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DDT42B - DDT %42B(412) User's Guide to New Features Page 1
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DDT42B - DDT %42B(412) User's Guide to New Features Page 2
Introduction and Overview
1.0 Introduction and Overview
This document is designed as a user's guide to DDT version 42/42B in
so far as it has changed from previous versions of DDT. It is not a
complete user's guide to all the wonders of DDT, just those new
features which have recently been implemented (although directed pri-
marily at new features only in DDT version 42/42B, some documentation
is included to describe other aspects of DDT which have been around
for a longer period of time, but were never fully understood or other-
Throughout this document it is assumed that the reader is already
familiar with DDT and the MACRO assembly language in general as well
as the appropriate operating system(s).
This (DDT42B) is the third revision of this document, incorporating
the additional changes to DDT version 42B as of edit 324, and com-
pletely supersedes all previous versions (DDT42).
DDT version 42B will run on KA-10's, KI-10's, KL-10's, and KS-10's,
using no paging, KI-paging, or KL-paging, with or without extended
addressing in user or executive mode (user and file DDT's run only in
user mode) with no special assembly needed. DDT version 42B must be
assembled to run under either the TOPS-10 or the TOPS-20 operating
It traditionally has been a goal to maintain one single set of source
files from which all flavors of DDT are built. This goal has been
TOPS-20 UDDT (and SDDT) now use memory locations 764000
through 777777 (previously 770000 through 777777), but the
starting address for DDT continues to be location 770000.
3.0 Memory and Address Control
The single biggest change to DDT version 42 from earlier versions is
in the realm of memory control and how the user addresses memory loca-
3.1 Extended addressing
All flavors of DDT except FILDDT will run in any memory section. Full
extended addressing is supported, as are "large" addresses - DDT will
now accept a full 36-bit expression as an address although obviously
only FILDDT can actually handle an address over 30-bits wide. In all
cases the actual address must be positive (i.e., effectively a 35-bit
DDT42B - DDT %42B(324) User's Guide to New Features Page 3
Memory and Address Control
3.1.1 Symbol table restrictions
There are certain restrictions however which must be adhered to in
order for DDT to function correctly. The first restriction is that the
symbol table logic is essentially section-dependent, i.e., the symbol
table and its pointers (.JBSYM=116 and .JBUSY=117, also .JBHSM=6 rela-
tive to the start of the "high segment") must reside (i.e., be mapped)
in the same section as that in which DDT itself is running. Further,
the symbol table can be no longer than 128K words in length and must
be RADIX-50 format.
Much thought is being given towards the implementation of a totally
new symbol table scheme which would address all of these problems, the
single biggest one of which is simply how is extended addressing going
to be used - as a single fixed address space with one or more "global"
symbol tables (like the TOPS-20 monitor currently works), or as a
collection of independent sections each of which has section-local
symbols/symbol tables (whatever that means), or what?
3.1.2 Breakpoint restrictions
The second restriction of which the user must be aware concerns break-
points. Since the hardware has no facility to unconditionally transfer
control to DDT using only 36-bits, DDT must be mapped into each
section (at the same relative address obviously) which contains code
into which the user wishes to place breakpoints.
3.1.3 Location examining restrictions
Even if running on an extended addressing machine if DDT is running in
section 0, then only locations within section 0 (addresses 0 to
777777) may be manipulated. DDT will make no effort to outsmart the
combined efforts of the user and the operating system by sneaking into
a non-zero section even momentarily to do the memory reference.
DDT version 42B, if it has previously been running in a non-
zero section, and is currently entered in section 0, will
blindly jump back into the last non-zero section in which
DDT was running. This is to allow DDT to be able to access
non-zero section locations (such as breakpoints).
3.2 Effective address calculation
DDT can calculate effective address references using either "local" or
IFIW (Instruction Format Indirect Word) or "global" or EFIW (Extended
Format Indirect Word) formats. In a normal DDT address-opening command
("/", "\", <TAB>, etc.) a single <ESC> delimiting the address expres-
sion (e.g., "MOVE 3,@200(10)$/" or just "$[") instructs DDT to treat
the expression as an IFIW word and calculate the effective address
exactly like the hardware would, were the hardware to execute that 36-
bit word as an instruction at location "." (whether or not location
"." is currently open).
Two <ESC>'s delimiting the address expression instructs DDT to treat
the 36-bit expression as an EFIW word and calculate the effective
address exactly as the hardware would, were the hardware to indirectly
DDT42B - DDT %42B(324) User's Guide to New Features Page 4
Memory and Address Control
address the 36-bit expression at location "." (whether or not location
"." is currently open). A strange case can come up about which the
user should be cautioned - there is an ambiguity as to where (i.e.,
what "section") to start the effective address calculation. DDT as-
sumes the left half of "." (i.e., the last location opened by the
user). If for example having opened location 0,,1234 which contains
7,,4321 the user issues the command "$$[" then DDT will calulate the
effective address as the contents of location 4321 in section 0 in-
dexed by the right half of register 7, and if bit 13 is on, treating
that word as an IFIW and continuing the address calculation. This,
although probably not what was expected, is in fact exactly what the
hardware would do since the indirect word came from section 0. Had the
user opened location 1,,1234 (containing 7,,4321) then DDT would take
the contents of location 7004321 and continue from there.
If no <ESC>'s delimit the address expression, then DDT simply uses the
full 36-bit expression as the address (e.g., "30,,30/" says open
location 30000030 and "-1/" says open location 777777777777). Again,
only FILDDT can actually reference an address greater than 30-bits
wide (not that anyone has that much disk space, but the hardware will
not permit an address space over 30-bits wide), and in any case the
address must be a non-negative 36-bit integer.
There is a special case in which DDT does something "kinky" - if a
space was typed in entering the address expression, or if no explicit
address was typed (i.e., the user is using the "last word typed" by
simply typing only (for example) <TAB>), DDT will form the 36-bit
actual address by using only the right half of the 36-bit address
expression plus the left half of "." as the section number. This not-
at-all-obvious behavior is so that the user can type in expressions
such as "JRST PAT<TAB>" and have DDT go to location PAT in the same
section as the JRST PAT instruction rather than going to address
254000000000+(PAT modulo 2**18). Another common usage of this "fea-
ture" would be in chaining down linked lists where the link pointer is
an 18-bit section-local address in the left half of a word. To do this
the user may type "sp$$Q/" (where "sp" means space). This is one of
those cases where usefullness outweighs cleanliness of implementation
3.3 Modifying memory
Two new commands have been added to facilitate DDT's manipulation of
the user address space.
