels are
         usually needed, and displays for terminated  class  object  are
         usually  deallocated.   Since  the  display  is adjacent to the
         block and addressed via the same ac (usually XCB,  the  current
         block  pointer), one instruction usually suffices to obtain the
         address of any block in the static environment.

 III. Dynamic link

         In addition to the display, ZDR also contains the dynamic  link
         or  reactivation  point  (ZDRZBI, ZDRARE).   For a procedure or
         attached class instance, the reactivation point is (address  of
         calling  block instance, return address).  For a detached class
         instance, ZDRZBI normally points to its own block,  and  ZDRARE
         points to the reactivation point within its code (program point
         after call on DETACH or RESUME or after a scheduling  statement
         in SIMULATION).  See III. Quasi-parallel Sequencing.

 III. FOR loop returns, thunk save areas, ZAC pointer

         Return addresses for FOR loops are also contained  in  the  ZDR
         record,  as  well as thunk save areas (see parameter handling),
         and the address of a ZAC record if one was created just  before
         the  ZDR record was created.  The existence of a ZAC record for
         the attached block is flagged by a bit in that  block  instance
         to  prevent  execution  of  unnecessary  instructions  if a ZAC
         record does not exist.  This is  relevant  because  the  ZDRZAC
         field  is accessed from the front end of the ZDR, and the front
         end address has to be calculated from  the  block  address  and
         prototype information, whereas the other information in the ZDR
         record is accessed conveniently via negative offsets  from  the
         block address.
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-12

741111    780302      6                           Lars Enderin

 III. Eventnotice record (ZER)

         ZER (eventnotice) records  are  used  in  SIMULATION  programs.
         Each  SIMULATION  block  has one or more ZER records allocated,
         chained via the ZSUZER field in the SIMULATION  block  and  the
         ZERZER field in the ZER record.  A ZER record contains a number
         of event notices (ZEV).

 III. Eventnotices (ZEV)

         Each ZER record contains several ZEV records (eventnotices), of
         which  some  are members of the sequencing set (SQS).  The free
         ZEV records form a chain via their ZEVZEV fields and the  first
         free  record  is  found via the ZERZEV field in the ZER object.
         For garbage collection purposes, the ZEVZER field of  each  ZEV
         gives the address of the ZER record of which the ZEV is a part.
         The ZEV also contains links to other ZEV instances in the  SQS,
         a  pointer to the associated process (ZEVZPS) and the scheduled
         simulated time for the next active phase of that process.   The
         process has a pointer to its associated ZEV record (ZPSZEV).

 III. ZYS records

         The ZYS record type is used  for  various  blocks  "behind  the
         scenes",  i.e.  not  directly related to any SIMULA concept.  A
         ZYS block may only contain information invariant under  garbage
         collection,  so  that it can be moved freely without relocation
         of internal information.  ZYS blocks are used e.g. for sub-file
         directories  (SFD),  and  SIMDDT  is marked as a ZYS record for
         garbage collection purposes.

 III. Extended lookup/enter block (ZXB)

         ZXB records are used for extended lookup/enter  blocks  in  the
         SIMULA  I/O subsystem.  A ZXB record may contain a pointer to a
         sub-file directory and is  thus  not  invariant  under  garbage
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-13

741111    780302      6                           Lars Enderin

 III.1.5.3 Garbage collector (GC)

         The garbage collector (GC) can be called for three reasons.

         1. The storage pool is exhausted, a new core request cannot  be
            honored  within  the  allocated  core (top location given by

         2. New buffers are to be allocated.  The  whole  pool  must  be
            moved upwards, leaving free space at the bottom.

         3. SIMDDT  calls  the  garbage  collector  when  executing  the
            VARIABLES  command  in  order  to remove garbage data before
            dumping the pool.

         In the first case ac  0  (X0)  contains  the  number  of  words
         required  above the limit given by YSALIM(XLOW).  In the second
         case, YSAREL(XLOW) contains the number of words needed  at  the
         bottom  of  the  pool,  and in the third case X0 and YSAREL are
         both zero, thus no more core is needed.

         The action taken when GC is  called  (entry  .SAGC  in  SA.MAC)
         depends  on  the allocation strategy chosen when assembling the
         RTS.  If the assembly constant QSASTE is non-zero, the pool  is
         allocated  in  steps  (i.e. the pool is expanded by one or more
         core  requests  between   garbage   collections   rather   than
         collecting garbage each time).  If a new core chunk (step) of a
         previously calculated size can be allocated  without  exceeding
         the  garbage  collection  limit  YSAL(XLOW)  calculated  in  an
         earlier  execution  stage,  the  step  is  taken,  otherwise  a
         complete  garbage  collection  is performed.  If X0 was zero on
         entry,  however,  a  complete  garbage  collection  is   always
         performed (cases 2 and 3 above).

         If QSASTE is zero,  the  whole  pool  given  by  the  value  in
         YSAL(XLOW)  is  allocated  initially  and  after  each  garbage
         collection, and a call to SAGC will always result in a complete
         garbage  collection (i.e. the limits YSALIM and YSAL coincide).
         This will probably be the best strategy for KA10  installations
         where CORE requests are fairly expensive.  QSASTE is defined in
         SIMMAC.MAC and set to 1 as default.  A  change  of  the  QSASTE
         value  only affects the SA.MAC module which must be reassembled
         after assembling SIMMAC.MAC.
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-14

741111    780302      6                           Lars Enderin

         Another algorithm has been added to take care of  big  programs
         which  may run out of real core memory and start to use virtual
         memory.  After calculating the optimal YSAL value, check to see
         if  the  job  would  go  virtual.   In  that  case,  modify the
         estimated limit  to  a  value  which  avoids  excessive  paging
         overhead.   This  algorithm is used only when virtual memory is
         available and it is currently  not  well  tested,  but  it  has
         reduced execution time significantly for a couple of programs.

         Garbage collector phases

         The garbage collector works in four phases.   Figure  III.1.5.3
         shows   the  interaction  of  GC  components  (subroutines  and
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-15

741111    780302      6                           Lars Enderin

         Figure  III.1.5.3  Subroutine  and  coroutine  linkage  in  the
         garbage collector.

         +------+    +------+    +------+    +------+
         +------+    +------+    +------+    +---:--+
                                     ! ^         V
                                     ! !     +------+
                                     ! +-----!SAGCCH!
                     +------+        !       +------+
                     !SAGCDR!        V
                     +------+<-->+------+    +------+
                     +------+<-->+------+    +--:---+
                     !SAGCFP!       ^  ^        V
                     +------+       !  !     +------+
                                    V  +-----!SAGCCH!
                                 +------+    +------+
             + ------------------!SACGN1!
             !                   +------+
         +------+    +------+    +------+    +------+
         +------+    +------+    +------+    +---:--+
                                     ! ^         V
                                     ! !     +------+
                                     ! +-----!SAGCUP!
                     +------+        !       +------+
                     !SAGCDR!        V
                     +------+<-->+------+    +------+
                     +------+<-->+------+    +--:---+
                     !SAGCFP!       ^  ^        V
                     +------+       !  !     +------+
                                    V  +-----!SAGCUP!
                                 +------+    +------+
             + ------------------!SACGN3!
             V                   +------+
          +------+   +------+
          !PHASE4!<->!.SANP !
          +---:--+   +------+
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-16

741111    780302      6                           Lars Enderin

 III. Phase 1

         The first action is to find all referenceable  records  in  the
         pool.   The  right half of the first word of all records in the
         pool is reserved as GC working space  (ZDNLNK  field).   During
         phase  1  this  field  is used to chain all referenced records.
         The chain starts with the outermost block instance,  which  can
         be  found  provided  XCB  points  to  a record with an attached
         display record.   The  outermost  block  instance  is  part  of
         generated  code  for the main program and is thus not placed in
         the dynamic storage pool.  Usually, though, the outermost block
         contains  pointers  into  the dynamic pool which may have to be
         updated during garbage collection.  If  no  display  record  is
         attached  to  the  XCB  record,  the  chain starts with the XCB
         record itself.  All global pointers in the static area (defined
         in  SIMRPA,  macro  STATIC) which may contain pointers into the
         pool are checked, and each record found via these  pointers  is
         attached  to the chain.  Since a dynamic record in the pool may
         contain other pointers,  each  record  in  the  chain  must  be
         searched  for  pointers.   Phase  1  is completed when the last
         record of the chain is checked without finding any new pointer.

 III. Phase 2

         At this stage there are two kinds of records in the pool:
         * Referenced records with a link address in ZDNLNK.
         * Unreferenced records with ZDNLNK = 0.
         New record addresses can now be  computed  for  all  referenced
         records,  assuming that the records should be moved towards the
         bottom of the pool in the same order by  increasing  addresses.
         If  YSAREL(XLOW)  is  non-zero,  its  value is added to the old
         bottom address.  Then the whole pool is  scanned,  placing  the
         new  address  of  each  referenced  record in its ZDNLNK field.
         Unreferenced neighbours are lumped  together  to  look  like  a
         single unreferenced record in the following phases.
         When all referenced records have been assigned  new  addresses,
         the new value of the first free location (the new YSATOP value)
         can be determined.  This plus the amount of core requested when
         calling  GC  gives  the  minimal  amount  of  core required for
         continued execution of the SIMULA program.  If not enough  core
         was  reclaimed  by  the  GC,  a  CORE  UUO  is executed for the
         required core size.  If that fails, execution  terminates  with
         an error message.
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-17

741111    780302      6                           Lars Enderin

 III. Phase 3

         All pointers that were checked during phase  1  must  again  be
         checked.   This  time any pointer into the pool will be updated
         to point at the updated location of the record  (given  by  its
         ZDNLNK  field).   The same routines are used as in phase 1, but
         the records are not treated in the same  order.   In  phase  3,
         referenced  records  are  treated  in  the  order in which they
         appear in the pool, starting at the bottom.  Instead of calling
         SAGCCH  for  each  record,  SAGCUP  is  called  to  perform the
         updating of addresses.  See figure III.1.5.3.
         Some dynamic records  contain  internal  pointers.   These  are
         relocated  as  soon  as the next record in the pool is found by
         the SAGCN3 routine.   Eventnotice  pointers  are  also  treated
         here.  They are found in SIMULATION and PROCESS block instances
         and in eventnotice records.
         At this stage, however, ZEVZER and ZERZER  pointers  cannot  be
         updated since they are used to define the relocation offset for
         eventnotice pointers not yet found.

 III. Phase 4

         Now the ZERZER chain and all ZEVZER pointers  can  be  updated.
         Each  referenced  record in the pool should now be moved to its
         new location given in the ZDNLNK field.  A complication  occurs
         if  the  bottom  of the pool should be moved upwards.  For some
         records moved towards a higher address, the old  and  new  area
         may overlap, prohibiting the use of a BLT instruction.  In that
         case, the record is moved word by word  starting  at  the  high
         end,  and  the records must be moved in reverse order.  This is
         simplified by first making a reverse chain of records  via  the
         ZDNLNK  fields.   After  all  records  have  been  moved to new
         locations, the reclaimed core is cleared to zeros.

 III. Determine and allocate new storage pool area

         After phase 4,  a  new  garbage  collection  limit  and  a  new
         allocation  step  size  if  applicable,  are  calculated.   The
         algorithms used in the calculations are given below.
         Finally, a CORE request is  made  if  necessary.   The  maximal
         available amount is used if the request is excessive.
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-18

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 III. Core size algorithm

         The core size algorithm is designed to minimize  running  costs
         for  the  SIMULA  program assuming an accounting formula of the
         following form:

             Cost = G * (L + A) * time

         where L is the storage pool size, G is a  constant,  and  A  is
         explained below.
         It is further assumed that the  time  required  for  a  garbage
         collection is proportional to the amount of active memory.  The
         proportionality  factor  B  is  approximated  with  a   divided
         difference  over  the  last  garbage  collection.   It  is also
         assumed that the allocation rate R is approximated by a divided
         difference  from  the end of the last garbage collection to the
         start  of  the  current  garbage  collection.   In   the   KI10
         production  system  core is released after a garbage collection
         and reclaimed when needed  in  constant  steps,  each  step  an
         integral number of 512-word pages (see next section).

         With the current accounting algorithm, A can be approximated by

                     5450         Q-La
             A =  --------- + 5 + ----  pages,
                  (La+Q+10)        2

         where Q is the size of high segment and code and  data  outside
         the  storage  pool,  and  La is the average size of the storage
         pool or (F+L)/2, where F is the size of active memory and L  is
         the  previous  storage  pool limit.  The following figure shows
         how A was computed:
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741111    780302      6                           Lars Enderin

         A = dA/kA.


                         !                                  .
                         !                            +  .
                         !                          + .
                         !                        +.:
                         !                   :  +   : Slope of tangent =
                         !   Accounting      +      :    kA = G
                         !   algorithm   +:  :      :
                         !     ! :   + . :   :      :
                         !     v +  .    :   :      :
                         !  +    :---    :   :      :
                      +  !    .  :  !    :   :      :
               +         ! .     :  !    :   :      :
                        .!       :  !    :   :      :
                     .   !       : dA    :   :      :
                  .      !       :  !    :   :      :
               .         !       :  !    :   :      :
            .            !       :  !    :   :      :
          ---------------+-------+-------+---+------+-----> Allocation
                         :.......:.......:   :      :
                             Q   :   F       :      :
                                 :...........:      :
                                 :     La           :

         The optimal storage pool limit minimises the sum of the cost to
         execute between garbage collections and the cost to execute the
         garbage  collections.   Assuming  stationary  program  dynamics
         (constant  allocation  rate  R,  constant active memory size F,
         constant garbage  collection  time  per  word  B  and  infinite
         execution time) the new optimal storage pool limit is:

             L = F  * (1 + sqrt[2*B*R*(1 + -)])                    (1)

         The expected time between garbage collections is  (L-F)/R,  the
         average  store  size  is (L+F)/2, which yields the cost between
         garbage collections:
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741111    780302      6                           Lars Enderin

             L-F    L+F
             --- * (--- + A)                                       (2)
              R      2

         The expected time for garbage collection is F*B, and  the  cost

             F*B*(L+A)                                             (3)

         The   cost   rate   per   effective   cpu   second   is    thus
         ((2)+(3))/((L-F)/R),  and  minimisation with respect to L gives

 III. Step size algorithm

         Assume that the accounting algorithm can be written

             COST = const * [A+W*(M+U)*M]*time

         where M is accounted memory.

         Assuming approximately linear memory size dependency  for  this
         execution phase we have

             COST = const * [A+(B*M+C)]*time

         Assuming average time for a CORE UUO is K, allocation rate is R
         and  that  core  is  expanded  in the interval (C0, C1), we can
         derive an approximation for the optimal step size:

             S = sqrt(K*R*[2 * --- + C0 + C1])

         Define m = W*(M+U)*M
         We then have

             -- = 2*W*M+W*U

         and the linearisation yields

             m = W*(2*Ma+U)*M-W*Ma^2
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741111    780302      6                           Lars Enderin

         where Ma is the average memory size.

         Thus B = W*(2*Ma+U)
         and  C = -W*Ma^2

         Define X = 2*Ma = C0+C1

         Thus B = W*(X+U)
              C = -0.25*W*X^2

         and the optimal step:

             S = sqrt(K*R*[------------- + X])

         The bracketed expression can be expressed as:

             2*A   X^2       2*A   X^2
             --- - ---       --- - --- + X^2 + X*U
              W     2         W     2
             --------- + X = --------------------- =
                X+U                   X+U

               X^2         2*A   2*A   U^2
               --- + X*U + ---   --- - ---
                2           W     W     2    X+U
             = --------------- = --------- + --- =
                    X+U             X+U       2

                  --- - U^2
               1   W
             = - [--------- + X+U]
               2     X+U

         The present values at our installation are
             U = 20 [pages]
             A = 1.1 [1/sec]
             W = 0.0001 [1/(sec*pages^2)]
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741111    780302      6                           Lars Enderin

                      K*R    44000 - 400
             S = sqrt(--- * [----------- + X+20])  [pages]
                       2        X+20

         Expressed in words and milliseconds and assuming K = 4 ms
         we will get
             S = sqrt(2*R*[----------- + X+10240])  [words]

             R = allocation rate in words/ms
             1.143*10^10 = [--- - U^2]*512^2  [words*words]
             10240 = U * 512  [words]

 III. Algorithm for virtual memory

         The algorithms described above are not suitable  when  the  job
         goes  virtual,  i.e. when the available real core memory is not
         sufficient for the job to run.  If the user has virtual  memory
         privileges,  the job will then start paging.  In this case, the
         assumptions on which the above algorithms are based will not be
         valid.   Essentially,  use of virtual memory should be avoided.
         An alternative allocation algorithm has  been  constructed  for
         the  case  where  virtual  memory  is used.  The assumptions on
         which the algorithm is  based  may  not  be  quite  valid,  but
         significant  cost  improvements have been obtained for programs
         using much virtual core.

