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DDT41 - DDT %41(260) Users Guide to New Features Page 1
Copyright (C) 1978, 1979
Digital Equipment Corporation, Maynard, Massachusetts, U.S.A.
This software is furnished under a license and may be used and copyed
only in accordance with the terms of such license and with the inclu-
sion of the above copyright notice. This software or any other copies
thereof may not be provided or otherwise made available to any other
person. No title to and ownership of the software is hereby trans-
The information in this software is subject to change without notice
and should not be construed as a commitment by Digital Equipment Cor-
Digital assumes no responsibility for the use or reliability of its
software on equipment which is not supplied by Digital.
DDT41 - DDT %41(260) Users Guide to New Features Page 2
Introduction and Overview
1.0 Introduction and Overview
This document is designed as a users guide to DDT version 41 in so far
as it has changed from previous versions of DDT. It is not a complete
users guide to all the wonders of DDT, just those new features which
have recently been implemented (although directed primarily at new
features only in DDT version 41, 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 otherwise
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 is the first revision of this document, incorporating the addi-
tional changes to DDT version 41 as of edit 260.
DDT version 41 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 41 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.
This requires version 4 of the PA1050 "compatibility pack-
3.0 Memory and Address Control
The single biggest change to DDT version 41 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
DDT41 - DDT %41(260) Users 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
breakpoints. 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.
3.2 Effective address calculation
DDT version 41 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-open-
ing command ("/", "\", <TAB>, etc.) a single <ESC> delimiting the
address expression (e.g., "MOVE 3,@200(10)$/" or just "$[") instructs
DDT to treat the expression as an IFIW word and calculate the effec-
tive 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
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 "$$[" then DDT will calulate the
effective address as the contents of location 4321 in section 0
indexed by the right half of register 7, and if bit 13 is on, treating
DDT41 - DDT %41(260) Users Guide to New Features Page 4
Memory and Address Control
that word as an IFIW and continuing the address calculation. This,
although probably not what was expected, is in fact exactly what the
hardware would do since the indirect word came from section 0. Had the
user opened location 1,,1234 (containing 7,,4321) then DDT would take
the contents of location 7004321 and continue from there.
If no <ESC>'s delimit the address expression, then DDT simply uses the
full 36-bit expression as the address (e.g., "30,,30/" says open loca-
tion 30000030 and "-1/" says open location 777777777777). Again, only
FILDDT can actually reference an address greater than 30-bits wide
(not that anyone has that much disk space, but the hardware will not
permit an address space over 30-bits wide), and in any case the ad-
dress must be a non-negative 36-bit integer.
There is a special case in which DDT does something "kinky" - if a
space was typed in entering the address expression, or if no explicit
address was typed (i.e., the user is using the "last word typed" by
simply typing only (for example) <TAB>), DDT will form the 36-bit
actual address by using only the right half of the 36-bit address
expression plus the left half of "." as the section number. This not-
at-all-obvious behavior is so that the user can type in expressions
such as "JRST PAT<TAB>" and have DDT go to location PAT in the same
section as the JRST PAT instruction rather than going to address
254000000000+(PAT modulo 2**18). Another common usage of this "fea-
ture" would be in chaining down linked lists where the link pointer is
an 18-bit section-local address in the left half of a word. To do this
the user may type "sp$$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
the user is trying to deposit into if it doesn't already exist (de-
fault for TOPS-20); the $$1W command instructs DDT to simply give an
error indication if the user attempts to write into a non-existant
page (default for TOPS-10). EDDT and FILDDT doing super I/O or
"DDT'ing" an .EXE file will NEVER attempt to create a non-existant
page. For FILDDT the user must specify patching the file when he
starts FILDDT in order to be able to create new pages (e.g., extend
the file or fill in a gap in the middle of the file (TOPS-20 only)).
DDT41 - DDT %41(260) Users Guide to New Features Page 5
Memory and Address Control
3.4 Page mapping and physical addressing
In DDT version 41 all flavors of DDT support page mapping and address
relocation as well as register and physical address manipulation. All
of these functions use some variation of the $U/$$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 is totally different (and main-
ly 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 $$U command instructs
DDT to manually track down the honest physical address rather than the
virtual address space in which DDT finds itself running. Physical ad-
dressing is really applicable only to EDDT or to FILDDT looking at
running monitor/memory (TOPS-10 only). User mode DDT (including EDDT
running in user mode, MDDT (TOPS-20 only), and VMDDT (TOPS-10 only))
and FILDDT looking at a disk all treat $U and $$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
directory 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-
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.
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Memory and Address Control
3.4.2 Page mapping
All flavors of DDT now support 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 fla-
vors 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$11U
command is issued; in either case if flg is zero then the paging
emulation is disabled, if flg is non-zero then the appropriate paging
emulation is enabled.
In executive mode EDDT or FILDDT looking at running monitor/memory
space DDT will internally utilize physical addressing in order to
provide the user the true mapped virtual address space desired.
18.104.22.168 KI-paging - For KI-paging (TOPS-10 default) the page mapping
command for the executive virtual addressing space is [upt<]ept$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 $U re-
turns DDT to regular unmapped virtual addressing.
22.214.171.124 KL-paging - For KL-paging (TOPS-20 default) the page mapping
command for the executive virtual addressing space is ept$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
command is either sec$2U where sec is the physical memory page number
of a KL-paging section map, or sex$$2U where sex is the index into the
SPT of the section map.
Basically, under KL-paging, $0U selects the ept, $1U selects the upt,
and $2U selects a single section. A single $ indicates the physical
memory page number and two $'s indicate an SPT index.