3.3.1 Automatic write-enable
The $W or $0W command instructs DDT to, if the user attempts to depos-
it into a write-protected memory location, automatically attempt to
write-enable the memory location, do the memory deposit, then finally
re-write-protect the memory location (default for TOPS-10); the $$W or
$$0W command instructs DDT to simply give an error indication if the
user attempts to change a write-protected memory location (default for
TOPS-20). For FILDDT the use of this command is restricted to non-file
usage such as "DDT'ing" the running monitor/memory space.
3.3.2 Automatic page-creation
The $1W command instructs DDT to automatically try to create the page
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Memory and Address Control
the user is trying to deposit into if it doesn't already exist (de-
fault for TOPS-20); the $$1W command instructs DDT to simply give an
error indication if the user attempts to write into a non-existant
page (default for TOPS-10). EDDT and FILDDT doing super I/O or
"DDT'ing" an .EXE file will NEVER attempt to create a non-existant
page. For FILDDT the user must specify patching the file when he
starts FILDDT in order to be able to create new pages (e.g., extend
the file or fill in a gap in the middle of the file (TOPS-20 only)).
3.4 Page mapping and physical addressing
All flavors of DDT (on TOPS-20, only FILDDT) support page mapping and
address relocation as well as register and physical address manipula-
tion. All of these functions use some variation of the $U/$$U DDT
command. In general these functions may be mixed together (for example
address relocation and page mapping).
*** Warning ***
The $U command syntax in DDT version 42/42B is totally dif-
ferent (and mainly incompatible) from previous versions of
DDT! The user is MOST strongly urged to carefully read this
section on memory mapping and addressing!
3.4.1 Physical addressing
DDT now has the concept of "physical" addressing in addition to its
normal "virtual" addressing. The $U command instructs DDT to use nor-
mal virtual addressing (what it used to do); the $$U command instructs
DDT to manually track down the honest physical address rather than the
virtual address space in which DDT finds itself running. Physical
addressing is really applicable only to EDDT or to FILDDT looking at
running monitor/memory (TOPS-10 only). User mode DDT (including EDDT
running in user mode, MDDT (TOPS-20 only), and VMDDT (TOPS-10 only))
and FILDDT looking at a disk all treat $U and $$U identically. In
physical addressing location 0 is not register 0 (i.e., DDT's internal
copy of user register 0) but rather physical memory location 0 page 0
bank 0 box 0 (that memory location on the hardware memory bus that
responds to all address bits = 0).
When the $$U DDT command is issued "physical" locations 0 to 17 become
"registers" 0 to 17. For user mode DDT this means locations 0 to 17
become DDT's registers rather than the user's registers (although the
user's registers will be properly restored on DDT-exit, $$U merely
directs DDT not to use the internal "fake" (i.e., user) registers).
For FILDDT this means file words 0 to 17 (as mapped by the .EXE direc-
tory if used) become locations 0 to 17 (normal for a data file).
Subsequent issuance of the $U DDT command will redirect locations 0 to
17 to being DDT's internal "fake" registers again, except for FILDDT
looking at an data file or doing super I/O to a disk.
Note that for executive mode EDDT to utilize physical addressing the
paging hardware must have been enabled PRIOR to DDT-entry. This re-
quirement exists because EDDT, in order to access all of physical
memory, needs to map the desired physical address into its own (execu-
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Memory and Address Control
tive) virtual address space, which it does by fondling the already-
extant page maps. For EDDT to provide physical addressing capability
without this restriction would require 2 (3 if KL-paging) more memory
pages be dedicated to EDDT for building temporary page maps, plus
support code etc.
For FILDDT to examine/modify physical memory a 7.00 or later release
of the TOPS-10 monitor is required; no release of TOPS-20 supports
FILDDT'ing physical memory.
3.4.2 Page mapping
DDT now supports page mapping in both the KI- and the KL-tradition.
EDDT in executive mode will dynamically figure out which style of
paging is in effect and operate accordingly. All other flavors of DDT
(including EDDT running in user mode) will assume the mode of paging
used by the operating system for which DDT was assembled - KI-paging
for TOPS-10 and KL-paging for TOPS-20. To select KI-paging emulation
the flg$10U command is used; to select KL-paging the flg$11U command
is issued; in either case if flg is zero then the paging emulation is
disabled, if flg is 1 then the appropriate paging emulation is
In executive mode EDDT or FILDDT looking at running monitor/memory
space DDT will internally utilize physical addressing in order to pro-
vide the user the true mapped virtual address space desired.
22.214.171.124 KI-paging - For KI-paging (TOPS-10 default) the page mapping
command for the executive virtual addressing space is [upt<]ept$U
where upt is the optional physical memory page number of the user
process table (for setting the "per-process" addressing space - exec
virtual addresses 340000 through 377777) and ept is the physical memo-
ry page number of the executive process table. The user virtual ad-
dressing space is selected by the upt$1U command. The command $U
returns DDT to regular unmapped virtual addressing.
126.96.36.199 KL-paging - For KL-paging (TOPS-20 default) the page mapping
command for the executive virtual addressing space is ept$U where
ept is the physical memory page number of the executive process table,
or epx$$U where epx is the index into the SPT of the executive
process table pointer. To select the user virtual addressing space the
command is upt$1U where upt is the physical memory page number of the
user process table, or upx$$1U where upx is the index into the SPT of
the user process table pointer. The command $U returns DDT to regular
unmapped virtual addressing.
To map a single section (256K address space) under KL-paging the com-
mand is either sec$2U where sec is the physical memory page number of
a KL-paging section map, or sex$$2U where sex is the index into the
SPT of the section map.
Basically, under KL-paging, $0U selects the ept, $1U selects the upt,
and $2U selects a single section. A single $ indicates the physical
memory page number and two $'s indicate an SPT index.