         - At start of GC, save number  of  niw  faults  in  YSANWA  and
         number  of niw faults since last GC in YSANWB.  GETTAB [%VMSPF]
         used.  Cumulative count of niw faults updated in left  half  of
         YSANWC.   This  should  be  job-related  data,  but only global
         system counts are available in this way, which means that other
         jobs  executing at the same time may add to the counts, causing
         more restrictive use of memory than necessary.
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         - Modification of core limit if CORE fails in SANP1:  If  first
         CORE  fails,  check  if  job  is going virtual, and if it does,
         subtract space (1K) for PFH.  GETTAB [-1,,.GTCVL] used.
         - Determination of new allocation limit for a job that runs  in
         virtual core:
         In SANP, if the page fault handler is in core, determine number
         of  niw  faults  between last two GC:s (from YSANWB) and change
         YSAL by 2K*SIGN(tgc-tswap*(nb+2*ng).  Here
         nb      niw faults before last GC;
         ng      niw faults during last GC;
         tswap   time (ms cpu) accounted for a page  swap  (currently  =
         tgc     time spent in this GC.
         However, if YSAL becomes less than minimal quantity needed, the
         latter  is  passed to SANP1.  The pool is not allowed to exceed
         the low segment limit (128K).
         - If job is not virtual, but the  new  estimated  optimal  YSAL
         value will make it virtual, make the new YSAL value so that the
         job just goes virtual:
         GETTAB[-1,,.GTCVL] is used to find out virtlim.
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         III.1.6  Execution control

         The execution of a SIMULA  program  is  controlled  on  several
         levels  - by the user through monitor commands, by the RTS, and
         by compiled code.

         1)  Monitor  commands  -  START  (CSTART),  REENTER,   CONTINUE
         (CCONTINUE), ^C.

         2) Overall program control by the RTS:
         High segments are swapped by OCSW.
         A program is set up and initialized by OCSP, OCIN, OCEI.
         Errors are handled by OCUU and SIMDDT, traps by OCTR and OCUU.
         SIMDDT interface is handled  by  OCUU,  .OCRE0,  .OCRE,  .OCLD,
         OCEP terminates program execution.  High segments  are  swapped
         by OCSW.

         3) Procedure calls are handled by CSSN, CSSW, CSEN, CSEP, CSES.
         Class  objects  are created and controlled by CPNE, CSEN, CPCD,
         CPCI, CPE0.  Subblocks are created by CSSB  and  terminated  by
         CSEU.   Reduced  subblocks  are  initialized by CSER.  Prefixed
         blocks are handled by CPSP, CSEN, CPPD, CPE0, CPCI.   Coroutine
         sequencing  and  quasi-parallel  systems  are  handled  by CPDT
         (detach),  CPCA  (call),  CPRS  (resume).   SWITCHes  and  GOTO
         statements  are  handled  by CSSC, CSES, CSGO.  Evaluation of a
         name parameter with a thunk may cause control to  go  anywhere.
         See  III.5  (parameter  handling).   SIMULATION  is  handled by
         special statements and procedures - see III.1.10.

         4) Conditional expressions and  statements,  WHILE  loops,  FOR
         loops  and  simple  GOTO  statements  are  handled  entirely by
         compiled code.

         III.1.6.1  Monitor commands for SIMULA program control

         III.  START (CSTART)

         A START or CSTART command causes a SIMULA program  in  core  to
         start  execution  at the entry point given by the right half of
         .JBSA.  LINK-10 sets the start address to .MAIN if  a  LOAD  or
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-28

741111    780302      6                           Lars Enderin

         EXECUTE  command  was issued.  If COMPIL recognizes the program
         as a SIMULA program or the /SIMULA switch  is  given,  a  DEBUG
         command  causes  LINK-10  to  set  the start address to .OCRE0,
         otherwise DDT will usually be loaded and the start address will
         be  .MAIN.  To understand what happens at the start of a SIMULA
         program it is necessary to know the general layout of a  SIMULA
         main program, as shown below:

         The first 4 or more words  are  copied  from  the  source  file
         LOOKUP    information.     The   interesting   information   is
         filename.ext, date and time of file creation and  last  access,
         and the access path:
                [proj,prog]    (zero    if    no    path    given)    or
                [proj,prog,sfd1,...]  if  SFD  files  are  part  of  the
                directory path.  See Monitor  Calls  manual  for  lookup
                block and SFD information.

                [source file lookup info]
                [text constants and any runswitches block]
                [Display (ZDR) for main program block]
        mainb:  [Block instance (ZPB) for main program]
        .MAIN:  JRST     1,.+1   ;Normal entry point
                TDZA     X1,X1   ;No SIMDDT address
                JRST     1,.+1   ;Enter here from .OCRE0
                MOVEI    XCB,mainb
                JSP      X16,.OCSP
                JFCL     runswitches address ;may be zero
                [compiled SIMULA statements]
                PUSHJ    XPDP,OCEP
                [prototypes, line number tables, symbol tables]

        If the program is started  at  .MAIN,  X1  is  set  to  zero  to
        indicate  that  SIMDDT is not in core.  The program will then be
        started without SIMDDT, unless this is a restart  after  program
        exit  and  SIMDDT  was  loaded  at  the  start  of some previous
        execution.  In that case, the address of SIMDDT has been  placed
        in  YOCDDT,  a  global  location  in  OCSP.  See REENTER command
        (III.   OCSP  loads  the  initial  high  segment  after
        allocating  core  for  the static data area and saving the ac's.
        Control is  then  transferred  to  OCIN  for  the  bulk  of  the
        initialization  -  setting up the RTS stack, allocating standard
        files SYSIN and SYSOUT and buffers,  reading  any  specification
        file, etc.  OCIN finishes by transferring control to OCEI in the
        OCEP module.  If the two high  segments  version  of  SIMRTS  is
        used,  invocation of OCEI from OCIN leads to a high segment swap
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-29

741111    780302      6                           Lars Enderin

        via the transfer vector and .OCSW.  When the program  starts,  a
        RESET  UUO stops any active I/O and the buffer space is released
        by moving .JBFF back to the  value  in  .JBSA(LH).   Any  active
        SIMDDT breakpoints are removed by OCSPDR.

        CSTART is identical to START except that the terminal remains in
        monitor  mode.   The  program  will  probably  wait for input or
        output on the terminal unless continued via ^C-CONTINUE.

        III.  REENTER command

        The REENTER command  has  different  effects  depending  on  the
        current  state  of execution in the SIMULA program.  The current
        address in .JBREN governs the actions.

        III.  REENTER before program start

        If issued directly after a LOAD or GET command,  REENTER  causes
        SIMDDT  to be loaded and entered before executing the statements
        of the SIMULA program.  REENTER starts the program at the  entry
        point  .OCRE0  in the OCSP module.  If COMPIL does not recognize
        the program as a SIMULA program, a DEBUG command will cause  DDT
        to  be  loaded instead of SIMDDT.  A ^C followed by REENTER will
        have the same effect as REENTER after LOAD in this case.  .OCRE0
        reserves  space  for  SIMDDT  if  not  already  loaded, modifies
        .JBSA(LH) to keep this space over RESET, and  transfers  control
        to  the  compiled  program  at  .MAIN+2  with  X1 containing the
        address of the SIMDDT area.  The first word of that area is  set
        to zero if SIMDDT was not loaded previously.
        In OCSP, the assigned SIMDDT address  is  saved  in  YOCDDT  for
        later  use  (restart).   When  OCEI eventually gets control from
        OCIN, SIMDDT is loaded if necessary and  entered  at  the  entry
        point  DSINI.   Breakpoints  may  then  be  set  before starting
        execution of compiled statements via a PROCEED command.
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-30

741111    780302      6                           Lars Enderin

        III.  REENTER during RTS initialization

        If the program is interrupted while .OCSP,  .OCRE0  or  OCIN  is
        active,  a  REENTER  command  has  the same effect as a CONTINUE
        command except for typing a message on the TTY.  The entry point
        OCRE1 is used in that program phase.

        III.  REENTER during execution of SIMULA statements

        During normal SIMULA program execution, the reentry  address  is
        .OCRE.  If the program is stopped by one or two ^C characters in
        this phase, a  REENTER  command  causes  .OCRE  to  be  entered.
        Depending  on  the  value  of the control variable YDSCSW(XLOW),
        SIMDDT may be entered
        a) directly,
        b) after returning to some well-defined point in the program,
        c) or the request to enter SIMDDT must  be  denied  because  the
        data  structures  are  being  changed  so that they are possibly
        inconsistent at the time of interrupt.
        In case (c), a new interrupt may be attempted after running  the
        program  for  some  time.   If  SIMDDT  was  not loaded with the
        program, it is loaded from  the  file  SIMDDn.ABS  (n  =  SIMULA
        version  number).   Once  within SIMDDT, breakpoints may be set,
        variables and the operating chain may  be  displayed,  but  some
        commands   can   not   be  performed,  chiefly  because  garbage
        collection may be involved.

        III.  Basic requirements for allowing  SIMDDT  to  be

        The following basic requirements must  be  met  when  SIMDDT  is
        executed in this mode:

        a) No garbage collection allowed

        Since there is no way to determine which registers and temporary
        cells  contain  pointers  into  the SIMULA storage pool, garbage
        collection cannot be allowed directly or as a side-effect,  i  e
        no storage may be claimed from the pool without making sure that
        no garbage collection will result.

        b) The data structure must be consistent

        The data structures used by SIMDDT must be consistent  in  order
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741111    780302      6                           Lars Enderin

        not  to  get  wild  results  or  cause  program  interrupts.  In
        particular, the current block pointer, XCB, must  be  consistent
        with  the program address used by SIMDDT to pinpoint the program
        state (program point and current data).  XCB may not point to  a
        block  which  is  partly  correct,  e  g  does not have a proper
        display vector.  If the interrupt was  not  in  compiled  SIMULA
        code,  the  bottom  stack  level  must  point to a program point
        consistent with XCB.

        c) SIMDDT may  not  call  non-reentrant  routines  which  change
        global data in use at the time of interrupt

        This requirement is met by not permitting SIMDDT  to  be  called
        while such a routine is active.

        III.  Controlling the effect of REENTER

        In order to conform to the basic requirements,  special  control
        features  have  been  implemented  for  determining  when and if
        SIMDDT can be called.  For the purposes of this  description,  a
        SIMULA  program  with  its  attendant  run  time  system  (RTS),
        possibly linked with some FORTRAN or MACRO-10 subroutines, is in
        one of three states:

        State A (allow).
        In this state, SIMDDT may be invoked directly.  The data in  the
        pool  is  consistent with the program point given by XCB and the
        interrupt address (usually .JBOPC).  A SIMULA program is in this
        state  when it is at code level, i e outside any RTS routines in
        code compiled by SIMULA.  State A also applies to some (sections
        of)  RTS  routines, e g mathematical subroutines.  The bottom of
        the stack (YOBJRT) then points into the SIMULA code.

        State D (defer).
        In some RTS routines, invocation of SIMDDT must be  deferred  to
        some  future instant, normally the return to user code.  If such
        a point can be easily identified by reference to  the  push-down
        stack,  the stack can be manipulated to effect the invocation of
        SIMDDT at the proper time.  Routines of this type should  always
        return  to  the  point  of  call, or the stack must be set up to
        allow for such treatment.  If a proper invocation  point  cannot
        be found easily, the state must be F.

        State F (forbid).
        In this state, invocation of SIMDDT is forbidden.   The  program
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741111    780302      6                           Lars Enderin

        is  restarted  with  a  message  that  a new attempt to stop and
        REENTER should be made.  A SIMULA program is in this state  when
        the  data  structure  could  be  inconsistent and/or the program
        point  cannot  be  identified,  etc.   State  F  is  avoided  if

        III.  Macros for controlling the effect of REENTER

        The state information is encoded in a global cell in the  static
        low  segment  area (based on the address in .JBOPS and addressed
        by XLOW, which is normally AC16).  This cell  is  called  YDSCSW
        and is manipulated by the following macros:

        This macro sets the state to A by clearing YDSCSW(XLOW).

        Adds 1 to YDSCSW(XLOW).  State A goes to  state  D.   Any  other
        state is unchanged.

        Subtracts  1  from  YDSCSW(XLOW).   Will   give   state   A   if
        YDSCSW(XLOW)  was  1,  otherwise the state is unchanged.  Proper
        use of the macros will ensure that YDSCSW(XLOW) is  not  0  when
        CENABLE is executed.  CDEFER and CENABLE must be properly nested
        so that state A is reached when an equal number of those  macros
        have  been executed, if the initial state was A, and state F was
        not set.

        Sets the left half of YDSCSW(XLOW) to -1.  The state is  thereby
        set to F (YDSCSW negative).

        In order for these macros to work, the global cell  YDSCSW  must
        be  addressable  via  the  register  designated XLOW (set by the
        LOWADR or SETLOW macro).
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        III.  Actions after REENTER

        If the program is interrupted and subsequently  REENTER-ed,  the
        RTS actions depend on the state as follows:

        If the ordinary reentry point is not in  effect,  the  state  is
        irrelevant,  either  because  the  RTS  is  not inintialized, or
        because  .OCRE  is  active  in  processing  a  REENTER  command.
        Execution merely continues with a message stating the reason.

        If the ordinary reentry point is in effect  (.JBREN=.OCRE),  the
        actions are:

        All registers are saved.  .JBREN is changed  to  an  alternative
        reentry point.  The PC value at interrupt (.JBOPC) is stacked as
        return address.   If  .OCRE  is  entered  from  OCRET  (deferred
        REENTER),  .JBOPC is the address of the return to SIMULA code, i
        e a faked PC value.  The state is  determined  and  governs  the

        III.  Calling SIMDDT directly after REENTER

        State A:  Set a  switch  to  disallow  garbage  collection.   If
        SIMDDT  is  not  in  core  (loaded with program), bring it in if
        possible.  Start at the point DSINR of SIMDDT with XDSBAS set to
        the  base  of  SIMDDT.   YDSCAD(XLOW) gives the program point of
        interrupt, if possible related to compiled  code.   When  SIMDDT
        returns, the switch to disallow garbage collection is reset.

        .JBREN is set to .OCRE, the registers are restored, and  control
        returns  to  the program via a "POPJ XPDP," instruction to where
        .JBOPC pointed.

        III.  Preparing for deferred SIMDDT call

        State D:  In this state, the interrupted instruction must be  in
        some RTS routine, and the stack must have the return to compiled
        code at the bottom.  Invocation of  SIMDDT  should  be  deferred
        until the return point.  This is achieved by fixing up the stack
        so that instead of returning to compiled  code,  a  special  RTS
        routine  will  be entered.  In some cases, the return address at
        the bottom of the stack does not point to the  next  instruction
        to  be  executed  at return but to an inline parameter.  In that
        case, the inline parameter must be copied to a cell  before  the
        start  of the special reentry routine, and the cell at the stack
        bottom must point to the copy.  The only inline parameters  used
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        in the present system at object code level are of the form
                XWD n,m
        where n is an integer whose absolute value  is  less  than  512.
        Thus,  the  op code part of the word is either 0 or 777 (octal),
        and the word cannot be  an  executable  instruction.   The  code
        return address is saved in YDSCAD(XLOW), and the state is set to
        F.  The accumulators are restored  and  return  is  effected  to
        where  .JBOPC  points.   When  the special routine (OCRET) whose
        address has been put into the bottom of the stack,  is  entered,
        YDSCAD(XLOW)  is  copied to .JBOPC, state A is set, and .OCRE is
        entered.  The following actions are the  same  as  for  state  A

        III.  Call on SIMDDT not allowed

        State F:  If REENTER is attempted in this  state,  execution  is
        continued  with  a  message.  A new attempt should be made after
        stopping the job again.

        III.  REENTER while  processing  REENTER  in  .OCRE  or

        The contents of .JBREN are OCRE2  in  that  case.   The  current
        SIMDDT   command  is  suppressed  if  possible.   If  SIMDDT  is
        executing, ^C^C followed by REENTER can thus  be  used  e.g.  to
        suppress the rest of a lengthy memory dump or source listing.  A
        message is typed and execution continued, or  SIMDDT  expects  a
        new command.

        III.  REENTER after program exit

        This has the same effect as REENTER after  LOAD  or  GET.   When
        SIMDDT  signals  that  it  is  ready to accept commands, any old
        breakpoint information has been reset.
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        III.  CONTINUE (CCONTINUE) command

        CONTINUE has the normal effect of continuing execution where  it
        was  stopped by a ^C or by execution of some instruction such as
        EXIT 1, HALT, or by some monitor error.

        CCONTINUE is  identical  to  CONTINUE  except  for  leaving  the
        terminal in monitor mode.

        When CONTINUE is given after normal program  exit,  it  has  the
        effect of invoking SIMDDT as if a ^C-REENTER had been given just
        after the final END of the main  program,  but  with  the  block
        state of the main program block set to 1 to allow examination of
        the global variables.  A MONRT.  (EXIT 1,) UUO returns  the  job
        to monitor mode again.
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        III.1.6.2      Overall SIMULA program control by the RTS

        The modules involved in  overall  execution  control  are  OCIN,
        OCIO, OCEP and OCSP.  OCIN and OCIO also contain some I/O code.

        III.    Setting up and initialization of a SIMULA program

        A SIMULA program is started either  directly  by  LINK-10  after
        loading  (EXECUTE  or  DEBUG  command), or by a START or REENTER
        command following a GET or LOAD  command.   The  SIMULA  program
        layout  shown  in  III.  (START  command)  indicates  the
        initial actions.

        III.  Getting the high segment into core (OCSP)

        OCSP (Overall Control, Start Program) is the first component  of
        the  RTS  to  be called from the SIMULA program.  OCSP is loaded
        with the main program, normally in the low segment.   Since  the
        low  segment  cannot  be shared, the functions of OCSP have been
        minimized, most initialization functions being deferred  to  the
        the  high  segment.   The  main  function of OCSP is to load the
        initial  high  segment  into  core  via  a  GETSEG  UUO,   after
        allocation  of a minimal amount of core for the so called STATIC
        area of the low segment.  If the high segment  was  loaded  with
        the  program  from REL files, and the main program was loaded in
        the high segment, OCSP uses SETUWP to turn off write protection,
        since  the main program contains a data area.  After getting the
        high segment, OCSP transfers control to OCIN to perform the rest
        of  the  initialization.   The ac's are saved starting at offset
        YACSAV in the STATIC area, after possibly getting the address of
        SIMDDT  from  YOCDDT  or from a REENTER call on .OCRE0.  OCSP is
        called by JSP instruction, saving the return address in  an  ac.
        In  the  non-sys  version,  it  is possible to look for the high
        segment on several disk areas if the first  GETSEG  fails.   See
        module listing for further details.
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        III.  SIMULA program initialization (OCIN)

        OCIN  starts  with  some  preliminary  initializations:   Clears
        static  area,  sets  standard value for LINESPERPAGE and LOWTEN,
        records time-of-day and runtime at program start for  later  use
        on  program  exit,  sets  up the run-time stack (based on XPDP),
        finds any RUNSWITCHES block, initializes trap and UUO  handling.
        The  rest  of OCIN is concerned with actions related to BASICIO,
        i.e.  SYSIN  and  SYSOUT,  specification  of  other  files   and
        allocation  of  i/o  buffers.  This is described in III.1.7 (I/O

        III.  End of initialization phase (OCEI)

        OCIN ends by transferring control to OCEI, which  opens  SYSOUT,
        loads  SIMDDT  if  space  was allocated and transfers control to
        SIMDDT or the first explicit SIMULA  statement.   If  SIMDDT  is
        first started, it transfers control to the program after its own
        initializations.  OCEI was introduced to define  a  point  where
        control  passes  from the initial high segment to the other high
        segment if two segments are used.

        III.    Swapping of high segments (OCSW)

        The high segment parts of the SIMULA RTS can be classified  into
        two categories:
        a) Routines which are used frequently  by  an  executing  SIMULA
        program,  such  as  block  creation  and  block  entry,  editing
        procedures, etc.
        b) Routines which are used only at the  start  of  execution  or
        very seldom during the execution of a normal SIMULA program.

        In order to reduce the  memory  space  normally  occupied  by  a
        SIMULA  program,  the  high segment routines (collectively named
        SIMRTS) are grouped into two distinct high segments, SIMRn1  and
        SIMRn2  (n  =  SIMULA version number, e.g. SIMR41 and SIMR42 for
        version 4 of SIMULA).  SIMRn1 contains  most  of  the  routines,
        those  of  category  a,  and  the  other  high  segment contains
        routines of category b and also  some  routines  of  category  a
        which are used by the category b routines.

        The routines of category b  are  those  concerned  with  program
        initialization  and file object creation.  Communication between
        the two high segments is via the transfer vector placed  at  the
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        start  of  the high segment and defined in the SIMRTS.MAC module
        via the RTSYMBOLS macro.  The transfer vector has the same  size
        in  both  high  segments.   The difference is in the contents of
        certain vector elements.  Each global  subroutine  in  the  high
        segment  has  a  fixed absolute position in the transfer vector.
        For those routines that are present in the current high segment,
        the  transfer  vector  contains a JRST instruction to the actual
        entry point.

        A subroutine which is not present in the current high segment is
        represented  by a "PUSHJ XPDP,.OCSW" instruction in the transfer
        vector, leading to the swapping routine.

        .OCSW handles a call in the following way:
        All ac's are saved starting at YUUOAC(XLOW).  If the call to the
        eventual  RTS routine was via a PUSHJ XPDP,xxxx instruction, the
        next lower level of the stack should point  to  the  instruction
        following  the PUSHJ.  The alleged PUSHJ instruction is saved in
        the stack over the GETSEG for some checking when  the  new  high
        segment  has  been brought in.  A GETSEG UUO brings in the other
        high segment.

        Since, by design, the transfer vectors and  the  OCSW  procedure
        are  identically  placed  in  their respective high segments and
        exactly similar except for a few locations, GETSEG  will  return
        to  the next instruction, which is now in the OTHER high segment
        (parallel worlds!!).  Since channel 0 is  used  for  GETSEG,  it
        must  be  restored if it was active for TTY I/O before the swap,
        and the buffers must be relinked.  The  address  of  the  called
        entry  in  the  transfer  vector  is  retrieved  from the return
        address found on the stack.

        If the original call was a PUSHJ  to  this  address,  OCSW  must
        prepare  to swap the other segment back in.  This is arranged by
        stacking as return address  the  address  of  L6,  where  L6  is
        defined differently in the two high segments:
        1)    L6: EXEC .OCSW        2)        POPJ XPDP,
        2)        POPJ XPDP,             L6:  EXEC .OCSW
        OCSW calls the ultimately  wanted  routine  by  jumping  to  the
        corresponding location in the current high segment, which should
        have a JRST to the actual code.  If the  subroutine  returns  to
        its caller, the return will be to L6 as defined above, causing a
        new swap and a final return to the caller,  since  the  location
        corresponding  to  L6  in  one  segment has a return instruction
        (POPJ XPDP,) in the other segment.
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        III.    RTS error handling

        A Run Time System error is normally reported via the RTSERR  UUO
        as defined in the UNIVERSAL parameter file SIMMCR.
        Some errors are detected by other means, e.g.  arithmetic  traps
        (III.,  errors  in routines from FORLIB (III.,
        errors  during   program   initialization   (III.   and
        Some errors are left for the monitor to handle, such as  failure
        to  find  a  high  segment  via GETSEG (a HALT instruction sends
        control to the monitor), push down list overflow, exceeded  time
        limit etc.
        In some situations which should not  occur  in  a  valid  SIMULA
        program + RTS, a RFAI UUO (defined in SIMMCR) is issued.

        III.  "Normal" RTS errors in SIMULA (RTSERR)

        An error in a running SIMULA  program  is  usually  reported  by
        execution  of  the RTSERR UUO.  The error number is indicated by
        the address field of the UUO (no index,  no  indirect  address).
        An  alternative  way  of indicating the error would be to use an
        ordinary subroutine linkage instruction such as PUSHJ, JSR or an
        XCT   applied   to   some  globally  accessible  location.   The
        disadvantage is that another word would be needed to specify the
        particular  error,  and  since  the number of possible errors is
        rather large, this approach was rejected.  The main disadvantage
        with the UUO approach is a possible clash with other uses of the
        same UUO, but since the SIMULA RTS must remain in control in any
        case, that disadvantage is not felt to be great enough.

        The error UUO and other UUO's are handled initially by  the  RTS
        routine OCUU.
        OCUU saves all ac's starting at YUUOAC(XLOW).
        The UUO opcode is checked  and  the  following  happens  if  the
        RTSERR UUO is recognized:
        The error number is placed in YDSENR(XLOW) and  the  address  of
        the  error  UUO is placed in YDSEAD(XLOW).  If the error did not
        occur in compiled code (RTS stack not  empty),  a  corresponding
        code  address  is computed from the bottom element of the stack,
        and that address is placed in YDSEAD instead.
        If SIMDDT is not already in core, OCLD attempts to  load  it  to
        the storage pool, possibly after expanding core or collection of
        garbage records.
        If SIMDDT cannot be loaded, the error number is reported  by  an
        inline  error  message  and  the  program is terminated via OCEP
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        (closing files), otherwise SIMDDT gets  control  via  its  DSINE
        entry  point,  writes  the  error message, identifies the source
        line and module, then expects commands from the user.  If SIMDDT
        returns  (via  an  EXIT  command),  the  program  is  terminated
        (closing files etc.).

        For some errors, continuation is  possible.   Those  errors  are
        distinguished  by  having  a non-zero ac field in the error UUO.
        The SIMDDT PROCEED command can be used  to  continue  execution.
        Error  recovery  is  either automatic (standard action) or a new
        text or integer value  is  requested  to  replace  faulty  data.
        Extensions of this scheme are possible.

        III.  RFAI UUO

        The RFAI UUO is used to signal an unusual situation in the  RTS;
        a  situation  which should not occur in a correct RTS, operating
        with a correct SIMULA program (without any  modules  written  in
        other  languages  such  as  MACRO-10 or FORTRAN).  An RTSERR UUO
        with a special error number is simulated when the  RFAI  UUO  is
        interpreted.  The ensuing treatment is described above.

        III.  Illegal UUO

        If OCUU encounters an unrecognized UUO code, an error message is
        again generated as for RFAI.

        III.  Errors in FORTRAN library routines

        In library routines taken from  the  FORTRAN  library  (FORLIB),
        e.g.  SQRT,  errors  are  reported  by the following instruction
                 XCT error-class, FORER.
                 CAI error-type, address(severity-code)
        Here error-class may be ER%LIB or ER%APR, where  the  errors  of
        class  ER%LIB  usually  have  an  associated ASCIZ error message
        string starting at "address", and  the  ER%APR  errors  simulate
        arithmetic traps, where address is where to go on recovery.

        An ER%LIB error is handled by first typing the routine  name  in
        an  informative  message  prefixed  by  "ZYQEIR" then typing the
        message string at "address" prefixed by "ZYQFLE".   An  ordinary
        error  message  (RTSERR  QFORER) is then simulated, transferring
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        control to OCUU.

        For an ER%APR error, error-type specifies  which  trap  (integer
        overflow, floating point underflow, divide check, etc) should be
        simulated.  Error-type is translated to  the  equivalent  SIMULA
        error number, and an ordinary error message is again simulated.

        III.  Errors during program initialization

        Some errors are detected in situations where  SIMDDT  cannot  be
        invoked because the necessary RTS environment is not yet set up.
        This occurs e.g. if the core size is too small to allow  program
        execution,  if  the  high segment cannot be loaded, or if SIMDDT
        cannot be loaded.  Any error during the initial setup of  SYSIN,
        SYSOUT  etc in OCIN is reported directly to the terminal without
        SIMDDT intervention.

        III.  Errors diagnosed by the monitor

        Some errors cannot be handled easily within the  RTS.   Examples
        are  push down list overflow (presently, the stack is needed for
        error handling), time limit exceeded.

        Attempted  execution  of  illegal  or  privileged  instructions,
        execution  of  a HALT etc, will also lead to an error on monitor
        level, as will some errors in  connection  with  file  handling.
        See Monitor Calls (in the Software Notebooks).
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        III.  SIMDDT - RTS interface

        SIMDDT, the interactive SIMULA debugger, is described  elsewhere
        in  this  document  (section  IV).   The  RTS is responsible for
        bringing SIMDDT into  core  and  calling  it  in  the  following
        - At program start in response to a DEBUG or REENTER command.
        - At program start to remove breakpoints if loaded earlier.
        - At a breakpoint inserted by SIMDDT.
        - Via REENTER after ^C interrupt.
        - When a RTS error has occurred.
        - After program exit  in  response  to  a  CONTINUE  or  REENTER

        III.  SIMDDT at program start

        SIMDDT will be called (entry point DSINI) before  the  start  of
        the   SIMULA   statements,  but  after  RTS  initialization,  if
        execution starts at the entry point .OCRE0 of the RTS.  This  is
        achieved  via the DEBUG command or via the REENTER command after
        a LOAD or GET or after program  exit.   See  III.1.6.2  (REENTER
        command).  When invoked in this way, SIMDDT stays in core during
        program execution and also over a new START or  REENTER  command
        issued after program exit.  A subsequent START command will thus
        have the same effect as REENTER, i e SIMDDT cannot  be  bypassed
        once  loaded  at  program  start.   After  possibly setting some
        breakpoints, the user starts normal  program  execution  with  a
        PROCEED command.

        III.  Removing breakpoints on restart

        If the program is restarted after successful execution or  after
        an  error,  SIMDDT  may  have  left  breakpoints lying around if
        invoked dynamically via ^C-REENTER.  Any  such  breakpoints  are
        removed  by  calling  OCSPDR  at  the  start  of OCSP or .OCRE0.
        OCSPDR determines if SIMDDT is in core  by  checking  the  right
        half  of .JBOPS, and if that is non-zero, the global cell YDSBAS
        based on the address in .JBOPS.  If YDSBAS is  non-zero,  it  is
        assumed  to  be the base of SIMDDT, which is called at the entry
        point DSINS to remove the breakpoints specified in its  internal
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        III.  Invoking SIMDDT at breakpoints

        When a breakpoint UUO is recognized by  the  UUO  handler  OCUU,
        SIMDDT  is  invoked at its entry point DSINR, provided SIMDDT is
        available.  Otherwise, an illegal UUO is indicated.

        III.  Invoking SIMDDT by ^C-REENTER

        SIMDDT may be invoked at almost any program  point  by  stopping
        the  program with one or two ^C commands, then issuing a REENTER
        command.  See III.

        III.1.6.3  SIMULA constructs for execution control - RTS support

        The  facilities  for  execution  control  covered   by   section
        III.1.6.2  have  no  direct  counterparts in the SIMULA language
        definition.   The  present  section  describes  those  execution
        control  features defined in SIMULA which are not implemented by
        compiled code.

        III.  The state of execution of a block

        The state of execution of, e g, a procedure block, is defined by
        the values of some items of information:
        - The values of all variables in the program.
        - The current program counter PC, also called  program  sequence
        control, PSC.
        - The current block CB, i e the nearest block  with  a  display.
        The  accumulator  designated  XCB  always  points to the current
        - Possibly, the current subblock, implicitly defined by PSC  and
        - The  reactivation  point  RP,  consisting  of  a  reactivation
        address  RA and a block instance address RBI, reactivation block
        instance.  In the RTS, RP=(RBI,RA) is represented in the display
        record by (ZDRZBI,ZDRARE).
        - The state is also  characterized  by  the  Boolean  conditions
        detached  and terminated.  A block which is not detached is said
        to be attached, and a block which is not terminated is  said  to
        be active.
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        - In a wider sense, the state of a block is also affected by the
        program  environment,  e  g  the  state  of  I/O  devices, other
        programs on the machine including the  monitor.   These  effects
        are not considered in this section.

        III.  Procedure call, procedure exit

        A procedure is invoked via a procedure statement or  a  function
        designator.   Some  procedures  are implemented as inline coding
        sequences.  These are not considered here.  Other procedures are
        implemented  as  standard  procedures  which give a well-defined
        result and return to the caller  without  essentially  affecting
        the  program  state  except  for changing some variables.  These
        procedures are described in the sections on I/O  handling,  TEXT
        handling and standard procedures respectively.

        III.  Calling a SIMULA procedure

        A SIMULA procedure is set up by one of the subroutines  CSSN  or
        CSSW.   CSSN is used when setting up a "normal" procedure, i e a
        procedure which is declared  and  statically  visible  from  the
        point  of  call,  and  which is not an attribute of an inspected
        class or a match for a virtual procedure.  Calling sequence:
                <save ac's>
                MOVEI    XSAC,prototype address
                PUSHJ    XPDP,CSSN
                <parameter transmission>
        The sequence <save ac's> is only needed if intermediate  results
        exist  in  some  accumulators  (and possibly pseudo ac locations
        .YXAi).  The sequence is:
                PUSHJ   XPDP,CSSA
                XWD     n,admap
        where n is the number of intermediate results starting by  XWAC1
        and admap is the address of a bit map specifying which ac's have
        object pointers (see III.1.5.2).  CSSN allocates  the  procedure
        block  and  its  attached  display record (ZDR) by calling SADB,
        then copies most of the display vector from that of CB.   RP  is
        set  to (CB,return address).  The ZAC address, if any, is copied
        by SADB to the display vector from YCSZAC which is then reset to
        zero.   If  the  procedure  has  no  parameters, CSEN is entered
        directly instead of returning to the calling program.   On  exit
        from CSSN, XRAC=XWAC1 holds the address of the procedure block.
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        The calling sequence to CSSW is
                [evaluate ZDP of procedure to top ac's]
                PUSHJ   XPDP,CSSW
                XWD     n,admap
                <parameter transmission>
        The top ac's are XWAC1+n and XWAC2+n.  If n is  zero,  admap  is
        also  zero,  signifying  that  no  intermediate results exist in
        ac's.  CSSW allocates a procedure block and display  vector  via
        SADB  like  CSSN.   The ZDP information is saved in YOBJAD(XLOW)
        over a  possible  garbage  collection  and  relocated  there  if
        necessary.   The innermost level (furthest from the block in the
        display) is the address of the procedure block itself, put there
        by  SADB.   The  next to innermost level in the display is taken
        from ZDPEBI of the input dynamic procedure address, and the rest
        of the display is copied from the display of ZDPZBI.