3.4.3 Setting the SPT
FILDDT will automatically define the start of the SPT from a disk file
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Memory and Address Control
(assumed monitor dump) from the symbol "SPT" if it exists (TOPS-20
only). The command spt$6U specifies to DDT that the SPT starts at
3.4.4 Register addressing
The command acs$5U instructs DDT to use the 20 consecutive locations
starting at acs as the registers (DDT maintains an internal copy of
the registers so changing "register" 3 will not affect (for example)
acs+3). FILDDT, when reading an .EXE file, will automatically load its
internal "fake" registers as though the user had typed CRSHAC$5U if
TOPS-10 or BUGACS$5U if TOPS-20. Note that if physical addressing mode
has been entered (the user has issued the $$U command) then the inter-
nal "fake" registers are ignored; if the user subsequently reenters
virtual addressing (via some form of the $U command) then an acs$5U
command may also have to be re-issued to get the registers back (this
does not affect the saving and restoring of the hardware registers in
user or executive DDT, only what DDT will use for typing out locations
0 to 17).
The command flg$3U explicitly controls the usage of DDT's internal
"fake" registers - if flg is 0 then the "fake" registers are ignored
(i.e., 0 to 17 are taken from the true current addressing space), if
flg is non-zero then addresses 0 to 17 are taken from DDT's internal
copies of the registers.
The $U command, except for FILDDT'ing a data file or doing super I/O
to a disk, will return DDT to its internal "fake" registers. The se-
lection of registers is completely independent of any page mapping in
effect. Changing virtual address spaces does not change the
In executive mode EDDT only the command n$4U will switch DDT to use
(and thus display) hardware AC block n (available only for KL-10's and
KS-10's). The user is warned that 7$4U on a KL-10 will bring rapid and
rabid death (the microcode uses AC block 7). On DDT exit DDT will
restore the ac block context to the state it was in at DDT entry.
3.4.5 Address relocation and protection
As an aid to looking at data structures which are formed using point-
ers as offsets rather than pointers as absolute values, DDT will allow
the user to set both a base relocation address to be added to all
addresses used in location examining commands and a protection address
beyond which the user "virtual" (note the use of "virtual" here as
meaning pre-relocated) address is illegal. This is (coincidently) ex-
actly analguous to the KA-10 hardware relocation and protection strat-
egy, and in fact could be used as such to "mimic" the $U KI/KL/KS-10
functionality on a KA-10 in executive mode. The form of this command
is bas$8U where bas is the base virtual address, and prt$9U where prt
is the maximum address the user will be allowed to type in. Note that
page mapping and address relocation and protection are independent
mechanisms, with address relocation and protection being performed
before any mapping is done. The protection address has no effect on
the final "physical" address generated by any mapping currently in
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Memory and Address Control
3.4.6 $U command summary
All $U/$$U commands take the following form:
1. $U Unmapped virtual addressing
2. $$U Unmapped physical addressing
3. ept$[$]U Select executive virtual addressing
4. upt$[$]1U Select user virtual addressing
5. sec$[$]2U Select single section
6. flg$3U Select (deselect) internal fake registers
7. acb$4U Select hardware ac block
8. acs$5U Load internal fake registers
9. spt$6U Select base of SPT
10. bas$8U Set base relocation address
11. prt$9U Set protection address
12. flg$10U Select (deselect) KI-paging
13. flg$11U Select (deselect) KL-paging
1. acb := integer ac block number
2. acs := address of 20-word register block
3. bas := base relocation address
4. ept := executive process table page number
5. flg := selection flag, zero to deselect, non-zero to select
6. prt := protection (maximum allowable) address
7. sec := section map page number
8. spt := address of SPT
9. upt := user process table page number
3.4.7 Address checking (Executive EDDT only)
EDDT, when running in executive mode, now is much more extensive in
validity-checking memory references. In particular, EDDT will not
cause a NXM (page fault) trap to the resident operating system if the
user types in an illegal (non-existent or unmapped) address, but rath-
er will simply type its ubiquitous ?<DINK><TAB> error message.
3.4.8 Address breaking
DDT will no longer cause an address break to occur when examining or
depositing a location at which an address break condition has been
set. This applies only to "user" examines and deposits, an address
break set in DDT will still cause an address break to occur.
4.0 Specifying the Start Address
The $G command now expects a 36-bit address (obviously with bits 0 to
5 off) at which to start the user program. This means that the users
of programs such as the TOPS-10 monitor which define symbols like
"DEBUG=:<JRST .>" can no longer go either DEBUG$G or DEBUG$X at the
user's whim but must decide on one form or the other (the default
obviously being to do nothing - i.e., to settle for the DEBUG$X form)
DDT42B - DDT %42B(324) User's Guide to New Features Page 9
Symbolic expression typein and typeout
5.0 Symbolic expression typein and typeout
DDT has expanded the range of both symbolic typein and symbolic type-
5.1 Symbolic typein
The JSYS opcode (opcode 104) has been added to TOPS-20 DDT, as have
all the TOPS-10 UUO's (but not the CALLI's etc.) for debugging pro-
grams which run under the compatibility package.
The "G" format floating point instructions GFMP, GFDV, GFAD, and GFSB
have been added to the opcode table for both typein and typeout.
5.2 Multiply-defined symbol typein
If the user types an ambiguous symbol (a symbol defined two or more
places outside of the current local symbol table and not in the
current local symbol table) DDT will issue an "M" error message.
5.3 Selecting no local symbol table
The $: command issued without an explicit module name to use as the
local (or "opened") symbol table will deselect any local symbol table.
This is the initial state in which DDT starts.
5.4 Symbol cache
DDT now has a symbol "cache" of symbols recently used to type out
values. This cache is used primarily for typeout; typein will check
the symbol cache for a matching symbol from the currently opened or
local symbol table, if no match is found the cache is ignored and the
regular symbol table is used. The symbol cache is "flushed" on the
issuance of any $: command.
5.5 Symbolic typeout
DDT now goes to great pains to find any possible user-defined symbol
(such as an OPDEF) to match the expression DDT is trying to type out.
The order in which DDT searches for a symbol match in symbolic typeout
mode for non-I/O instructions is:
1. Full 36-bit match; OP, AC, I, X, and Y fields (e.g., the TOPS-20
monitor calls such as GTJFN)
2. OP, I, X, and Y fields (e.g., the TOPS-10 monitor calls such as
3. OP and AC fields (e.g., the TOPS-10 monitor calls such as INCHWL
or "instructions" such as HALT)
4. OP field only (e.g., user UUO's or "OPDEF XMOVEI [SETMI]")
5. DDT's internal hardware opcode table
The order in which DDT searches for a symbol match in symbolic typeout
mode for I/O instructions is:
1. I/O OP and DEV fields (bits 0 to 12 - e.g., KL-10 APRID or KS-10
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Symbolic expression typein and typeout
2. Regular (non-I/O) OP field (e.g., KS-10 UMOVE)
6.0 Command typeout
6.1 ASCII typeout
DDT adds the typeout mode commands $8T and $9T to typeout 8 bit ASCII
or 9 bit ASCII respectively (i.e., pick up 8 or 9 bit bytes and "type"
them straight as is - which with current TOPS-10 and TOPS-20 operating
systems means as 7-bit ASCII).