        III.  Code for parameter transmission

        The sequence <parameter transmission>  is  only  needed  if  the
        procedure has parameters and has the following form:
                <parameter coding>
                PUSHJ   XPDP,CSEN       ;Enter procedure coding
        If the procedure has parameters, CSSN or CSSW returns to  SIMULA
        code  for  evaluation and transmission of the actual parameters.
        The coding sequence, <parameter coding>,  compiled  to  transmit
        the  parameters  must preserve the value of XWAC1 by using XWAC2
        as top ac and causing a ZAC object to be  created  whenever  the
        garbage   collector  may  be  called.   The  <parameter  coding>
        consists of coding to evaluate and transmit a representation  of
        each  actual  parameter.   The  code  generated  for a parameter
        normally consists of a straightforward expression evaluation  to
        one  or two ac's and a store operation which places the value in
        the  formal  location.   For  name   parameters,   a   parameter
        descriptor  (ZFL instance) is computed and stored, and any thunk
        coding is compiled inline and bypassed with a  JSP  instruction.
        For   parameters   by   reference   or   by  value,  the  normal
        representation is  used  for  variables  and  expressions.   For
        procedure,  label or switch parameters, a dynamic representation
        is computed (ZDP, ZDL or ZDS instance) and stored.  For  a  TEXT
        parameter  by  value, the system routine COPY is called, and for
        an ARRAY by value, CSCA  is  called.   When  calling  a  formal,
        virtual  or  external  NOCHECK  MACRO-10 procedure, however, the
        parameter transmission sequence following the PUSHJ to CSSW  has
        a  special  form  involving a call on the RTS routine PHPT.  See
        parameter handling.  CSEN is called directly from CSSN  or  CSSW
        if  no  parameters exist, otherwise it is called after parameter
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        transmission to start the operations of the procedure.

        III.  Starting the actions of a SIMULA procedure

        CSEN changes XCB and jumps to  the  declaration  coding  of  the
        procedure.   A  procedure,  unlike  a class object, can never be
        detached.  When it terminates via CSEP for a function  procedure
        or  CSES  for  a  pure procedure, its data area can be reclaimed
        directly or by subsequent garbage collection.

        III.  Calling FORTRAN and MACRO-10 procedures

        OPTIONS(/E:FORTRAN (or F40),entrypoint);
        The coding sequence for calling an external FORTRAN procedure is
        essentially  the same as that for any SIMULA procedure, with the
        restriction that REF, LABEL, SWITCH,  and  PROCEDURE  parameters
        are  not  allowed.  A procedure block is allocated in accordance
        with a special prototype generated from the specification coding
        for   the   procedure.   The  special  routine  PHFO  serves  as
        intermediary between the SIMULA-generated  procedure  block  and
        the  FORTRAN  code.   See  parameter handling.  When the FORTRAN
        code is active, XCB is saved in the global cell YFOXCB.

        A NOCHECK MACRO-10 procedure is handled like a formal or virtual
        SIMULA  procedure,  i  e  by a special calling sequence to CSSW,
        including a call on PHPT.  See parameter handling.  Strict rules
        must  be  followed  by  the  MACRO-10  procedure in order not to
        compromise the security of the SIMULA program.

        A  MACRO-10  procedure  with  specified  parameters  is  handled
        exactly as a SIMULA procedure.

        OPTIONS(/E:QUICK [,NOCHECK] ,entrypoint);
        A MACRO-10 procedure which is specified as "QUICK" (not  "CODE")
        takes  all  its  parameters  in  ac's  and  is called by a PUSHJ
        XPDP,entrypoint instruction.  The calling sequence is  the  same
        as for e.g. MAIN, STRIP.  The RTS is not involved, only compiled
        code.  See SIMLH2.MAN Chapter 7 and appendix E.
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        III.  Procedure exit

        The exit from a procedure is via the final statement or possibly
        via  a  GOTO  statement.   A  function procedure exits via CSEP,
        which restores any intermediate results and  puts  the  function
        value on top.  The presence of intermediate results is signalled
        by the ZDNACS bit in  the  object,  and  the  saved  values  are
        retrieved  via  the  ZDRZAC  pointer in the display record.  Two
        ac's are always transmitted to avoid testing for result type  in
        CSEP.   The  compiled code knows which ac(s) to use according to
        the stack discipline for register allocation.  A pure  procedure
        exits via CSES, which is similar to CSEP except that no function
        value is returned.  The procedure block may be deallocated if no
        block  which  is still referenceable was allocated on top of the
        procedure block.  A procedure always returns to the  environment
        given  by (ZDRZBI,ZDRARE), the dynamic link, except for a direct
        or indirect GOTO statement.

        III.  Creation of a class object

        A class object is generated  by  an  "object  generator"  -  NEW
        <class-id>  [<parameter  list>].   An  object  generator is very
        similar to a function designator  and  may  also  serve  like  a
        procedure  statement if the value is discarded.  Since name mode
        parameters are not allowed, the calling sequence is simpler than
        a general procedure call.  Calling sequence:
                <save ac's>
                PUSHJ   XPDP,CPNE
                XWD     m,prototype address
                <parameter transmission>
        where m=offset in display of the block whose display  should  be
        copied.   Usually, the display of XCB can be copied, but in some
        cases of inspection statements all relevant levels  may  not  be
        correct.   CPNE works in a fashion very similar to CSSN.  If the
        class has local classes declared (ZCPKDP bit set in  prototype),
        the  ZDNKDP  bit in the block must be set to prevent the display
        from being deallocated when the class object is terminated.  The
        display  may be needed when calling a procedure attribute of one
        of the local classes, which may again be terminated and  without
        a  display.   Any  REF  or ARRAY locations on the current prefix
        level of  the  object  are  initialized  to  NONE  via  SADB  on
        allocation.  CPIN is called to initialize REF and ARRAY cells in
        the outer prefix levels.  If parameters exist, control goes back
        to  SIMULA  code  for  parameter transmission, otherwise CSEN is
        called directly.   Parameter  transmission  is  done  as  for  a
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        procedure  but  is  simpler  since only value and reference mode
        parameters  are  possible  and  label,  switch   and   procedure
        parameters  are  not allowed.  See III.  CSEN enters
        the declaration coding of the outermost prefix.  CPCD  transfers
        control to the declaration coding at the next inner level or the
        statements at the outermost level.  CPCI  transfers  control  to
        the  statements of the next inner prefix level at an explicit or
        implicit INNER statement.  At the end of the statements  at  the
        innermost  level  without  an  explicit INNER, CPE0 is called to
        terminate and detach the class object.   If  an  explicit  inner
        exists  for  some  prefix,  compiled  code  effects the transfer
        either directly or via ZCPIEA of the prefix, if the code address
        is  not  available  at  compile time (external or system class).
        Control returns with an object reference in the  top  ac  either
        when the class terminates or when an explicit or implicit detach
        is performed.  See below (quasi-parallel sequencing).

        III.  Entering a prefixed block

        A prefixed block is set up in a way  very  similar  to  a  class
        object.  Calling sequence:
                MOVEI   XSAC,prototype address
                PUSHJ   XPDP,CPSP
                <parameter transmission>
        The display is  copied  from  the  statically  enclosing  block,
        adding the address of the prefixed block itself at the innermost
        level.  Initialization is done as for a class object.  In  order
        to  implement quasi-parallel sequencing as described by CBL 9.2,
        an enclosing detached block must be linked dynamically  to  this
        prefixed  block.   This  is  done  by  setting RP of the nearest
        enclosing detached block to (address of this prefixed block, 0).
        If this is the outermost prefixed block, the dynamic link is set
        to point to itself.  After any parameter transmission, which  is
        similar  to  that  for  a  class,  CSEN transfers control to the
        declaration coding at the outermost prefix level as for a class.
        Instead  of  the  last  call to CPCD, however, CPPD is called to
        transfer control to the statements of the outermost prefix.  The
        block  terminates  via compound tails of prefixes and ultimately
        via CPE0, which restores  the  XCB  of  the  static  environment
        before exiting to the instruction after the prefixed block.
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        III.  Implementation of quasi-parallel systems

        Quasi-parallel systems (QPS) are described in CBL 9.2.   In  the
        present  section  the  actual  implementation  of  a QPS control
        structure is outlined.

        III.  Information needed to define a QPS

        A QPS consists of
        a) A prefixed block, called the main program of the QPS.
        b) Any enclosed detached class objects or prefixed blocks.
        c) Any other blocks enclosed by the main program.
        The COMPONENTS of the QPS are the detached instances.  Blocks of
        category  (c)  are  relevant only to define "enclosed", which is
        interpreted according to CBL 9.1, q v.   The  QPS  structure  is
        defined  by  the  reactivation  points  of  its components.  The
        reactivation point RP = (RBI,RA) of a component  is  interpreted
        in the following way:
        If RA is zero, RBI points to an enclosed  prefixed  block  which
        contains  the  actual reactivation point.  If RA is non-zero, it
        points to the program address  where  execution  should  resume,
        with XCB=RBI.

        The basic means for controlling  the  execution  of  a  QPS  are
        provided  by  the  system procedures DETACH and RESUME, CBL 9.2.
        The procedure CALL has been implemented to simplify  programming
        of coroutines.

        III.  DETACH statement

        The detach statement works according to CBL 9.2.1.  The  effects
        described  there  are  implemented  by the CPDT procedure in the
        following way:

        a) The smallest operating block instance, X, is attached, either
        because  it  has  just  been generated via NEW or because it has
        been reattached via CALL.
        Actions:  RP := (X,RA), where RA is program point after call  on
        detach.   ZDNDET  is set, and control returns to old RP (calling
        block,return address).

        b) X is already detached.
        Actions:  RP := (X,RA)  as  above.   The  main  program  of  the
        innermost  enclosing QPS is then found by following static links
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        for detached objects, dynamic links for attached objects,  until
        a  prefixed  block is found.  The static link SL of a block B is
        the block address found at offset B.ZBIZPR.ZCPSBL relative to B.
        RP  =  (BI,RA)  of  the  main  program is found.  If the program
        address RA is zero, the reactivation  point  is  inside  BI  (an
        enclosed prefixed block), and RP := BI.RP, whose program address
        RA is then examined, etc until a  non-zero  program  address  is
        found.   This  is where execution resumes, with XCB set to RP.BI

        III.  RESUME statement

        The resume procedure has the effects  described  in  CBL  9.2.2.
        The  RTS  routine  CPRS  implements  resume.   Assume  that  the
        detached object to be resumed is Y, component of a QPS called S.
        Resume is called inside a currently operating component X of the
        same QPS.

        Actions:  X, the nearest detached block instance,  is  found  by
        dynamic  links  starting from XCB.  RTS error (RESUME operating)
        if X=Y.  X.RP := (CB,RA), where CB is current block, pointed  to
        by  XCB  and  RA = address after resume statement.  To check for
        errors, all enclosing detached instances are found by  following
        static and dynamic chains from X as in the detach procedure (the
        operating chain  is  followed)  until  the  outermost  block  is
        reached.   If  any  of  the  encountered blocks is Y, we have an
        error.  Starting from  Y,  find  actual  reactivation  point  by
        following  RP  = (BI,RA) until RA is non-zero.  Resume execution
        at that RP.

        III.  CALL statement

        The CALL procedure (CPCA) is not part of the Common Base but  is
        a  recommended  extension.  CALL essentially reverses the effect
        of DETACH, as follows:
        Assume the statement CALL(Y), executed within a QPS component X.
        X  and  Y  are  components  of  the  same  QPS  S.  Y may not be
        attached, terminated or operating.  X  is  found  starting  from
        XCB,  following  RP  until  a detached block is found.  Error if
        X=Y.  Copy Y.RP to X.RP to set correct reactivation  point.   As
        in  CPRS,  all  detached blocks enclosing X are found via static
        and dynamic links and checked against Y  (error  if  identical).
        Attach  Y to caller by setting off the ZDNDET bit and setting RP
        = (CB,RA), where CB is current  block,  RA  is  program  address
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        after  CALL  statement.  The reactivation point of Y is found as
        in CPRS and execution resumed there.

        III.  Object termination

        Ignoring any compound tails after explicit INNER statements, the
        effect  of  passing  through  the  final  END of an object is to
        detach and terminate the object.  CPE0 is called.  For  a  class
        object,  CPE0  sets the ZDNTER bit and transfers control to CPDT
        (see detach).  The same is done for a prefixed block, but  since
        detach has no effect, control is returned to CPE0 which restores
        XCB and transfers control to the statement  after  the  prefixed

        III.  GOTO statement leading out of an object

        See CBL 9.2.4.

        III.  INNER statement

        The INNER statement is implemented by the  CPCI  routine,  which
        transfers  control  to  the  statement  coding at the next inner
        prefix level.  Has no effect if there is no inner level.

        III.  GOTO statements, switches

        GOTO statements are normally handled by compiled code.   If  the
        target of the GOTO implies change of current block (CB), CSGO is
        called, however.  The parameter to CSGO is a dynamic label  (ZDL
        instance).   CSGO  follows  the operating chain from the current
        block until the target block is found or the outermost block  is
        reached.   Class  objects  passed  on the way are terminated and
        subblock addresses eliminated from displays when passing.  Since
        the target must be operating, a GOTO out of a detached object is
        allowed only if it leads out of the nearest  enclosing  prefixed
        block,  and  a  transfer to a connected label will be allowed if
        and only if the inspected block is operating.  A SWITCH  element
        is  evaluated by CSSC, which takes a dynamic switch address (ZDS
        instance) and a switch index as parameters and returns a dynamic
        label  address  (ZDL).   If  the  switch element is a statically
        visible label, the  static  description  is  translated  to  the
        dynamic   form.   If  the  switch  element  is  a  designational
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        expression, a dummy procedure block is set  up  and  the  switch
        thunk  is  entered,  to  return later via CSES.  CSES works like
        CSEP except that the dynamic label is already in the top ac's.

        III.1.6.4  Execution control by compiled code

        Apart from calling sequences  to  RTS  routines,  the  principal
        means  of  execution  control  in  compiled code are conditional
        statements and expressions, WHILE loops, FOR  loops  and  simple
        GOTO  statements.   Only  FOR loops are interesting from the RTS
        point of view, because the return  address  of  a  FOR  loop  is
        stored  in  the  display  vector.   One  word is required in the
        display vector for each FOR loop nesting level  in  the  current
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        III.1.7  I/O handling

        I/O handling in a SIMULA  program  covers  the  following  basic

        BASICIO functions, i e setting up and opening SYSIN and  SYSOUT.
        In  DECsystem10  SIMULA,  file  specifications  may  be read and
        runtime switches interpreted.

        Creation and setup of file objects of the various file  classes,
        allocation and handling of buffers.

        Implementation of the basic I/O functions OPEN, CLOSE,  INIMAGE,

        I/O editing functions:  INCHAR, OUTCHAR, ININT etc.

        III.1.7.1  BASICIO functions

        The functions related to the definition of BASICIO in CBL 11 are
        mostly  implemented  in the OCIN module of the RTS.  Pointers to
        the standard file objects SYSIN and SYSOUT  are  placed  in  the
        static  lowseg  area at offsets YSYSIN and YSYSOUT respectively.
        FILE prototypes  are  placed  in  the  transfer  vector  (SIMRTS
        module),  whereas  declaration  and  statement coding and symbol
        tables are placed in the IO module.  A special naming scheme  is
        adopted for quantities relating to file subclasses:
        Each class is denoted by 4 letters:  IOFI (FILE), IOIN (infile),
        IOOU  (outfile), IOPF (printfile) and IODF (directfile).  SIMRPA
        defines a macro for each prototype.  If the class code  is  xxxx
        (=IOIN  etc),  then  .xxxx  is  the  global name (prototype base
        address), xxxx%S is  the  address  of  the  initial  statements,
        xxxx%D is the start of declaration coding, D%xxxx is the name of
        the macro, xxxx%I is  the  first  address  after  inner  in  the
        statement  part,  xxxx%M is the block map address, and xxxx%Y is
        the address of the symbol table.

        Before allocating SYSIN and  SYSOUT,  a  possible  specification
        file  defined via a compile-time /R switch is read.  Encountered
        switches are acted  upon,  buffer  space  is  allocated,  and  a
        specification  table  for  files  is  built.  This table will be
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        consulted whenever a file subclass is created.

        When any specification file has been processed, .SAGI is  called
        to initialize the storage allocation system, leaving the buffers
        and specification tables below the bottom of the storage pool.

        Next, the SYSIN infile object is set up.   Normally,  SYSIN  and
        SYSOUT  will  both be the user's TTY, but the specification file
        may define SYSIN and/or SYSOUT differently.  In the  absence  of
        specifications  for  SYSIN  or SYSOUT, the logical name SYSIN or
        SYSOUT is checked to find out if  it  has  been  assigned  as  a
        device  name.   If  so,  the  file named SYSIN or SYSOUT on that
        device is used,  otherwise  the  TTY  is  used.   See  OCIN  for
        complete  algorithm. is performed, where
        n=80 or the width of the TTY line in the case when  SYSIN  is  a

        The SYSOUT printfile object is then set up,  and  LINELENGTH  is
        determined  as  132  or the width of the TTY line if applicable.
        The actual execution of is  left
        to OCEI which is then entered.

        The final actions of BASICIO, SYSIN.close and SYSOUT.close,  are
        performed at program end via OCEP.

        III.1.7.2  File object generation, file setup, buffer allocation

        The calling sequence for a file object generation is similar  to
        that for a SIMULA-coded class, for example:
                PUSHJ   XPDP,CPNE       ;NEW infile("INPUT")
                XWD     -n,IOIN
                DMOVE   XWAC2,[XWD 0,-2+[ASCII/INPUT/]
                               XWD 5,0]
                PUSHJ   XPDP,TXCY
                XWD     1,[1B0]
                PUSHJ   XPDP,CSEN
        This calling  sequence,  when  executed,  causes  the  following
        - The file object is created via CPNE and SADB.  The display  is
        copied  from the block referenced at offset -n in the display of
        the current block.
        - CSEN  changes  XCB  and  sets  the  return  information,  then
        transfers  control  to  the declarations of the outermost prefix
        level, IOFI%D.  Neither IOFI%D  nor  IOIN%D,  which  is  entered
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        next,  contains  any  real  declaration coding, only calls CPCD.
        The statement  coding  part  of  IOFI,  IOFI%S,  which  is  then
        entered,  is also empty, transferring control to IOIN%S via CPCI
        (inner).  At IOIN%S, the real initial  actions  for  the  infile
        start  by  setting  flags  and  clearing  certain switches, then
        calling the SETUPFILE routine of the IONF module

        - SETUPFILE  uses  the  information  in  ZFISPC  (copy  of  text
        parameter  to  NEW  ...)  and any specification table created at
        BASICIO   initialization   (see   above)   to   determine    the
        characteristics  of the file which is to be input and/or output.
        If sufficient information is not  available  or  a  dialogue  is
        expected,  the user is interrogated for the missing information.
        A free software channel is claimed and OPENed.  Device and  file
        name,  number  and  size  of  buffers,  etc  is  determined,  if
        necessary by consulting  the  user.   CREATEFILE  allocates  and
        links  the  buffers,  performs  new  OPEN on the same channel as
        before, and performs LOOKUP and/or ENTER.

        III.1.7.3  Other basic I/O functions

        Apart from the I/O  functions  mentioned  above,  the  following
        procedures  contained  in  the  IO  module are essential:  OPEN,
        remaining  procedures  are  merely applications of text handling
        procedures.  Most of the procedures conform to  the  definitions
        in CBL chapter 11.  Special information:

        - OPEN (IOOP):  Special actions must be  done  via  the  routine
        REOPEN  in  IONF  if  the  file was once opened and then closed.
        Buffers must be allocated and chained, information fetched  from
        the  specification  table,  an  OPEN  UUO must be performed etc.
        These things are normally done at file creation (see above), and
        the  SIMULA  procedure OPEN simply does what CBL specifies.  For
        DIRECTFILES, however, OPEN  computes  file  record  length  from
        image.length,  allowing  for  <CR><LF>  and  rounding  up  to  a
        multiple of 5 (word alignment), and computes max  LOCATION  from
        file size and record length.

        - CLOSE (IOCL), in addition to doing what CBL  prescribes,  also
        outputs  the  last  image for an outfile, provided pos > 1.  The
        software channel  is  released  and  YIOCHTB  updated.   If  the
        channel  table  entry  refers to two file objects representing a
        bi-directional device such as a terminal,  the  channel  is  not
        released until the last of the two files is closed.
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        - INIMAGE (IOIG) has the following special properties:  Carriage
        returns  and null characters are ignored.  Line feed is taken as
        end of image but is not transmitted to the  image.   Form  feed,
        vertical  tab  and altmode (octal 14, 13, 33) are taken as image
        delimiters and are also taken as part of the image.  If  inimage
        tries  to  read  past  the end of the file or reads an unwritten
        record  inside  a  directfile,  a  special  end-of-file   record
        starting  with  /*  and padded with blanks is transmitted to the

        - OUTIMAGE (IOOG).  Only image.strip is moved to the  buffer  to
        save  file  space and time.  Carriage-return is tagged on to the
        end.  If the buffer is output immediately, as for a TTY file,  a
        line  feed  is added before output.  Pending line feeds from the
        last line, if any, are output to the buffer before  the  current
        line  is  output.   This  is  because  of  our interpretation of

        - BREAKOUTIMAGE (IOBO).  Works as OUTIMAGE, except that no CR-LF
        is  added.  To enable spaces at the end to be significant, image
        is not stripped shorter than image.pos-1.

        - LASTITEM (IOLI) works according to CBL, except that horizontal
        tab (octal 11) is treated like the space character.

        - ENDFILE is implemented as an ordinary variable in  the  infile
        or  directfile object.  ENDFILE is set to -1 (TRUE) when inimage
        finds that the file is exhausted, i.e. when the  last  character
        in the file was read by the previous inimage call.

        - LOCATE merely checks if the argument is in range  and  if  so,
        sets  off endfile flags and sets a new LOC value = argument.  No
        operations directed to an external device are performed here.

        Special considerations for directfile:

        - The images in a directfile are all of the same  length,  given
        by  the  size  of  the  text  given  as IMAGE when calling OPEN.
        Images are buffered within disk buffers  (only  DSK  allowed  as
        device),  and  the  disk  buffers  are  held  in core as long as
        possible to minimize actual I/O.  When inimage  or  outimage  is
        performed  on  a  disk block not currently in core, the block is
        read first if it exists.  If no  outimage  was  performed  on  a
        buffer,  it  is  not  rewritten  on  disk when replaced in core.
        Close does not imply that the  last  image  is  written  as  for
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        III.1.7.4  Editing procedures for input/output.

        The editing procedures, e g INCHAR, ININT, OUTFRAC, are  defined
        in  terms of the corresponding text attributes.  The definitions
        given in CBL usually cover those  functions  sufficiently.   The
        "inaccessible"  function  FIELD  of CBL 11.3.1 is implemented as
        .IOFD.  Deviations from CBL:
        INCHAR (IOIC) and INTEXT (IOIT) test for ENDFILE before  calling
        inimage.   In this way characters from the end-of-file image can
        be read.
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        III.1.8  Text handling

        Text handling is performed by the  modules  TX  and  TXBL  which
        contain  a)  RTS  procedures  and functions corresponding to the
        text attributes found in section 10 of  CBL,  b)  the  non-local
        procedures  BLANKS  and  COPY.   Text  relations  and text value
        assignment are also handled by special  procedures.   A  routine
        for creating a temporary text variable (ZTT) is also part of the
        TX module.  Some attributes are implemented  by  inline  coding.
        The representation of text information is described in I.5, data
        structures, and LH2 appendix B.

        III.1.8.1  Text attributes

        Most  text  attributes  are   implemented   by   straightforward
        translation   of   the   CBL  definition.   Some  implementation
        dependent details are outlined below.
        Pos is ZTVCP+1, i e pos=1 implies ZTVCP=0  which  is  convenient
        since it allows NOTEXT to be represented by two zero words.  The
        attributes LENGTH, POS and MORE are implemented by inline  code.
        The  function  attributes  MAIN  (TXMN),  GETCHAR (TXGC), GETINT
        (TXGI), GETREAL (TXGR) and GETFRAC (TXGF) are called as follows:
                MOVEI   XTAC,Xtop
                PUSHJ   XPDP,function
        where Xtop and Xtop+1 contain the text reference  on  entry  and
        the  returned  function  value on return.  The other attributes:
        PUTFIX  (TXPX)  and  PUTFRAC  (TXPF)  are  called  with the text
        reference in XWAC1 and XWAC2, other parameters in XWAC3,  XWAC4,
        etc.   The  additional  procedure LOWTEN (TXLT) is called in the
        same way.

        III.1.8.2  Other text procedures

        Texts are generated by COPY (TXCY), BLANKS (TXBL) and by  INTEXT
        (IOIT),  which  is  part  of the I/O sub-system (III.1.7).  Text
        assignment of the form T1  :=  T2  is  performed  by  TXVA,  and
        comparison  of  text  values, e g T1 < T2, is performed by TXRE.
        TXRE and TXVA are called with XTAC pointing to the first  ac  of
        the  4  consecutive  ac's  that  contain  the  texts  T1  and T2
        respectively.  The relation between T1 and T2 is  coded  in  the
        result  of  TXRE  as -1, 0 or +1.  TXCY is called with an inline
        parameter specifying any intermediate results:
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-59

741111    780302      6                           Lars Enderin

                PUSHJ   XPDP,TXCY
                XWD   n,admap
        Thus an accumulator stack may have  to  be  saved  over  storage
        allocation.   The  parameter  to  TXCY is saved in YTXZTV over a
        possible garbage collection.  TXBL is  called  like  TXCY.   The
        text  procedure TXDA, called like TXCY, in certain cases creates
        a ZTT object containing a dummy text variable which stands for a
        text  expression.   The output of TXDA is the dynamic address of
        this text variable i e [XWD 1,ZTT object address] and  also  its
        absolute address in the next ac.
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-60

741111    780302      6                           Lars Enderin

        III.1.9  Parameter handling

        Since  value  and  reference   mode   parameters   to   ordinary
        procedures,  classes and prefixed blocks are handled entirely by
        compiled code, parameter handling  in  the  RTS  is  limited  to
        unusual  circumstances  -  treatment  of  name  mode parameters,
        transmission of  parameters  to  formal,  virtual,  FORTRAN  and
        NOCHECK MACRO-10 procedures.

        III.1.9.1  Access to name mode parameters

        Inside a procedure, a name parameter is  accessed  in  different
        modes  and contexts.  The PH module contains run-time procedures
        for handling name mode parameters, calling thunks, etc.

        III.  The value of a name mode parameter

        If the value of the  parameter  is  required,  PHFV  is  usually
        called.   PHFV handles parameters of kind simple, i e not array,
        procedure or switch.  Label parameters are handled by PHFM,  and
        text  parameters  are handled by PHFV only in contexts where the
        text descriptor will not be modified, as in T.strip.

        PHFV starts by calling PHINIT (see below).  The  parameter  does
        not have a thunk if the actual parameter is a simple variable or
        constant.  The absence of a thunk is signalled by the ZFLNTH bit
        of  the  formal  location.   In  this  case,  the  value  of the
        parameter is loaded directly from the ZFL instance, from a block
        instance  or  from  the literal pool.  See ZFL format.  In other
        cases, the parameter must be computed by a thunk.  This is  done
        via   ENTERTHUNK,   PROCVALUE   (if   actual   parameter   is  a
        parameter-less procedure) and THUNKRETURN.   Since  calling  the
        thunk implies loss of control (the only link back to PHFV is via
        the return address for ENTERTHUNK), care must be taken  to  call
        ENTERTHUNK  in  such  a way as to retain all useful information.
        If the thunk returns a dynamic address, i e the thunk is  not  a
        value  type  thunk (ZFLVTD bit), the value must be retrieved via
        the dynamic address.  Conversion of the value is  done  by  PHCV
        (entry  PHCV1)  if  necessary  (ZFLCNV  bit).  PHEXIT returns to
        compiled code with the value in the proper ac(s).
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-61

741111    780302      6                           Lars Enderin

        III.  The dynamic address of a name mode parameter

        The dynamic address of a parameter X is needed

        a) when X is used on the left hand side of an assignment (X:=...
        or  X:-...).   The actual parameter in this case may be a simple
        variable, a value  or  reference  mode  parameter  or  an  array
        component,  possibly  accessed  via  remote referencing (<object
        expression>.<attribute>).  PHFA is used in this case, except for
        type  TEXT,  which  is  handled by PHFV for value assignment and
        PHFT for text denotes (X:-...).

        PHFA works as follows:  PHINIT is called.  Then, if the ZFL type
        for  the  parameter  signifies  a  value  rather  than a dynamic
        address, an error message is issued.  If no  thunk  exists,  the
        dynamic address is computed from the ZFL, otherwise the thunk is
        evaluated via ENTERTHUNK and THUNKRETURN.  If the parameter type
        is  REF,  the  prototype  pointer  is  taken  from ZFLZQU before
        returning to compiled code via PHEXIT.

        b) when X is specified PROCEDURE, ARRAY, SWITCH or LABEL.   PHFM
        is used in this case.  PHFM works like PHFA except that no check
        for illegal assignment need be made.

        c) when X is specified TEXT and used in a context where the text
        descriptor  may be affected as with text editing procedures like
        GETCHAR, PUTINT.  PHFT is used  in  these  cases  and  for  text
        denotes (X:-...).
        PHFT starts by PHINIT and works essentially  like  PHFA  if  the
        parameter  representation  yielded  via  the ZFL or a thunk is a
        dynamic address.  In this case, the actual  parameter,  possibly
        accessed by remote referencing, is a text variable (or parameter
        by value or reference) or a text array component.
        If the actual parameter is a constant, a RTS error is signalled.
        This  is  so  if  ZFLVTD=1  and no thunk has been compiled.  The
        remaining case is when the actual parameter is a text expression
        computed  by a thunk.  Evaluation is done as in PHFV, but before
        returning to compiled  code,  a  dummy  text  variable  must  be
        created.   TXDA  creates  a  ZTT  record  with  a  copy  of  the
        descriptor  computed  for  the  text  expression.   The  dynamic
        address  of  this  dummy  descriptor  is  returned  from PHFT to
        compiled code.
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741111    780302      6                           Lars Enderin

        III.  Assignment to formal parameters by name

        If X is a formal parameter by name, an assignment  of  the  form
        X:=<rhs>  is  carried  out  as  follows:   a)  PHFA computes the
        dynamic address to one or two ac's.  These  ac's  are  preserved
        over the execution of <rhs>.
        b) The assignment is carried  out  by  PHFS.   Assume  that  the
        dynamic  address  of X is in XWACi (and possibly a qualification
        in XWACi+1), and the value of <rhs> in  XWACj  (+XWACj+1).   The
        calling sequence to PHFS is:
                HLLZ    X0,X   ;ZFL instance
                PUSHJ   XPDP,PHFS
                XWD     XWACj,XWACi
        PHFS carries out qualification checking and possible  conversion
        before transmitting the value.

        III.  Accumulator handling in PH procedures

        The procedures PHFA, PHFM, PHFT and PHFV interface with compiled
        code  via  the  two  local  subroutines  PHINIT and PHEXIT.  The
        calling sequence to any of the procedures is
                [dynamic address of X to XWAC1+n]
                PUSHJ   XPDP,PHFy
                XWD     n,admap
        where y is A, M, T or V, n is the number of intermediate results
        and  admap  is zero or the address of a relocation map for the n
        results.  PHINIT calls .CSSA.  to save these results if n  >  0,
        then  puts the dynamic address of X in XWAC1 (normal case).  The
        absolute address of X, the formal location or ZFL  instance,  is
        loaded  to  XFAD,  and  the two ZFL words are loaded to XFL0 and
        XFL1.  After computing the parameter, possibly via a thunk, each
        routine  returns  to  compiled code via PHEXIT.  PHEXIT modifies
        the stack to skip the inline parameter.  If intermediate results
        were  saved  (YCSZAC(XLOW)  nonzero),  PHEXIT returns via .CSRA,
        otherwise directly.
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741111    780302      6                           Lars Enderin

        III.  Thunk evaluation

        The execution of a thunk is started  via  ENTERTHUNK,  which  is
        called  by  JSP  XRET,PHET.   Before  executing  the thunk code,
        information must be placed in the thunk save area  allocated  in
        the  display  of  the  block where the actual parameter belongs.
        The offset of the thunk save area (ZTS) is found at the head  of
        the  thunk  as  shown  by  the following general coding sequence
        compiled for a thunk:

                XWD     d,0
                <code to compute the actual parameter>
                MOVEI   XSAC,d(XCB)
                JSP     d+OFFSET(ZTSRAD)(XCB)

        where d is the offset of the thunk save area in relation to  the
        end of the display vector.  In the thunk save area are saved the
        dynamic address of the formal location, the  return  address  to
        object  code  (found  in  XPDP stack), the return address to the
        present RTS routine (value of XRET), the  acs  pointer  (YCSZAC)
        and   XCB.   This  information  is  sufficient  to  restore  the
        environment on return from the thunk,  provided  that  the  save
        area  itself  can  be  found,  which is guaranteed by having the
        thunk return its address in XSAC as seen above.  The new XCB  is
        taken from ZFLZBI, and the thunk is entered with the ZTS address
        in XSAC.

        In  the  rare  case  when  the  value  of  a  procedure  without
        parameters  is  required, the thunk makes an intermediate return
        with the dynamic address of the procedure in  XWAC1  and  XWAC2.
        PROCVALUE  (called  by  JSP  XRET,PHPV)  checks if the procedure
        should have parameters after all (error if so), and then returns
        to  the thunk with a new return address taken from XRET put into
        the save area.  X0 is stacked on entering PHPV  so  that  SIMDDT
        can get the correct code address for the error message.