The adr$0T command has been added to type out a 7-bit "ASCIZ" format
text string (i.e., a string of 7-bit ASCII characters terminated by a
null byte) starting at address "adr" or "." if adr is not specified.
Upon issuance of the $0T command, DDT will type out consecutive 7-bit
bytes until either the terminating null is encountered or the user
types a character (similar to stopping a search).
6.2 Floating point typeout
The [$]$2F command (TOPS-20 FILDDT/SDDT/UDDT only) selects double-pre-
cision floating point typeout mode; in other respects it is identical
to the [$]$F command.
6.3 Mask and value typeout
In FILDDT the various masks and internal values (such as $U) may be
examined by issuing the appropriate command without an argument. For
example, typing "$M" to FILDDT will result in the typeout of the cur-
rent $M mask value.
7.0 Command files
The $Y command (TOPS-10 DDT only) has been changed somewhat, both in
input and output (logging) functions.
7.1 Command input
If the user does not type a 36-bit expression to be used as a file
name (such as $""FILNAM"$Y) but just types $Y by itself then DDT will
prompt with "File: ". After the prompt the user can enter a TOPS-10
file specification in the form dev:name.type[directory]/switches where
[directory] can of course contain SFD's.
The echoing of the $Y command input may now be supressed via the TTY
control "mask" - see section 14.2.
7.1.1 /A switch
The /A switch instructs DDT to abort the command file if a DDT-detect-
ed command error occurs (such as reference to an undefined symbol).
DDT42B - DDT %42B(324) User's Guide to New Features Page 11
7.2 Command output (logging)
When reading a command file ($Y command) DDT will no longer "log" all
output onto device LPT: but rather just type out onto the user
8.0 Automatic patch insertion
The automatic patch insertion facility ($< and $> commands) are basi-
cally the same as in version 40 of DDT with only minor differences.
8.1 Patch opening
The user may specify patching either by sym$< where sym is the name of
a symbol (which will be automatically updated at the termination of
the patch) or via exp$< where exp is any 36-bit expression represent-
ing the address of the resultant patch. If the later form of the patch
command is used no symbol will be updated to the end of the patch.
With DDT version 42B, the user can no longer open a patch if one is
already in progress - the currently open patch must be either closed
($n> command) or aborted ($$0> command) first.
8.2 Default patching symbol
The list and order of default patching symbols which DDT uses when the
user does not supply an explicit patching symbol is now:
1. PAT (TOPS-10 EDDT only)
2. FFF (TOPS-20 EDDT/KDDT/MDDT only)
3. PAT.. (all flavors)
4. PATCH (all flavors)
8.3 Default patching address
If the user does not supply an explicit patching symbol and DDT is
unable to find one of the default patching symbols then the address
specified by the right half of location .JBFF (even on TOPS-20) is
used. On patch close ($> command) if the patching address was default-
ed via .JBFF, then both the right half of location .JBFF and the left
half of location .JBSA are updated to point to the end (+1) of the
8.4 Patch closing
The patch close command has been expanded to a more general form of
$n>, where the "n" is used to control "skipness" of the patch close,
i.e., how many JUMPA instructions to insert at the end of the patch to
return to the patched code. If "n" is 0 then no JUMPAs are inserted
(usefull for patching a JRST instruction); if "n" is 1 then one JUMPA
instruction is inserted (as in patching a non-skip instruction); etc.;
the default remains 2.
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Automatic patch insertion
8.5 Patch closing confusion and restriction
It no longer matters how (when) the user types the $> command, either
immediately after the final word expression, or after a <CR> or <LF>
to terminate the final word expression - DDT will never generate a 0
word for free.
There is a very obscure restriction however on the use of the #
command in conjunction with the $> command. If the user is referencing
an undefined symbol in the expression for the last word of the patch
then that expression must explicitly be terminated in such a fashion
as to close the location before terminating the patch. For example,
"MOVE T1,BLETCH#$>" is illegal but "MOVE T1,BLETCH#cr$> (where "cr"
indicates a carriage return) is ok.
8.6 Patch abortion
A patch may be aborted with the $$0> command.
The breakpoint logic in DDT version 42/42B has been extensively re-
vamped in order to support extended addressing. The default number of
breakpoints is now 12 (decimal); and can be set (by defining the sym-
bol NBP=number of breakpoints) arbitrarily high (within memory space
limitations) rather than being limited to 9 or 36 (decimal) depending
on which code restriction one choose to believe.
9.1 Setting breakpoints
DDT can now set a breakpoint in code running in any section with two
1. If DDT is currently running in section 0 then breakpoints can
only be set in section 0 (see section 3.1.3 above).
2. DDT must be mapped in the section containing the code in which
breakpoints are to be placed (the logic of this is that since
there is no way for DDT to cause unconditional transfer of con-
trol to DDT with only 36 bits some portion of the section address
space must be devoted to DDT; therefore, given this restriction,
one might just as well put all of DDT in that section since it
makes for a cleaner and simpler implementation). Note that this
does not mean DDT must be running in that section, but merely
that DDT must be mapped in that section!
It does not matter into how many different sections the same code is
mapped as long as DDT is mapped into the same sections since DDT is
"section-independent". For example (taking the TOPS-20 monitor which
maps section 0 and section 1 identically) if a breakpoint is set at
address 1004567 (or 1,,4567) but the PC was 4567 (or 0,,4567, i.e., in
section 0 rather than section 1) when the breakpoint was executed DDT
does not care (as long as DDT is mapped in that section, which in the
example of the TOPS-20 monitor it is).
The syntax for setting a breakpoint is now opn<cmd>bpt$nB where n is
DDT42B - DDT %42B(324) User's Guide to New Features Page 13
optional and, if specified, declares the breakpoint number to be
assigned to that address; bpt is the 36-bit address at which to place
a breakpoint; opn is an optional 36-bit address to open and display
upon execution of the breakpoint; and cmd is the address of a DDT
command string to execute upon breaking.
The "cmd" string address is the address of an ASCII (7-bit byte)
string of characters terminated by either a ^Z or a null character.