        When the thunk returns to the RTS, which may be after  execution
        of a substantial amount of code including execution of the thunk
        itself recursively, THUNKRETURN  uses  the  information  in  the
        thunk  save  area based on XSAC to restore the environment where
        the parameter was accessed.  On exit  from  THUNKRETURN  (PHTR),
        XFAD points to the formal parameter X (ZFL instance), the return
        to  SIMULA  code  is  stacked,  YCSZAC  points  to   any   saved
        intermediate  results,  XCB  is  restored, and XFL0 contains the
        first ZFL word.
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741111    780302      6                           Lars Enderin

        III.1.9.2  Parameter transmission to formal, virtual and NOCHECK
        MACRO-10 procedures

        The  parameters  to  formal,  virtual   and   NOCHECK   MACRO-10
        procedures  cannot  be  checked fully at compile time, since the
        formal parameter specifications are not  known.   Procedures  in
        this   category  are  called  via  CSSW.   The  computation  and
        transmission of actual parameters is  carried  out  by  the  RTS
        procedure PHPT, which has a very special calling sequence:

                [compute dynamic address of formal or virtual
                procedure to top ac's]
                PUSHJ   XPDP,CSSW
                XWD      n,admap
                Z        npar
                PUSHJ   XPDP,PHPT
                [Z       prototype address]      ;Only for REF
        ZAP1:   actual parameter descriptor (ZAP) for first param.
                XWD      d,ZAP2
                [thunk for first parameter, if any thunk is needed]
                [Z       prototype address]      ;Only for REF
        ZAP2:   ZAP for second parameter
                XWD      d,ZAP3
                [thunk for second parameter]
                [Z       prototype address]      ;Only for REF
        ZAPn:   ZAP for n'th parameter
                XWD      d,ZAPEND
                [thunk for n'th parameter]
        ZAPEND: XWD      0,0
                PUSHJ   XPDP,CSEN        ;enter procedure body

        In the above sequence, d is the offset of the last word  of  the
        thunk save area in the display vector, and npar is the number of
        actual parameters.  CSSW is informed that PHPT is to  be  called
        by  the [Z npar] instruction placed as a second inline parameter
        to  CSSW.   In  any  other  situation   involving   CSSW,   this
        instruction  must  be  an executable instruction.  CSSW uses the
        absence of this information to give  an  error  message  if  the
        procedure  expects  parameters but has not been provided with an
        actual parameter list.

        The number of actual parameters is checked against the number of
        formal  parameters,  except  for  a  NOCHECK  MACRO-10 procedure
        (ZPCNCK bit set in the prototype).  The parameters  are  treated
        in  sequence.   Information from the ZAP or the ZFL in case of a
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741111    780302      6                           Lars Enderin

        passed-on name mode parameter is loaded to  standard  registers.
        In  the  NOCHECK  case,  the  formal parameter type and kind are
        copied from those of the actual parameter.  The prototype  of  a
        NOCHECK  MACRO-10  procedure  has  formal  parameter descriptors
        (ZFP) for 31 formal parameters all specified name.   This  means
        that   ZFL  instances  will  be  computed  and  stored  for  all
        parameters.   The  actual  parameter  is   first   checked   for
        compatibility.   If ZAPNTH is set, the ZAP specifies the address
        or value of the quantity directly (generally, by effective block
        level  and  offset).   The  quantity  can  be  loaded,  possibly
        converted or qualification checked, and  stored  in  the  formal
        location.    Note  that  the  qualification  of  an  actual  REF
        parameter is found in the word preceding the ZAP.  If the formal
        parameter  is  specified  NAME,  a  ZFL instance is computed and
        stored.  A NOCHECK MACRO-10 procedure is treated as having  only
        name  mode  parameters.   If  a thunk has been compiled, and the
        formal parameter  is  not  of  name  mode,  the  thunk  must  be
        evaluated  to  yield the value.  This is done in the same way as
        in PHFA or  PHFV,  with  the  additional  requirement  that  the
        current   positions   in  the  actual  (ZAP)  and  formal  (ZFP)
        descriptor lists must be remembered.  This is  done  by  storing
        the  addresses  of the ZAP and the ZFP descriptors in the formal
        location until the value has been computed.  Since  the  dynamic
        address  of the formal location is saved in the thunk save area,
        the formal and actual descriptor positions can be  recovered  on
        return  from the thunk.  The dummy ZAP of all zeros finishes the
        parameter list.

        III.1.9.3  Parameter transmission to FORTRAN procedures

        A FORTRAN procedure is identified to the SIMULA system  via  the
        attribute   file  produced  by  the  SIMULA  compiler  when  the
        interface code is compiled.  See LH2 appendix E.  The  prototype
        of  the  procedure  and  its  associated core map (ZMP) contains
        information assuming a special block layout  documented  in  the
        PHFO  module.  The value of ZPCDEC is the address of PHFO, which
        is thus entered after block allocation and  parameter  transfer,
        which  is  done exactly as for a SIMULA procedure.  PHFO has the
        task of adapting the parameters to FORTRAN formats  and  calling
        the  actual FORTRAN procedure.  If the returned parameter values
        have to be converted, PHFO does this before returning to  SIMULA
        code.   Both old (F40) FORTRAN and new FORTRAN-10 procedures can
        be handled.  During execution of the FORTRAN  procedure,  YFOXCB
        in  the  static  area  has  the  XCB  value corresponding to the
        procedure block.  More information  is  available  in  the  PHFO
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-66

741111    780302      6                           Lars Enderin

        module listing.
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741111    780302      6                           Lars Enderin

        III.1.10  SIMSET and SIMULATION

        The standard classes SIMSET and SIMULATION  are  implemented  in
        the  SS  and  the  SU  modules,  respectively, where prototypes,
        symbol tables and RTS procedures implementing the  main  classes
        and  their attributes, are found.  Names for quantities referred
        by the prototypes are generated in SIMRPA in the same way as for
        the  FILE  subclasses,  see III.1.7.1.  The classes involved are
        represented by the symbols (fixup numbers used by the compiler):
        D%SUSI  is  thus  the  macro  for  implementing  the  SIMULATION
        prototype, SUSI%S  is  the  statement  coding  (SUSI.ZCPSTA)  of
        SIMULATION, etc.  The present RTS design places SS and SU in the
        low segment to be  loaded  with  the  compiled  SIMULA  program.
        Since some information in the class prototypes is dependent upon
        the level where the SIMULA program uses SIMSET or SIMULATION  as
        a  prefix,  the SIMULA compiler must supply this level as global
        symbols to be used for relocation  by  LINK-10  or  the  LOADER.
        Because  both  the  effective  block  level and its negation are
        involved, two symbols, .SIMLV and .SIMVL, are  defined.   .SIMVL
        defines  ZPREBL  of the SIMSET and SIMULATION prototype, i e the
        offset from the block address where the display element pointing
        to  the class object itself can be found at run time.  .SIMLV is
        the complement of .SIMVL, i e a positive quantity, which is used
        to  define  the  display  vector lengths (ZPCDLE) of the classes
        involved.  .SIMVL is also used  as  ZCPSBL  (static  environment
        block  level)  for  the  prototypes  of  SIMSET  and  SIMULATION

        III.1.10.1  SIMSET implementation

        SIMSET  is  defined  in  CBL  14.1.   The  implementation  is  a
        straightforward translation of this definition.  FIRST, LAST and
        PREV (recommended  extension)  are  implemented  inline  by  the
        compiler.   The  SIMSET  prototype  is  SSST  (entry .SSST).  No
        special actions  are  performed  for  declarations  (SSST%D)  or
        initial  actions (SSST%S).  Coding to set up a block prefixed by
        SIMSET is no different from any prefixed block setup by CPSP.
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741111    780302      6                           Lars Enderin

        III.  Classes local to SIMSET

        SIMSET has the class attributes LINKAGE, LINKAGE class HEAD  and
        LINKAGE  class  LINK.  LINKAGE (SSLG) objects have the two links
        ZLGPRE and ZLGSUC implementing the  inaccessible  variables  SUC
        and  PRED  defined  in  CBL  14.1.2.   ZLGSUC and ZLGPRE share a
        36-bit word.  No additional data is needed for HEAD and LINK.

        III.  Procedure attributes of LINKAGE, HEAD and LINK

        The procedure attributes are implemented  in  a  straightforward
        manner following CBL definitions.  Parameters to pure procedures
        are passed in XWAC1, XWAC2, etc, and parameters to functions are
        passed  in  ac's starting with XWACn, where XTAC has the address
        of XWACn.

        III.1.10.2  SIMULATION

        The standard class SIMULATION is defined in CBL  14.2.   The  SU
        module  contains  prototypes  and actions of SIMULATION, PROCESS
        and  MAIN  PROGRAM,  procedure  attributes  of  SIMULATION   and
        procedures   to   implement   the   activation  statement.   The
        sequencing set is implemented as a binary tree of EVENT  NOTICES
        (ZEV).   Event  notices are referred by process objects (and the
        event notice refers back to the process)  which  are  scheduled.
        For  storage  management reasons, event notices are contained in
        event notice records (ZER) and chained to free lists when not in
        use.   The  event  notice  records are chained to the SIMULATION
        block and to each other.  If several  SIMULATION  blocks  exist,
        they  are  chained to each other for garbage collection reasons.
        The  SQS  tree  is  rooted  at  the  last  eventnotice  (largest
        scheduled  time),  which  is located via ZSULT of the SIMULATION
        block.  The current event notice (smallest  scheduled  time)  is
        found  via  ZSUFT.  The three links ZEVZBL (upward link), ZEVZLL
        (left) and ZEVZRL (right) are used to define the tree in such  a
        way that the EVTIME (ZEVTIM) values of the left hand subtree are
        always smaller than those of the right hand subtree and thus the
        path from last to first event is via left links.  ZEVZBL is used
        for backtracking when the left or right  link  does  not  exist.
        Depending   on  the  order  of  scheduling  statements,  several
        possible trees exist for a certain sequence of events.  Example:
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-69

741111    780302      6                           Lars Enderin

        First                                Last
        (ZSUFT)                              (ZSULT)  (dummy)
        ------   ------   ------   ------   ------   ------
        [  1 ]<--[  2 ]<--[  3 ]<--[  8 ]<--[ 10 ]<--[ZZZZ]
        ------   ------   ------   ------   ------   ------
                                     !        !
                                     V        V
                 ------   ------   -------   ------
                 [  4 ]<--[  6 ]<--[  7  ]   [  9 ]
                 ------   ------   -------   ------
                          [  5 ]

        where left arrows denote left links and down arrows denote right
        links.   ZSUFT  (current)  refers  to the ZEV labeled 1, and the
        root is the last  event  (10).   ZZZZ  stands  for  the  largest
        possible  real  value  [3777777777777  octal]  and  this node is
        placed in the tree to facilitate ranking of times.  The tree  is
        manipulated   at   the   request   of   scheduling   statements:
        Eventnotices  are  allocated  by  SANE,  which  allocates  a new
        eventnotice record (ZER) when  no  more  free  eventnotices  are
        found  in the current record.  Special measures have to be taken
        to preserve event  notice  addresses  over  a  possible  garbage
        collection.   Since  the  garbage  collector  can  only relocate
        addresses that point to the start of recognizable records,  i  e
        with  a ZDN part, eventnotice addresses in global locations have
        to be split  into  two  parts,  ZER  address  +  offset,  before
        allocating  a  new ZER record.  The global locations that can be
        used are YSUPCP and  YSUSCP.   See  SU  and  SANE  coding.   The
        address  of  the  (current)  SIMULATION  block  is found via the
        global location YSULEV in the static area, which is  set  up  by
        the  initial  actions  of  SIMULATION to contain the instruction
        MOVE XSAC,.SIMVL(XCB).  The effective block level of  SIMULATION
        is  thus  found  in  the right half of this word.  The procedure
        ACTIVATE described in CBL 14.2 is implemented as SUAC.  CURRENT,
        IDLE,  MAIN,  TERMINATED  and  TIME are implemented inline.  The
        rest is straightforward translation of the CBL definition, where
        the  SQS  is implemented as described above without referring to
        the CBL definition of  class  EVENT  NOTICE.   To  simplify  SQS
        handling,  a dummy ZEV with ZEVTIM=max real value is inserted at
        the start.  ZEVTIM is represented in single  precision  floating
        point  format.   SUAC  is  called with an activation mask in X0,
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741111    780302      6                           Lars Enderin

        process reference in XWAC1, second parameter (process  or  time)
        in XWAC2.  See SU module for details.
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741111    780302      6                           Lars Enderin

        III.1.11  Standard procedures and functions

        The following standard procedures and functions are available in
        DEC-10 SIMULA:

        SIMULA name      Fixup (1) Access (2)    result  param.  call
                                         type    types           (3)

        ABS              inline    G             R/L     R/L
        ACCUM            SUAM      SU            -       R,R,R,R  (5)
        ARCCOS           MAAC      G             R       R        F
        ARCSIN           MAAS      G             R       R        F
        ARCTAN           MAAT      G             R       R        F
                 MAATD     G             L       L               F
        BLANKS           TXBL      G             T       I        A2
        BREAKOUTIMAGE    IOBO      IOOU                           P
        CALL             CPCA      G             -       O        A1
        CANCEL           SUCA      SU            -       SUPS     P
        CARDINAL SSCA      SSHD          I                       A1
        CHAR             inline    G             C       I
        CLEAR            SSCL      SSHD                           P
        CLOSE            IOCL      IOFI                           P
        COPY             TXCY      G             T       T        A2
        COS              MACO      G             R       R        F
                 MACOD     G             L       L               F
        COSH            MACH      G             R       R         F
        CURRENT          inline    SU            SUPS
        DETACH           CPDT      G                              P
        DIGIT            inline    G             B       C
        DISCRETE RDDI      G             I       A,I             R1
        DRAW             RDDR      G             B       R,I      R1
        EMPTY            SSEY      SSHD          B                A1
        EJECT            IOEJ      IOPF                           P
        ENDFILE          inline    IOIN,IODF     B
        ENTIER           inline    G             I       L/R
        EVTIME           SUEV      SUPS          R                A1
        ERLANG           RDER      G             R       R,R,I    R1
        EXP              MAEX      G             R       R        F
                 MAEXD     G             L       L               F
        FIRST            inline    SSHD          SSLK
        FOLLOW           SSFW      SSLK          -       SSLG     P
        GETCHAR          IOGC      TEXT          C                A1
        GETFRAC          IOGF      TEXT          I                A1
        GETINT           IOGI      TEXT          I                A1
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741111    780302      6                           Lars Enderin

        GETREAL          IOGR      TEXT          L                A1
        HISTD            RDHI      G             I       A,I      A1
        HISTO            RDHO      G             -       A,A,R,R  P
        HOLD             SUHO      SUPS          -       R        P
        IDLE             inline    SUPS          B
        INCHAR           IOII      IOIN,IODF     C                I1
        INFRAC           IOIF      IOIN,IODF     I                I1
        INIMAGE          IOIG      IOIN,IODF                      I1
        ININT            IOIG      IOIN,IODF     I                I1
        INREAL           IOIR      IOIN,IODF     L                I1
        INTEXT           IOIT      IOIN,IODF     T       I        I2
        INTO             SSIT      SSLK          SSHD             P
        Integer to       MACL      G             L       I        F
        long real
        LAST             inline    SSHD          SSLK
        LASTITEM IOLI      IOIN,IODF     B                       I1
        LETTER           inline    G             B       I
        LINEAR           RDLI      G             R       A,A,I    R1
        LINE             inline    IOPF          I
        LINESPERPAGE     IOLP      IOPF          -       I        P
        LOCATE           IOLT      IODF          -       I        P
        LOCATION inline    IODF          I
        Long real to     MACI      G             I       L        F
        LOWTEN           TXLT      G             -       C        P
        MAIN             TXMN      TEXT          T                T1
        MOD              inline    G             I       I,I
        MORE             inline    TEXT          B
                 inline    IOFI          B
        NEGEXP           RDNE      G             R       R,I      R1
        NEXTEV           SUNE      SUPS          SUPS             A1
        NORMAL           RDNO      G             R       R,R,I    R1
        OPEN             IOOP      IOFI          -       T        P
        OUT              SSOU      SSLK                           P
        OUTCHAR          IOOC      IOOU,IODF     -       C        P1
        OUTFIX           IOOX      IOOU,IODF     -       L,I,I    P1
        OUTFRAC          IOOF      IOOU,IODF     -       I,I,I    P1
        OUTIMAGE IOOG      IOOU,IODF     -                       P
        OUTREAL          IOOR      IOOU,IODF     -       L,I,I    P1
        OUTTEXT          IOOT      IOOU,IODF     -       T        P1
        PASSIVATE        SUPA      SUPS          -                P
        POISSON          RDPO      G             I       R,I      R1
        POS              inline    TEXT,IOFI     I
        PRECEDE          SSPC      SSLK          -       SSLG     P
        PRED             SSPD      SSLG          SSLK             A1
        PREV             inline    SSLG          SSLG
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-73

741111    780302      6                           Lars Enderin

        PUTCHAR          TXPC      TEXT          -       C        T1
        PUTFIX           TXPX      TEXT          -       L,I      T1
        PUTINT           TXPI      TEXT          -       I        T1
        PUTFRAC          TXPF      TEXT          -       I,I      T1
        PUTREAL          TXPR      TEXT          -       L,I      T1
        RANDINT          RDRA      G             I       I,I,I    R1
        RANK             inline    G             I       C
        RESUME           CPRS      G             -       O        P
        SETPOS           TXSE      TEXT          -       I        P
                 TXSE      IOFI (6)      -       I               P
        SIGN             inline    G             I       R/L
        SIN              MASI      G             R       R        F
                 MASID     G             L       L               F
        SINH             MASH      G             R       R        F
        SQRT             MASQ      G             R       R        F
                 MASQD     G             L       L               F
        STRIP            TXST      TEXT          T       T        A1
        SUB              TXSU      TEXT          T       I,I      A1
        SUC              SSSC      SSLG          SSLK             A1
        TAN              MATA      G             R       R        F
        TANH             MATH      G             R       R        F
        TERMINATED       inline    SUPS          B
        Text assignment TXVA       G             -       T,T      T4
        Text relation    TXRE      G             B       T,T      T4
        TIME             inline    SU            R
        UNIFORM          RDUN      G             R       R,R,I    R1
        WAIT             SUWA      SUPS          -       SSHD     P
        ** (power)       MARR      G             R       R,R      F
                 MALL      G             L       L,L             F
                 MARI      G             R       R,I             F
                 MALI      G             L       L,I             F


        Type codes used:  B=Boolean, C=character, A=real  array,  L=long
        real,  I=integer, O=ref(any class), R=real.  SUPS etc - see note

        (1) Fixup is the identifying code given to  the  symbol  by  the
        initial  expansion  of  the RTSYMBOLS macro in SIMMCR.  Values <
        400000 octal are used  as  fixup  numbers  for  external  lowseg
        references.   Values  > 400000 are used as absolute addresses in
        the high segment, referring to the  transfer  vector  in  SIMRn0
        (SIMRn1 and SIMRn2).
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-74

741111    780302      6                           Lars Enderin

        (2) If the access code is G, the procedure is available  in  any
        SIMULA  program  and not an attribute.  Procedures marked SU are
        available only in SIMULATION  blocks.   TEXT  signifies  a  text
        attribute, and the other four-letter codes signify attributes to
        the corresponding classes:
        PROCESS (SUPS).