This string will be treated as DDT command input, just as if it came
from an $Y command file (and in fact the $Y command file facility must
be enabled - symbol FTYANK defined non-zero when assembling DDT - for
the cmd string to work).
DDT will no longer assign two different breakpoints to the same ad-
dress, either accidentally or under user control - if the user at-
tempts to set a breakpoint at a location at which a (different) break-
point is already set, the old breakpoint is cleared first.
9.2 Breakpoint typeout
Upon execution of a breakpoint DDT will now type the user instruction
(in instruction format regardless of the permanent typeout mode) at
that breakpoint and set "." to the breakpoint address. If, further,
opn was specified as in section 9.1 above, then DDT will also display
the contents of location opn in the permanently set typeout mode and
"." will be updated to opn (with the breakpoint address itself
becoming the previous PC sequence and so available via the $<CR> etc.
The breakpoint instruction typeout is under the control of a flag bit
set in the TTY control mask - see section 14.2 for details.
9.3 Examining breakpoint locations
The $nB command continues to be the "address" of breakpoint n's data-
base, but $nB is no longer equal to $n-1B+3. The breakpoint database
of interest to the user now has the following format:
1. $nB+0/ If nonzero the address for breakpoint n
2. $nB+1/ The conditional break instruction (break if skips)
3. $nB+2/ The proceed count (break on transition to 0)
4. $nB+3/ If greater than or equal to zero then the address to be
The rest of the breakpoint data base should not be of use to the user.
9.4 Unsolicited breakpoints
DDT has a new breakpoint facility - the ability to handle unsolicited
breakpoints (i.e., breakpoints that DDT did not itself set). If con-
trol passes to location $0BPT+1 ($0BPT is a global DDT symbol) then
DDT will act as if a breakpoint had been set at the address-1 con-
tained in location $0BPT. The address in $0BPT must be setup as if the
cpu executed a JSR $0BPT instruction - if in section 0 then flags,,PC
otherwise just global 30-bit PC. After "hitting" an unsolicited
DDT42B - DDT %42B(324) User's Guide to New Features Page 14
breakpoint the user can proceed with program execution with the $P
command (all arguments to the $P command such as proceed count or
auto-proceed ($$P) are ignored).
Although this facility gives programs the ability to cause breakpoints
at any time (thus getting into DDT with the program state carefully
preserved) it is intended to be of most use in conjunction with an as-
yet-unimplemented monitor command (such as control-D) to "force" a
breakpoint on a program without having to control-C/DDT the program.
Then the user could simply continue with the program by typing $P.
10.0 Single-stepping the program
The $X DDT command has been significantly modernized (and sped up in
general) with version 42/42B of DDT.
10.1 New opcodes
The ADJSP, DADD, DSUB, DMUL, and DDIV instructions have been added to
DDT's $X table although double- and quad-word integers (for DADD etc.)
are still typed out as two or four single words rather than one big
multiple precision integer. All of the extended JRST-class instruc-
tions are correctly simulated/traced. A user-UUO being executed in a
non-zero section is simply XCT'ed and is not traced.
10.2 Byte-manipulation typeout
A rudimentary byte-manipulation-instruction typeout facility was added
(to DDT version 40 actually) to display the byte pointer and the con-
tents of the effective address of the byte pointer. The EXTEND-class
instructions are not handled.
10.3 Effective address calculation
DDT now always calculates the effective address of the instruction
being $X'ed rather than just blindly "doing it" in order to both pre-
vent DDT from getting an illegal memory reference as well as to make
DDT be independent of the section in which the user PC resides (i.e.,
DDT does not have to be mapped into the user PC section to handle
$X'es although if the user PC is in a non-zero section then DDT must
be in a non-zero section). Besides, it's usually faster too!
10.4 KS-10 I/O instruction trace
The KS-10 specific I/O instructions which reference the UNIBUS
(executive mode only) are not traced, only the contents of the regis-
ter specified in the AC field are displayed. Since the UNIBUS device
registers can be reference-volatile (i.e., merely referencing one can
cause it to change - such as the DL-11 data registers) DDT does not
typeout the contents of the referenced UNIBUS address. Further, since
the effective address of the instruction is not calculated in a stan-
dard format (at least as far as DDT is concerned) the effective ad-
dress itself is not even displayed.
DDT42B - DDT %42B(324) User's Guide to New Features Page 15
Single-stepping the program
10.5 PC skipping
If the user instruction being $X'ed skips then DDT will now typeout
"<SKIP>" if the PC skips by one location, or "<SKIP n>" if the PC
skips by n locations, where n is less than or equal to the DDT
assembly parameter SKPMAX (by default 3). If the PC changes more
drastically than that (e.g., goes to a smaller address) DDT will type
DDT (TOPS-20 only) will now handle instructions followed by either an
ERCAL or an ERJMP instruction (which is really just a 72-bit instruc-
tion with two effective addresses). If the instruction being executed
does not take the error jump then DDT will print "<ERSKP>" after the
normal instruction trace to indicate to the user that an ERCAL or
ERJMP was just skipped (i.e., the PC incremented by 2 rather than 1)
and will not display the ERCAL or ERJMP instruction. If the instruc-
tion does take the error jump then the ERCAL or ERJMP instruction will
be displayed, if an ERCAL instruction then register 17 will also be
displayed, and the PC will be changed to the error address.
DDT will print "<ERSKP>" rather than showing the ERCAL or ERJMP in-
struction since DDT has no way of telling whether or not the
instruction itself caused the skip (as in a SKIPA) or if the PC merely
"fell through" the ERCAL or ERJMP instruction (as in a successful
Users of EDDT and MDDT should be cautioned about $Xing instructions
followed by an ERCAL or ERJMP in non-zero sections - the monitor has a
tendency to transfer control to the error address in section 0, which
will cause a BUGHLT because DDT (running in executive mode) does non-
zero section things thinking it is still in a non-zero section.
10.7 $X'ing an INIT
DDT will now let the user $X an INIT (TOPS-10) monitor call. DDT will
print out <SKIP 2> if the INIT fails or <SKIP 3> if the INIT succeeds.
10.8 $X speed up
By building into DDT a table of instructions which can cause the state
of the known world to change, and assuming the state of the world does
not change if the instruction being $X'ed is not so marked, the time
required to $X an instruction is cut by roughly a factor of 10. This
results in a dramatic performance increase especially for EDDT on KL-
10's where waiting for the console front end to switch between secon-
dary and primary protocol is very time-consuming.