        (3) Calling sequences:

        In all  cases  but  F  (FORTRAN  sequence),  parameters  are  in
        successive  ac's  starting  with  the  current  top of stack ac,
        called Xtop.  For pure procedures and for most  function  calls,
        Xtop=XWAC1.  Most procedures are optimized for this case and are
        coded to place the parameters in standard ac's before processing
        and  restore  ac's  on  exit.   In  the  case  of text and class
        attributes, the first actual parameter (in Xtop and  Xtop+1)  is
        the  class  or  text  reference.   The  result is placed in Xtop
        (+Xtop+1) or where Xtop points.

        A1 is used for most functions which are attributes of a class or
        of TEXT:
                 [load address of top ac to XTAC]
                 [object reference in Xtop or text reference in Xtop and
                 PUSHJ   XPDP,f
        where f is the function fixup code.

        A2 is used for a global function  when  garbage  collection  may
                 [Any arguments to Xtop, Xtop+1 etc]
                 PUSHJ   XPDP,f
                 XWD     n,admap
        where n is Xtop-XWAC1 and admap points  to  a  bit  map  showing
        which  ac's  XWAC1  etc contain pointers.  Admap is zero if n is

        F is the calling sequence used for  arithmetic  functions  taken
        from the FORTRAN library and for TAN (copied from ALGLIB):
                 [Put the first parameter in .YFARG]
                 [any second parameter in .YFAR2]
                 MOVEI   XFP,.YFADR
                 PUSHJ   XPDP,f
                 (D)MOVE Xtop,X0
        where XFP=X16,  and  .YFADR,  .YFADR+1  have  the  addresses  of
SIMULA FOR DEC SYSTEM 10             TD, RTS                  III.1-75

741111    780302      6                           Lars Enderin

        .YFARG, .YFAR2.

        I1 is used for certain I/O functions:
                 [File ref in Xtop]
                 [parameters in Xtop+2, Xtop+3, ...]
                 MOVEI   XTAC,Xtop
                 PUSHJ   XPDP,f
        Note that Xtop+1 is not used.  Xtop.image.pos is affected.

        I2 is similar to A2,  with  a  file  reference  in  Xtop,  first
        parameter in Xtop+1.

        P is the sequence for a pure procedure statement:
                 [object reference or first parameter in XWAC1]
                 [Rest of parameters in XWAC2, ...]
                 PUSHJ   XPDP,p
        where p is the procedure address.

        P1 is used for output editing procedures:
                 [File ref in XWAC1]
                 [parameters in XWAC3, XWAC4, ...]
                 PUSHJ   XPDP,p

        T1 corresponds to P1 for text editing, with XWAC1 a pointer to a
        text descriptor (ZTV instance).

        R1 is used for random drawing functions.  As A1 except that  the
        last integer parameter (random stream number) must be an integer
        variable (possibly value parameter).

        (5) The interpretation of "NAME" for the 3 first  parameters  is
        "address of cell".

        (6) <file ref>.setpos is implemented as <file ref>.image.setpos.

                               CHAPTER 2

              SIMULA FOR DEC SYSTEM 10             TD, RTS


The RTS is coded as several source modules, each containing one or  more
RTS  subroutines  or  system class prototypes.  The source modules, when
compiled by MACRO-10 together with  the  required  UNIVERSAL  and  other
parameter files, yield REL files which are combined into:

1) SIMLIB.REL, created by  FUDGE2  (or  MAKLIB)  from  those  REL  files
compiled  with  a  zero  origin (relocatable) and thus meant for the low
segment, plus some chosen routines from SYS:FORLIB.

2) SIMRTS, the high segment, which may exist as SIMRn0.EXE (n  =  SIMULA
version  number, e.g. 4) in a one high segment version, or more commonly
2a)  SIMRn1.EXE,  containing  the  most  frequently  used  high  segment
routines.  Created by LOAD - SSAVE.
2b) SIMRn2.EXE, which contains routines for program initialization, file
creation  and  initialization.   Created  by  LOAD  - SSAVE.  SIMRn1 and
SIMRn2 are not in core simultaneously.  The transfer  from  one  to  the
other  is  handled  by  the  transfer  vector  and the OCSW routine (see
III.1.6.2).  Some routines are included both in SIMRn1 and SIMRn2.

3)  SIMDDn.ABS,  containing  self-relocating  code  for  the   debugging
sub-system.  Created by executing SUTABS.REL loaded with SIMDDT.REL.

4) SIMHGH.REL, containing the  same  REL  files  as  used  for  creating
SIMRn0.EXE (those marked 0 or 1,2 in III.2.2).  To be used for debugging
or for loading a SIMULA program (partly) in the high segment.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.2-1

III.2.1  SIMLIB components

Module   Global symbols

MA       .MACI   .MACL
TAN      TAN.
SU       .SUSI
SS       .SSST
[The following routines are taken from SYS:FORLIB]
EXP1     EXP1.   EXP1.0  EXP1.2  EXP1.4  EXP1.6
EXP3     EXP3.   EXP3..  EXP3.0  EXP3.2  EXP3.4  EXP3.6
EXP2     EXP2.   EXP2..  EXP2.0  EXP2.2  EXP2.4  EXP2.6
EXP.     EXP.
ALOG.    ALG10.  ALOG.
SIN.     COS.    COSD.   SIN.    SIND.
DLOG.    DLG10.  DLOG.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.2-2


OCSP is loaded with all SIMULA main programs, the others  on  demand  by
library search.  OCSP loads the initial high segment.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.2-3

III.2.2  SIMRTS components

The following is contained in SIMRTS:

Module   Segment Global symbols

SIMRTS   0       .IODF   .IOFI   .IOIN   .IOOU
         .IOPF   .PDERR
SIMRT1   1       .IODF   .IOFI   .IOIN   .IOOU
         .IOPF   .OCSW   .PDERR
SIMRT2   2       .IODF   .IOFI   .IOIN   .IOOU
         .IOPF   .OCSW   .PDERR
CS       1       .CSCA   .CSEN   .CSEP   .CSER
         .CSES   .CSEU   .CSGO   .CSNA
         .CSQU   .CSRA   .CSSA   .CSSA.
         .CSSB   .CSSC   .CSSN   .CSSW
CSSADM   2       .CSRA   .CSSA.
CP       1       .CPCA   .CPCD   .CPCI   .CPDT
         .CPE0   .CPNE   .CPPD   .CPRS
IO       1       IODF%D  IODF%I  IODF%M  IODF%S
         IOPF%Y  .IOBO   .IOCS   .IOCL
         .IOCLA  .IOCS   .IOEJ   .IOFD
         .IOIG   .IOLI   .IOLN   .IOLP
         .IOLT   .IONB   .IOOG   .IOOP
         .IORB   .IOSP
IOED     1       IOTXR   .IOIC   .IOIF   .IOII
         .IOIR   .IOIT   .IOOC   .IOOF
         .IOOI   .IOOR   .IOOT   .IOOX
IONF     2       .IOCF   .IOCOM  .IOENT  .IOLOK
         .OCINE  .OCINK
PH       1       .PHCV   .PHFA   .PHFM   .PHFS
         .PHFT   .PHFV
SA       1,2     .SAAB   .SAAR   .SACL   .SADB
         .SADE   .SAGC   .SAGI   .SAIN
TX       1       .TXCY   .TXDA   .TXGC   .TXGF
         .TXGI   .TXGR   .TXLT   .TXMN
         .TXPC   .TXPF   .TXPI   .TXPR
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.2-4

         .TXPX   .TXRE   .TXSE
TXBL     1,2     .TXBL   .TXST   .TXSU   .TXVA
OCIN     2       YOCSWL  .OCIN   .OCIN4  .OCIN5
         .OCIN6  .OCIN7  .OCINB  .OCINF
OCIO     1,2     .IOERF  .IOFER  .IOTYS  .OC8T
         .OCDT   .OCIN1  .OCIN2  .OCIN8
         .OCIN9  .OCINC  .OCIND  .OCING
         .OCINH  .OCLA   .OCOO
OCEP     1       .FORER  .FOREX  .OCEI   .OCEP
         .OCLD   .OCTR   .OCUU

Modules marked  0  are  only  used  in  the  one  high  segment  version
SIMRn0.EXE.   Those  marked  1 are used in SIMRn1.EXE, those marked 2 in
SIMRn2.EXE.  All modules except CSSADM.REL,  SIMRT1.REL  and  SIMRT2.REL
are used in SIMRn0.EXE.

                               CHAPTER 3

              SIMULA FOR DEC SYSTEM 10             TD, RTS


Normally (1), the low segment of an executing SIMULA program contains:

1) The compiled code of the SIMULA main program.

2) Code for any external classes or  procedures,  possibly  compiled  by
MACRO-10 (or an equivalent processor, e g BLISS) or FORTRAN.

3) RTS routines loaded from SIMLIB by library search.   OCSP  is  loaded
with all SIMULA programs, the other routines are loaded as needed.

4) If SIMDDT is loaded directly at the start of execution (DEBUG or LOAD
-  REENTER),  it  occupies the space directly following the last routine
loaded by LINK-10 or LOADER.

5) The so-called STATIC area is allocated next.  See SIMRPA.  The static
area  is  addressed  by  loading the right half of .JBOPS to a register,
usually called XLOW (default XLOW=X16) and using  the  symbols  declared
via  the STATIC macro in SIMRPA as offsets.  The RTS stack based on XPDP
is part of the static area.

6) The dynamic low segment area starts with
6a) an area for buffers and buffer headers,
6b) the IOSPEC table,
6c) more buffers if needed.
See IO.MAC for table layouts and record definitions.

(1) Part or all of the program can also be loaded in the high segment if
a  shared  SIMULA  program  is desired or if the program is very big and
more space in the low segment is needed for data.  See SIMHGH.HLP.  Also
useful for debugging the RTS.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.3-1

7) The storage pool for dynamically allocated objects is at the  end  of
the low segment.  SIMDDT and its work areas may be allocated in the pool
if not loaded at program start.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.3-2

III.3.1  Low segment layout

-------------------------  <---  0
!   JOBDAT area          !
!-----------------------!  <--- 140 (octal)
!        main            !
!        program         !
!  external procedures   !
!  and classes           !
!-----------------------!  <--- OCSP
!        RTS             !
!        routines        !
!-----------------------!  <--- YOCDDT if non-zero
.  (SIMDDT if loaded)    .
:-----------------------:  <--- .JBSA,   .JBOPS
!   Static area          !
!-----------------------!  <--- YOCBST
!   Initial buffer       !
!   area         !
!-----------------------!  <--- YIOSPC
!   IOSPEC table !
!   Buffer area          !
!   continued            !
!-----------------------!  <--- YSABOT
!   Storage pool !
!   for SIMULA           !
!   objects              !
! - - - - - - - - - - - !  <--- YSATOP
!   Unused part of       !
!   pool         !
!-  -   -   -   -   -  -!  <--- YSALIM
! Space for a ZAC obj.   !
-------------------------  <--- .JBREL

                               CHAPTER 4

              SIMULA FOR DEC SYSTEM 10             TD, RTS


The RTS interfaces with a) the user, b) the monitor and file system,  c)
the  compiled  program.  Within the RTS, there are interfaces d) between
low and high segment, e) between SIMDDT and the rest of the RTS.

III.4.1  RTS interface with the user

The user communicates with the RTS by the following means:

- The user TTY, which is used by the RTS for error  messages  and  other
diagnostics,   for   the   file  initialization  dialogue  if  specified
(III.1.7), to receive SIMDDT commands, etc.  The TTY  is  used  both  as
SYSIN and as SYSOUT if no other specification has been given.

- A specification file may  be  read  if  the  /R  switch  is  given  at
compile-time.  See III.1.7.

III.4.2  RTS interface with monitor and file system

This is described mainly in III.1.6 and III.1.7.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.4-1

III.4.3  RTS interface with compiled program and internally

SIMRTS, the high segment of RTS, interfaces with  the  compiled  program
via the transfer vector and the static area.  The low segment of the RTS
interfaces with compiled program via global symbols and the static area.

III.4.3.1  Low segment - high segment interface

Since the high segment (SIMRTS) is  created  independently  of  any  low
segment  that is present at run time, communication between the segments
must go through fixed  addresses  or  via  a  base  register  and  fixed
offsets.  Both methods are used, as explained below.

III.  References to high segment from low segment

High segment routines are called via a transfer  vector,  consisting  of
JRST  and  PUSHJ  instructions.   The  transfer  vector  is  created  by
expansion of the RTSYMBOLS macro, with  a  suitable  definition  of  the
macro which is called for each entry.  No global variables are placed in
the high segment.

III.4.3.2  References from the high segment to the low segment

The (static) area for global variables  and  tables  is  placed  at  the
initial  .JBFF  location.  .OCSP saves the address of the global area in
.JBOPS, right half.  When referring from either  the  high  or  the  low
segment  to  those global locations, a base register must be used, which
is loaded by a HRRZ instruction from .JBOPS when needed.   The  symbolic
name  of  that register should always be XLOW.  The actual value of XLOW
may change throughout the run time system, but usually XLOW=XFP is used.
The global variables are thus always indexed by XLOW.

Some low segment data must be loaded together with the  SIMULA  program.
These are:

*        Pseudo or extended  accumulators  which  are  global  locations
defined  at  load  time, with the names .YXA1, .YXA2, etc.  The compiler
refers to those  locations  by  consecutive  fixup  numbers,  which  are
translated  to  external  references.   In order to have access to those
locations also from the high segment,  the  address  of  .YXA1  must  be
placed  in  the  static  area based on XLOW.  YXACAD(XLOW) contains that
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.4-2

*        Address and value list for communication with  FORTRAN  library
subroutines, e g SIN, COS, and the exponentiation routines.  The address
list consists of two words at address  .YFADR.   Those  words  have  the
addresses  of the two doublewords .YFARG and .YFAR2.  When calling, e g,
the SQRT function, the argument is placed in .YFARG and XFP is  made  to
point  to  .YFADR,  before  executing  the  PUSHJ.   As  an example, the
following calling sequence is generated for the statement y:=sqrt(x):

 MOVE    XWAC1,x
 MOVE    XWAC1,0

III.4.3.3  References from the RTS to the compiled program

SIMDDT uses the main line number table address to find line  number  and
symbol  tables  for  the  main  program  and  any external procedures or
classes.  The RTS stack, starting at YOBJRT of the static area, is  used
for  referring  to  compiled code in several RTS routines and in SIMDDT.
.JBSA points either to .MAIN or .OCRE0 (DEBUG command).  The  data  area
for  the  outer  program  block  and  its  display  vector are placed in
compiled  code.   Program  objects  (subblocks,   procedures,   classes,
prefixed  blocks)  refer  to  compiled  code  via the prototype pointer.
Other  blocks  may  also  contain  references  to  compiled  code.   The
SIMULATION  or  SIMSET  block  level  is  defined by global symbols (see

III.4.3.4  Access to SYSIN, SYSOUT objects, to SIMULATION block

The SYSIN file object address is found at offset YSYSIN  in  the  static
area.   Code  is  compiled  to  load  an ac from .JBOPS before accessing
SYSIN.  Access to SYSOUT is  via  YSYSOUT.   In  a  SIMULATION  program,
YSULEV  contains  information  (MOVE  XSAC,offset  of  simulation  block
address in current display) to find the SIMULATION block.   Since  these
three offsets are used by compiled code, they must not be changed unless
absolutely necessary (old SIMULA rel files will be useless).