10.9 Repetitive $X'es
The $$X command now takes an optional address range. Normally $$X will
terminate when the user PC inclusively enters the range .+1 to .+
SKPMAX (default value of SKPMAX is 3). The user may specify
lwr<upr>$$X where lwr is the lower address boundary and upr is the
DDT42B - DDT %42B(324) User's Guide to New Features Page 16
Single-stepping the program
upper address boundary which, if the user PC ever inclusively enters
the range so specified, terminates the $$X. If only lwr is specified
then upr defaults to lwr+SKPMAX. This command is very useful for re-
covering from having $X'ed a (for example) PUSHJ instead of having
$$X'ed the (for example) PUSHJ.
10.10 Fast $$X'es
The $$X command has been enhanced to allow the user to specify that
much of the $$X "paranoia" be bypassed. The high-speed $$X command is
$$1X, which causes DDT to not "restore the state of the world" prior
to each instruction $Xed. This is usually not a problem unless the
code being $$Xed modifies the PSI system, terminal characteristics, or
10.11 $X'ing from instr$X
If the user $X'es a return from a subroutine which was entered by
doing an instr$X (for example "PUSHJ P,SUBRTN$X where SUBRTN has a
breakpoint in it) then DDT simply "returns" from the original instr$X
rather than proceding to $X the internals of DDT itself. This is a
very obscure condition so don't worry too much about it.
10.12 $$X status
DDT will now respond to a ? character being typed during an $$X se-
quence by typing "Executing: " followed by the current user "pc" and
instruction being executed. Typing any other character terminates the
10.13 $X PC
The $. command now acts like the . command only $. returns the value
of the $X PC (i.e., the address of the next instruction to be $X'ed).
The $$. command returns the previous $. value (useful for $$.<$$X as
in section 10.9 above).
Most of the differences in how DDT handles searches are simply bug
fixes, not major changes in the logic of searching.
11.1 Non-existant pages
DDT now simply skips over pages which don't exist in the address space
being searched, rather than terminating the search as soon as a hole
has been found.
11.2 Effective address searches
Since almost all address calculations start with an IFIW basis (with
the exceptions being such things as interrupt vectors and the like on
KL-10's or KS-10's), DDT will assume that each word it examines is an
instruction and perform an IFIW effective address calculation. The
final result must match in all 30 bits (actually internally DDT will
DDT42B - DDT %42B(324) User's Guide to New Features Page 17
do a full 36-bit compare so the address being searched for had better
not contain anything in bits 0 to 5).
11.3 Address limit defaults
With the advent of extended addressing and physical addressing the
address limits are defaulted somewhat differently than from previous
versions of DDT:
1. EDDT, MDDT (TOPS-20 only), UDDT, and VMDDT
1. Lower Limit: <current section>,,0
2. Upper Limit: <current section>,,777777
2. FILDDT looking at an .EXE file
1. Lower Limit: 0
2. Upper Limit: highest virtual address mapped
3. FILDDT looking at a data file
1. Lower Limit: 0
2. Upper Limit: highest word written in file
4. FILDDT looking at disk structure/unit
1. Lower Limit: 0
2. Upper Limit: highest word in disk structure/unit
5. FILDDT looking at runing monitor
1. Lower limit: 0
2. Upper limit: 777777
6. FILDDT looking at physical memory (TOPS-10 only)
1. Lower Limit: 0
2. Upper Limit: Highest extant memory address
As with any defaults not all cases will be properly "guessed" by DDT.
In particular if the user has mapping or address relocation in effect
the virtual address range so produced may have nothing whatsoever in
common with the address limit defaults chosen by DDT.
11.4 Search matches
DDT will leave each address matched by its search on the "pc stack"
available to $<CR> etc. commands. When the search is terminated DDT
will set "." to the last address searched.
11.5 Searching status
DDT will now respond to a ? character being typed during a search by
typing "Searching: " followed by the current location and value being
searched. Typing any other character terminates the search immediate-
DDT42B - DDT %42B(324) User's Guide to New Features Page 18
DDT allows the user to "watch" a location, waiting for it to change.
Although primarily useful for FILDDT'ing the running monitor, it is
present in all flavors fo DDT for completeness. The syntax of the
watching command is exp$V, where exp is the address to be watched. If
no explicit address is specified the last location opened by the user
will be used.
Upon initial issuance of the $V command the location is displayed.
Thereafter the location is continuously monitored, and will be
displayed every time its contents change. In user mode DDTs (and this
includes TOPS-20 MDDT as well) the location is checked once a clock
tick (approximately 50 to 60 times a second), in exec mode EDDT the
location is continuously being monitored - no "pause" is attempted.
DDT will respond to a ? character being typed during an $V sequence by
typing "Watching: " followed by the current location and contents be-
ing watched. Typing any other character terminates the $V immediately.
13.0 Zeroing memory
The algorithm used by DDT previous to version 42/42B has only limited
usefulness in today's modern virtual world (especially on TOPS-20). A
new command has been implemented - lwr<upr>exp$z where lwr is the
lowest (starting) address, upr is the highest (ending) address, and
exp is the 36-bit quantity to deposit in each word inclusively bounded
by lwr and upr. Both lwr and upr must be specified. If exp is not
specified then 0 is used as the default. This command completely su-
persedes the older $$Z command, which has been removed from DDT ver-
A special note: The creation of zeroed pages (which formerly were non-
existent) by the $Z command is under the control of the automatic page
create flag (i.e, the $1W and $$1W commands - see section 3.3.2).
DDT will now respond to a ? character being typed during an $Z
sequence by typing "Zeroing: " followed by the current location and
value being "zeroed". Typing any other character terminates the $Z
14.0 Special masks
DDT version 42/42B (it actually started with DDT version 40) has sev-
eral new "masks" (for lack of a better name and/or command) of inter-
est to the user.
14.1 $0M - Search mask
The operation of the search mask continues unchanged. The search mask
may now be referenced by either the $M (old style) or the $0M com-
mands. The default value remains 777777777777.
DDT42B - DDT %42B(324) User's Guide to New Features Page 19
14.2 $1M - TTY control mask
This mask controls special TTY behavior (primarily TOPS-10 and exec
14.2.1 $Y command echo
Bit 15 controls the echoing of command input from $Y command files. A
0 selects command file echoing (the default case); a 1 supresses
command file echoing.
14.2.2 Breakpoint printout
Bit 16 controls the automatic printing of the breakpoint instruction.
A 0 selects the printing of the breakpoint instruction on occurance of
a breakpoint (the default case); a 1 supresses the printing of the
14.2.3 Tab separator display
Bit 17 controls whether DDT will print its usual <TAB> or three spaces
for the <TAB> separator. A 0 (the default) selects three spaces, a 1
selects a <TAB>.
14.2.4 Tab simulation
Bit 34 controls tab simulation. A 0 selects literal <TAB> characters
(i.e., the terminal handles <TAB>'s directly, a 1 selects space-fill
instead. This condition is automatically set for user mode DDT's (in
user mode <TAB>s are always output literally) - it is only useful to
manually set tab simulation in exec mode EDDT.
14.2.5 Rubout control
Bit 35 controls rubout (and ^W) operation. A 0 selects "hardcopy"
operation (DDT will echo a "\" character and the character being
deleted), a 1 will cause rubouts to echo as a backspace, space, back-
space sequence. This condition is automatically set for user mode
DDT's (if TTY DISPLAY (TOPS-10 only) is set then rubouts echo as
<BS><SP><BS>) - it is only useful to manually set fancy rubouts in
exec mode EDDT.
14.3 $2M - Offset range
The 36-bit "mask" in this case is really a value, used as the maximum
offset allowable for typing addresses in the form symbol+offset. The
default offset is 1000 (octal).
14.4 $3M - Byte mask
This mask is used in conjunction with the $O command for typing bytes
in a word that are not necessarily evenly spaced. Whenever an $O com-
mand is issued without an explicit byte size the byte boundaries are
determined by one-bits in the byte mask - each one bit in the byte
mask marks the low order bit of a byte. Bit 35 is always considered
on. The default value is 0 (i.e., one 36-bit byte). For example the
DDT command 040100200401$3M sets the byte mask for typing right-justi-
fied 8-bit bytes (preceded by the leading 4-bit byte).
DDT42B - DDT %42B(324) User's Guide to New Features Page 20
RADIX-50 symbol typein
15.0 RADIX-50 symbol typein
Since prehistoric times DDT has supported RADIX-50 symbol typein, but
that fact was never documented. The syntax for using a RADIX-50 symbol
as an 36-bit item in an expression is sym$5" where sym is the desired
RADIX-50 symbol. For example, to search for all occurences of the
symbol PAT.. the DDT commands 37777,,-1$M (only look at low-order 32
bits) and PAT..$5"$W suffice.
16.0 New DDT runtime information
16.1 Exec-mode machine state
Several new words have been added to DDT's runtime table describing
the state of the machine upon (executive mode only) DDT-entry. These
words are all accessible via the DDT command $I+offset (not available
1. $I-01/ APR CONI word
2. $I+00/ PI CONI word
3. $I+01/ Mask of PI channels turned off by EDDT
4. $I+02/ Executive virtual address of EPT
5. $I+03/ Executive virtual address of UPT
6. $I+04/ Executive virtual address of CST
7. $I+05/ Executive virtual address of SPT
8. $I+06/ Original AC-block word (DATAI PAG) if acb$4U
16.2 Error typeout
DDT will now sporadically issue a short textual error message for
certain error conditions (e.g., trying to write in a write-locked
The $? command will type out the last DDT error; the $$? command
(TOPS-20 only) will type out (ERSTR JSYS) the last process (JSYS)
17.0 Obsolete commands
The executive mode paper tape facilities (^R, $J, and $L DDT commands)
are no longer supported. The code has been removed from the source
18.0 New exec-mode support
The exec-mode support for EDDT has been enhanced in several ways.
The exec-mode user can now typeahead while EDDT is typing out, EDDT
will periodically "poll" the command input terminal to see if anything
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New exec-mode support
has been typed, and will "buffer" any such typeahead internally. Note
that this does not apply to such things as searchs where typeahead
will still cause the search to be aborted (unless "?" is typed).
EDDT now supports the XON/XOFF protocol. Typing XOFF (^S) will cause
EDDT to freeze terminal output; Typing XON (^Q) will cause EDDT to
resume terminal output.
18.3 Output suppression
EDDT now supports output suppression a la ^O. Typing ^O will toggle
the output suppression - the first ^O causes output to be discarded,
the second ^O causes output to resume, etc.
19.0 FILDDT startup and commands
FILDDT is a special version of DDT with the facilities for "DDT'ing"
address spaces other than its own, such as disk files and in particu-
lar .EXE files. FILDDT has existed for years but has always been off
in the background as a specialized "tool" for the exclusive use of
monitor programmers looking at crash dumps. With DDT version 42 FILDDT
is now a general purpose utility for use by the "general public",
particularly people who have databases resident in disk files (.REL
files for example).
Out of efficiency considerations FILDDT builds the symbol table(s) it
will actually use at runtime in its own address space. Virgin FILDDT
has no symbols (the symbol table (if any) for FILDDT in FILDDT.EXE is
completely independent of the address space being FILDDT'ed and does
not count). There are special commands to instruct FILDDT to extract
(and build internal-to-FILDDT copies of) symbol tables from .EXE files
(see below). Once FILDDT has setup its internal symbol table(s), it
may then be SAVEd with the internal symbol table(s) for later use by
exiting to monitor level (with the ^Z FILDDT command) and typing the
When FILDDT is started it will prompt "File: ". The user may at this
time optionally enter a standard TOPS-10 file specification in the
form dev:name.type[directory]/switch. At least one function switch is
mandatory. SFD's are of course legal in the directory specification.
19.2.1 /D command
The /D command or function switch instructs FILDDT that the file spec-
ified is a data file - i.e., do not map the file as an .EXE file and
use real file words 0 to 17 for locations 0 to 17.
19.2.2 /F command
The /F command or function switch instructs FILDDT to "DDT this file
anyway". It is useful only in conjunction with the /S command or func-
DDT42B - DDT %42B(324) User's Guide to New Features Page 22
FILDDT startup and commands
tion switch which normally re-prompts for another file specification.