                               CHAPTER 5

              SIMULA FOR DEC SYSTEM 10             TD, RTS


MACRO-10 modules in the RTS refer to  the  UNIVERSAL  files  SIMMAC.UNV,
SIMMCR.UNV  and  SIMRPA.UNV,  created from the corresponding .MAC files.
SIMMAC is described elsewhere (I.6).  SIMMCR contains  information  used
at  code  generation,  chiefly record definitions for static and dynamic
records used at run time.  SIMRPA contains additional  definitions  used
mostly at run time but also for code generation in some cases.


SIMMCR contains the following:
- Definitions of register names XIAC, XFP, XCB, XSAC, XTAC, XRAC, XRAC1.
- TYPZDN macro, used to define the constants for run-time dynamic record
discrimination  (ZDNTYP  field)  and  also  in  the garbage collector to
define a jump table for record processing.  The  last  tag  value  used,
QZDNTM, is defined.
- RTS error codes used at compile time.
- Parameter descriptor codes used in ZFL and ZAP records.
- Record definitions (DR, DF) for dynamic records ZDN,  ZBI,  ZBP,  ZCL,
ZDA, ZRV, ZDP, ZFL, ZSU, ZEV, ZLG, ZPS, static records  ZPR,  ZPC,  ZCP,
ZMP,  ZSL,  ZSR,  ZTD,  ZPD,  ZFP,  ZFR,  ZAP,  ZLN,  ZSD,  ZSM, ZTH and
miscellaneous definitions:   ZTS  (thunk  save  area),  YFOFAD,  YFOAAD,
- The RTSYMBOLS macro, which gives values to most symbols used to access
RTS  routines and system class prototypes.  The entries in the RTSYMBOLS
macro are calls of an X macro, used  for  procedure  entries,  and  a  Y
macro, used for prototypes.  The X macro is invoked:
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.5-1

ROUT is the mnemonic used for the entry.  SEG  is  L  (low  segment),  1
(SIMRn1),  2  (SIMRn2)  or  3 (both SIMRn1 and SIMRn2).  An empty second
parameter stands for 1.  SIM is the letter S if the symbol has a  SIMULA
name  (exists in initial symbol table), empty otherwise.  N, if present,
gives the global name to be used for the entry  point  to  the  routine,
otherwise the global name is formed by prefixing ROUT with a dot.
The Y macro is invoked:
PROT corresponds to ROUT.  If SEG is 1,2 or 3,  the  prototype  will  be
expanded  in  the  transfer vector directly to be accessible to compiled
code via PROT.  The initial expansion reserves space for  the  prototype
to  get  the  correct  values  of the following entries.  F, if present,
shows the size of the variable part of the prototype  (after  the  fixed
ZPC  part, i e parameter descriptors for formal parameters).  RTSYMBOLS,
when expanded with the initial definitions of X  and  Y,  defines  fixed
high  segment  addresses  if  SEG=1,2  or  3,  consecutive fixup numbers
starting with 1 if SEG=L.  The high segment addresses are used  directly
in  compiled  code  as entry addresses to RTS procedures and prototypes.
The fixup numbers are coupled with an external symbol table  defined  in
PASS  3  of the compiler by another expansion of RTSYMBOLS (with X and Y
redefined).  In the SIMRTS module used to produce SIMRTS.REL, SIMRT1.REL
or  SIMRT2.REL,  another  expansion  of  RTSYMBOLS produces the transfer
vector used at run time.   RTSYMBOLS  also  defines  fixup  numbers  for
pseudo  ac's and the FORTRAN subroutine interface cell - .YFADR, .YFARG,
.YFAR2.   A  readable  representation  of  RTSYMBOLS  is   produced   by
compilation (with listing) of the following MACRO-10 code:

 PRINTX NA       VAL     S1      EN
 PRINTX ------------------------------
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.5-2


SIMRPA contains definitions  needed  mostly  in  the  RTS  and  in  some
compiler modules.

III.5.2.1  SIMRPA macros

The following macros are defined:

LOWADR(X) - defines XLOW=X and loads XLOW from .JBOPS.

SETLOW(X) - defines XLOW=X, does not change X.

IFNONE(X) - skips if X==NONE.

TRIMSTACK - removes top of XPDP stack.

ERRMAC(A) - defines the error message macro to be used  in  component  A

RIGHTHALF(A) - compile time message if A is  not  a  right  half  field,
where A is a field designator.

SAVEALLACS - All ac's are saved at offset YUUOAC of the static area.

(see III.

TEXT(T), RTEXT(T), RRTEXT(T) - used for garbage collector debugging.

ERRCODE(N0,A) - Defines the initial  error  number  used  in  module  A,

RTSMODULE(M,M1,SEG) - Defines TITLE,  ERRMAC(M),  high  or  low  segment

PROCINIT(A) - Defines declarations  for  IO,  OCIO,  OCIN  modules  when
expanded.  A is module name.

RDINIT(e) - Defines macros and initial info for submodule RD'e of RD.MAC
(submodules are separated by PRGEND).

DCLASS(NAM,...) - Defines D%'NAM  (prototype  macro)  for  the  standard
class with index code NAM, e g IOFI (FILE).
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.5-3

DSZCML - Defines symbol table record (ZSM) for a standard class.

DZSD - Defines symbol table entry for local quantity.

SYSCLASS - Used to define codes QCLPB, QSUSI, QSSLG,  QSUPS,  QIOFI  for
the  ZCPGCI  field  in  the  prototype.   Used  by  garbage collector to
indicate special relocation.  SYSCLASS is used there to  define  a  jump

III.5.2.2  Constants defined in SIMRPA

Alternative ac names are defined.  Compile  time  constants  like  QIOLP
(default   LINESPERPAGE   value).    Entry  point  offsets  for  SIMDDT.
Constants referring to monitor tables, FORTRAN  error  codes,  interrupt

The offsets in the STATIC area are defined via the STATIC macro.   These
offsets begin with the letter Y.

III.5.2.3  Record definitions in SIMRPA

The following records are defined:

III.5.2.4  Switch definitions in SIMRPA

Switches used in OC, IO and SA modules are defined.

                               CHAPTER 6

              SIMULA FOR DEC SYSTEM 10             TD, RTS


Special debugging tools have been developed for the I/O  subsystem  (see
I.10,  test  standard),  and the garbage collector (SAGC, III.6.2).  For
the rest of the RTS, the standard debugging tool,  DDT  has  been  used.
SIMDDT  can also be used (in conjunction with DDT).  At present (version
4 of SIMULA) the debugging tools for SAGC cannot be used, however.

III.6.1  Debugging with DDT

III.6.1.1  Debugging using rel files for all RTS components

DDT debugging is simplified if all REL  files  both  for  low  and  high
segment  are  available.   In  this  case  the required files are loaded
directly and possibly saved as an EXE  file.   SIMLIB  may  be  used  in
library  search  mode.   Note  that  local symbols are not available for
routines in SIMLIB.  If all required rel files are loaded directly  with
DDT via a DEBUG command, breakpoints may be set anywhere before starting
the program.  The following command sequence may be used:
 [set breakpoints]
 ^[G (or .OCRE0^[G if SIMDDT start is wanted)
where ^[ stands  for  <altmode>  and  SIMHGH.REL  contains  the  modules
from  SIMHGH.REL  may  also  be  loaded  separately.   SIMRTS.REL   (the
one-segment  version)  must  always  be  the  first  module  in the high
segment.  If possible, REL  files  compiled  with  QDEBUG=1  and  QSYS=0
should  be  used  (see  implementation  guide).   For  complete  garbage
collector debugging, SADEB.REL  must  be  included.   See  III.6.2.   If
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.6-1

execution  is  made  to start in SIMDDT, DDT may be entered by typing ^C
when a command is expected, then issuing the DDT  command.   Control  is
returned  to  SIMDDT  by executing "JRST 2,@.JBOPC^[X" and then typing a
SIMDDT command.  Note that  SIMDDT  may  be  used  to  force  a  garbage
collection  via  the  VARIABLES  command.  This is useful when a garbage
collection error is expected.  See  III.6.2  for  ways  to  dump  SIMULA
objects and to trace garbage collector actions.

III.6.1.2  Debugging the SYS version of the RTS

Debugging the SYS version of the RTS high segment is  rather  difficult.
If  possible,  the  rel  files should be used as described in III.6.1.1.
With DDT, breakpoints can be set in the low segments without difficulty,
but  breakpoints  cannot  be  set  in the high segment at all if the SHR
files from SYS are used at user level.  One possible way is to  GET  and
SAVE  SIMRn1  and  SIMRn2 from SYS, creating unsharable EXE files on the
user area, then load the program with DDT (DEBUG/D SIMPRO) and SAVE  it,
then  ASSIGN  DSK  SYS  (after  setting  the SYS switch via SETSRC).  If
breakpoints must be placed in SIMRn1, set a breakpoint at .MAIN+6,  then
set the required high segment breakpoints after stopping at .MAIN+6.  It
is also possible to start in SIMDDT and enter DDT  at  a  breakpoint  as
shown  above.   If breakpoints in SIMRn2 are needed, set a breakpoint at
.OCSP+73 (may be changed, check for JRST 400047) and start.   SIMRn2  is
then  in  core.   If you need to set breakpoints at other invocations of
SIMRn1 or SIMRn2 than the first one, patches must be made  in  the  high
segment.  It is possible e g to place "EXIT 1,", which works as ^C, in a
high segment location, noting the replaced instruction.  If the replaced
instruction  is  is  HRRZ  16,.JBOPS  (LOWADR),  it  may  be  redundant,
otherwise the replaced instruction has to be executed in DDT  explicitly
(^[X  command)  before  returning  via  .JBOPC  as  shown  above.   This
debugging mode should not be used unless it is necessary to reproduce an
error.   If  the  error  can  be  shown  to occur when all rel files are
loaded, debugging should be done as in III.6.1.  If the SYS version  has
only one high segment, debugging is somewhat simplified.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.6-2

III.6.2  Testing facilities in the garbage collector


               The  testing  facilities   are   currently
               (version 4) not usable.

In the test version (QDEBUG=1, QSYS=0),  the  garbage  collector  (SAGC,
shortened  as  GC)  processing can be traced.  The global cell at offset
YSASW in the static area contains 5 switches to determine the amount  of
test output produced.  The switches are:

Switch   bit no  Meaning

SAGCPE   0       1 if no output on nnnGCP.TMP
SWGCTE   1       1 for test output from GC
SWGCT2   2       1 for test output on TTY
SWGCT3   3       1 for test output on SYSOUT
SWGCT4   4       1 if GC runtime and low seg limit
         should be logged.

If SAGCPE is off, the GC  parameters  used  in  the  storage  allocation
algorithms  are  output in append mode on the file nnnGCP.TMP after each
garbage collection, where nnn is job number.  The data on this file  can
be  analysed  by  the utility program SUTGCA (see chapter V).  SAGCPE is
set off by default if QDEBUG=1.  Initialization is done by .SAGI,  which
opens the file.  SAGCPE is set off if I/O errors occur on the file.

If SWGCTE is on, the test version logs GC actions.  In PHASE1 the  start
addresses  of  all chained records are output.  In PHASE2 all referenced
records in the pool are logged with old  address,  new  address,  record
length given.  In PHASE3 all updated pointers are given with old and new
values.  Finally, in PHASE4, the old address, new  address,  and  length
for each moved record are output.  SWGCTE is off by default.

SWGCT2 and SWGCT3  specify  the  output  destination  for  test  output.
Default is TTY (SWGCT2 and NOT SWGCT3).

If SWGCT4 is set a short log message is given on each GC execution.
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.6-3

The default switch  settings  can  be  changed  by  patching  the  .SAGI
routine.   The  appropriate  bits  in the first instruction compiled for
SETON SWGCT2(XLOW) can  be  set.   The  bits  may  also  be  changed  in
YSASW(XLOW) at a DDT breakpoint.

The SADEB.MAC module contains a number of dump routines to be  used  for
GC  testing  as  well  as  the  output routines for logging as specified

.SAPD dumps the whole object pool.

SAPDRE dumps a single record in the pool.

.SQSDU dumps the sequencing set of SIMULATION (SQS).

The routines are invoked by the ^[X command in SIMDDT, executing a PUSHJ
17,r instruction, where r is the appropriate routine.

When calling SAPDRE, XCUR==X6 must contain  the  start  address  of  the
record.   For  easier  handling  of the dump routines, note that the DDT
command "PUSHJ 17,.SAPD<PD:", for example, defines PD as PUSHJ 17,.SAPD.
Thus PD^[X may be used to get a pool dump.

                               CHAPTER 7

              SIMULA FOR DEC SYSTEM 10             TD, RTS


As an illustration of what happens at run time,  the  execution  of  the
following  SIMULA  program is analysed in some detail, referring to line
numbers as allocated by the compiler:

B1     1   BEGIN
       2      CLASS pb1;;
B2     3      pb1 BEGIN
       4         CLASS c1;
B3     5         BEGIN
       6            CLASS c2;
B4     7            BEGIN
       8               detach;
       9               rc3:-NEW c3;
      10               resume(rc3);
E4    11            END c2;
      12            CLASS c3;
B5    13            BEGIN
      14               detach;
E5    15            END c3;
      16            REF(c2)rc2;
      17            REF(c3)rc3;
      18            CLASS pb2;;
      19            rc2:-NEW c2;
      20            detach;
B6    21            pb2 BEGIN
      22               CLASS pb3;;
B7    23               pb3 BEGIN
      24                  resume(rc2);
E7    25               END pb3;
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.7-1

E6    26            END pb2;
E3    27         END c1;
      28         REF(c1)rc1;
      29         rc1:-NEW c1;
      30         resume(rc1);
      31         resume(rc1);
E2    32      END pb1;
E1    33   END program;

Tracing with SIMDDT gives the following execution order by  source  line
number:  3, 2, 29, 19, 8, 20, 30, 21, 18, 23, 22, 24, 9, 14, 10, 15, 31,
25, 26, 27, 32.

This is explained as follows:

Start before line 1 at the entry point .MAIN given by .JBSA.  Call OCSP,
which  swaps  in  SIMRn2,  transfers  control to OCIN for initialization
(SYSIN and SYSOUT setup etc).  OCIN transfers control to .OCEI  via  the
transfer vector in SIMRn2, swapping in SIMRn1 to replace SIMRn2 (.OCSW).
OCEI returns to compiled code.

CSER is called to initialize the reduced subblock starting  at  line  1,
using .SAIN to initialize data according to subblock map.

At line 3, CPSP, using SADB and .SAIN, sets up the prefixed  block  with
its   display  vector  and  dynamic  link.   CSEN  is  called  to  start
declarations in the prefix (ZPCDEC of the pb1 prototype) then  via  CPCD
to  ZPCDEC of the prefixed block, then via CPPD to line 2 (statements of
the prefix), continuing via CPCI (implicit inner) to ZCPSTA of the block
(line 29).

At 29 a c1 object is generated and entered:  CPNE [SADB [.SAIN] CSEN]  -
ZPCDEC - CPCD - ZCPSTA (line 19).

At 19, create c2 like c1, ending up at line 8.

At line 8, detach (CPDT) detaches c2 and returns the  reference  to  the
caller,  continuing  at line 20, where c1 is detached, returning to line

At line 30, resume (CPRS) of c1 leads to the reactivation point at  line

At line 21 a prefixed block again starts via CPSP,  CSEN,  prefix  class
(line  18), continuing at line 23, where a new prefixed block is started
SIMULA FOR DEC SYSTEM 10             TD, RTS                   III.7-2

(lines 22, 24).

At line 24 c2 is resumed at  its  reactivation  point  line  9.   C3  is
resumed (line 14) and detaches to line 10.

At line 10, c2 terminates via CPE0.  CPE0 calls CPDT, leading to the QPS
reactivation point at line 15, where c3 terminates, detaching to line 31
of the surrounding prefixed block.  The resume (CPRS) at line  31  leads
to  line 25 in the innermost prefixed block via operating chain and back
down via dynamic links.

The prefixed blocks terminate in turn via CPE0 and ZCPIEA links:   lines
25,  26.  C1 terminates via CPE0 and CPDT, leading to line 32, where the
pb1 prefixed block terminates via CPE0 and ZCPIEA.

OCEP terminates program execution, closing files via .IOCLA  and  .IOCL.
Returns to monitor level via EXIT 1,.