Used in conjunction with /S (which implies an .EXE file) FILDDT will
use the file from which symbols were extracted as the file to be
19.2.3 /H command
The /H command or function switch instructs FILDDT to type out a brief
help text, abort the current command, and prompt the user for another
19.2.4 /J command
The /J command or function switch is applied to a job number rather
than a file specification and instructs FILDDT to "DDT" the address
space of the job specified. Since FILDDT uses JOBPEK monitor calls to
access the specified job's address space the success or failure of any
given memory reference is dependent on the job being resident in main
memory - if the job is swapped out or if the memory reference is to a
page which is paged out the memory reference will fail. This is a
19.2.5 /M command
The /M command or function switch instructs FILDDT to "DDT" the
currently running monitor and physical memory address space (con-
trolled by use of the $U and $$U commands). This is a privileged
19.2.6 /P command
The /P command or function switch instructs FILDDT to enable for writ-
ing as well as reading the specified address space. Note that DDT's
internal fake registers are always writable.
19.2.7 /S command
The /S command or function switch instructs FILDDT to only extract the
symbol table from the file specified, replacing any symbol table
FILDDT may already have. Unless overridden by the inclusion of a /F
command FILDDT will, after having read the symbol table, again prompt
the user for the next FILDDT command.
19.2.8 /U command
The /U command or function switch is applied to a file structure or
disk unit only rather than a complete file specification and indicates
to FILDDT that the user wants the entire physical address space repre-
sented by that file structure or disk unit name independent of any
"file structure mapping" normally imposed by the monitor. This is a
FILDDT on TOPS-20 runs in native mode, and in particular, uses the
PMAP monitor call for all regular file access. FILDDT will also type a
brief message telling what address space is about to be "DDT'ed" be-
fore going into DDT mode.
19.3.1 DRIVE command
The format of the DRIVE command is:
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FILDDT startup and commands
DRIVE (FOR PHYSICAL I/O IS ON CHANNEL) c (CONTROLLER) k (UNIT) u
The DRIVE command allows examination of the disk unit u on system
channel c on channel controller k without regard for whether it is
mounted as part of a file structure, or indeed whether it even has the
necessary information so that it could be so mounted (as if the HOME
blocks were wiped out). If, however, the drive is part of a mounted
file structure, FILDDT will type a message indicating the structure to
which it belongs. This is a privileged command.
19.3.2 ENABLE DATA-FILE command
The ENABLE DATA-FORMAT command instructs FILDDT to treat the file as
pure data, even if a valid .EXE directory is detected, and in particu-
lar to use real file words 0 to 17 as locations 0 to 17.
19.3.3 ENABLE PATCHING command
The ENABLE PATCHING command instructs FILDDT to enable any
subsequently specified address space for patching (writing). This com-
mand is ignored when looking at the running monitor since there is no
monitor call to "poke" the running monitor.
19.3.4 EXIT command
The EXIT command instructs FILDDT to return to command level. If
FILDDT has an internal symbol table (due to a previous LOAD or GET
FILDDT command) then a SAVE command will save FILDDT with the symbols
19.3.5 GET command
The format of the GET command is:
GET (FILE) filespec (optional switches)
The GET command instructs FILDDT to set up the disk file filespec as
the address space to be "DDT'ed", as modified by the optional switches
or previous ENABLE commands. The available switches are:
188.8.131.52 /DATA - The /DATA switch is equivilent to a previous ENABLE
184.108.40.206 /PATCH - The /PATCH switch is equivilent to a previous ENABLE
220.127.116.11 /SYMBOL - The /SYMBOL switch instructs FILDDT to extract sym-
bols from the specified .EXE file before "DDT'ing" the file, discard-
ing any symbols that FILDDT may already have. This switch is legal
only with .EXE files.
19.3.6 HELP command
The HELP command instructs FILDDT to type out a short summary of the
available FILDDT commands.
19.3.7 LOAD command
The format of the LOAD command is:
LOAD (SYMBOLS FROM) filespec
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FILDDT startup and commands
The LOAD command instructs FILDDT to extract symbols from the disk
file filespec, which must be an .EXE file, then to return to FILDDT
command level. This command is legal only for .EXE files.
19.3.8 PEEK command
The PEEK command instructs FILDDT to use the currently running monitor
as the address space to be "DDT'ed". The address space so available is
currently limited to monitor executive virtual addresses 0 to 777777,
since the PEEK monitor call will only accept 18-bit address arguments
for executive virtual addresses. Physical memory addressing is not
available. This is a privileged command.
19.3.9 STRUCTURE command
The format of the STRUCTURE command is:
STRUCTURE (FOR PHYSICAL I/O IS) str:
The STRUCTURE command instructs FILDDT to use as the address space to
be "DDT'ed" the entire disk file structure str independent of any
"file structure mapping" normally imposed by the monitor. This is a
Following is a list of the various defaults supplied by FILDDT:
1. DSK: is the default file device unless super I/O is specified
(which requires an explicit file structure or disk unit name).
2. .EXE is the default file type or extension unless either a data
file or super I/O is specified, in which case there is no default
file type or extension.
3. The default directory is the user's default directory.
4. The specified address space is read-only.
5. If "DDT'ing" an .EXE file and FILDDT does not already have a
symbol table, extract the symbol table (if any) from the .EXE
6. If "DDT'ing" an .EXE file and the symbol CRSHAC (if TOPS-10) or
BUGACS (if TOPS-20) exists, give a "free" CRSHAC$5U or BUGACS$5U
command. If the CRSHAC/BUGACS symbol does not exist then use file
words 0 to 17 (if any) as mapped by the .EXE directory for loca-
tions 0 to 17. For TOPS-20 only, if the symbol SPT exists then
also give a free SPT$6U command as well.
19.5 Other FILDDT-specific commands
Following are the commands which are unique (or different) to FILDDT.
19.5.1 ^E command
The ^E command instructs FILDDT to exit the current address space and
prompt the user for a new address space. The ^E command is equivilent
to a ^Z, START command sequence.
19.5.2 ^Z command
The ^Z command instructs FILDDT to exit to monitor level after having
written out any changes to the current file (if any). It is most
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FILDDT startup and commands
important that the user exit only via ^Z (or ^E which does an implicit
^Z) in order to guarantee the integrity of the file data (if any) - a
^C can leave a file in an indeterminate state (some changes written
out to the disk and some not).
19.5.3 I/O errors
Should FILDDT incur an I/O error reading or writing a disk file, a
warning message will be issued but FILDDT will otherwise ignore the
error. This is to allow the user the ability to manually fix a file
with bad data by rewriting the data correctly (hoping the rewriting
operation clears the error condition - if the physical disk surface
itself is at fault, then it is probably hopeless).
[End of DDT42B User's Guide]