3.4.3 Setting the SPT
FILDDT will automatically define the start of the SPT from a disk file
(assumed monitor dump) from the symbol "SPT" if it exists (TOPS-20
only). The command spt$6U specifies to DDT that the SPT starts at
3.4.4 Register addressing
The command acs$5U instructs DDT to use the 20 consecutive locations
starting at acs as the registers (DDT maintains an internal copy of
the registers so changing "register" 3 will not affect (for example)
acs+3). FILDDT, when reading an .EXE file, will automatically load its
internal "fake" registers as though the user had typed CRSHAC$5U if
DDT41 - DDT %41(260) Users Guide to New Features Page 7
Memory and Address Control
TOPS-10 or BUGACS$5U if TOPS-20. Note that if physical addressing mode
has been entered (the user has issued the $$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 "regis-
In executive mode DDT 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
pointers as offsets rather than pointers as absolute values DDT ver-
sion 41 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) exactly analguous to the KA-10 hardware relocation and
protection strategy, 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 be-
ing performed before any mapping is done. The protection address has
no effect on the final "physical" address generated by any mapping
currently in effect.
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
DDT41 - DDT %41(260) Users Guide to New Features Page 8
Memory and Address Control
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 version 41, 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 sys-
tem if the user types in an illegal (non-existent or unmapped)
address, but rather will simply type its ubiquitous ?<DINK><TAB> error
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$X at the
users whim but must decide on one form or the other (the default
obviously being to do nothing - i.e., to settle for the DEBUG$X form)
5.0 Symbolic expression typein and typeout
DDT version 41 has expanded the range of both symbolic typein and
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.
DDT41 - DDT %41(260) Users Guide to New Features Page 9
Symbolic expression 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 cur-
rent 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
2. Regular (non-I/O) OP field (e.g., KS-10 UMOVE)
6.0 ASCII typeout
DDT version 41 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).
DDT41 - DDT %41(260) Users Guide to New Features Page 10
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 $Y by itself then DDT will
prompt with "File: ". After the prompt the user can enter a TOPS-10
file specification in the form dev:name.type[directory]/switches where
[directory] can of course contain SFD's.
7.1.1 /A switch
The /A switch instructs DDT to abort the command file if a DDT-detect-
ed command error occurs (such as reference to an undefined symbol).
7.2 Command output (logging)
When reading a command file ($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$< where exp is any 36-bit expression represent-
ing the address of the resultant patch. If the later form of the patch
command is used no symbol will be updated to the end of the patch.
8.2 Default patching symbol
The list and order of default patching symbols which DDT uses when the
user does not supply an explicit patching symbol is now:
1. PAT (TOPS-10 EDDT only)
2. FFF (TOPS-20 EDDT and MDDT only)
3. PAT.. (all flavors)
4. PATCH (all flavors)
5. PAT (all flavors except TOPS-10 EDDT)
8.3 Default patching address
If the user does not supply an explicit patching symbol and DDT is
unable to find one of the default patching symbols then the address
specified by the right half of location .JBFF (even on TOPS-20) is
used. On patch close ($> command) if the patching address was default-
ed to via .JBFF, then both the right half of location .JBFF and the
DDT41 - DDT %41(260) Users Guide to New Features Page 11
Automatic patch insertion
left half of location .JBSA are updated.
8.4 Patch closing confusion and restriction
With DDT version 41 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$> (where "cr"
indicates a carriage return) is ok.
The breakpoint logic in DDT version 41 has been extensively revamped
in order to support extended addressing. The default number of break-
points is now 12 (decimal); and can be set (by defining the symbol
NBP=number of breakpoints) arbitrarily high (within memory space limi-
tations) rather than being limited to 9 or 36 (decimal) depending on
which code restriction one choose to believe.
9.1 Setting breakpoints
DDT can now set a breakpoint in code running in any section with two
1. If DDT is currently running in section 0 then breakpoints can
only be set in section 0 (see section 3.1.3 above).
2. DDT must be mapped in the section containing the code in which
breakpoints are to be placed (the logic of this is that since
there is no way for DDT to cause unconditional transfer of con-
trol to DDT with only 36 bits some portion of the section address
space must be devoted to DDT; therefore, given this restriction,
one might just as well put all of DDT in that section since it
makes for a cleaner and simpler implementation). Note that this
does not mean DDT must be running in that section, but merely
that DDT must be mapped in that section!
It does not matter into how many different sections the same code is
mapped as long as DDT is mapped into the same sections since DDT is
"section-independent". For example (taking the TOPS-20 monitor which
maps section 0 and section 1 identically) if a breakpoint is set at
address 1004567 (or 1,,4567) but the PC was 4567 (or 0,,4567, i.e., in
section 0 rather than section 1) when the breakpoint was executed DDT
does not care (as long as DDT is mapped in that section, which in the
example of the TOPS-20 monitor it is).
The syntax for setting a breakpoint is now opn<bpt$nB where n is
optional and, if specified, declares the breakpoint number to be as-
signed to that address; bpt is the 36-bit address at which to place a
DDT41 - DDT %41(260) Users Guide to New Features Page 12
breakpoint; and opn is an optional 36-bit address to open and display
upon execution of the breakpoint. The syntax was changed because two
full 30-bit addresses could not be squeezed into two halfwords.
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)
breakpoint is already set, the old breakpoint is cleared first.
9.2 Breakpoint typeout
Upon execution of a breakpoint DDT will now always typeout 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 it-
self becoming the previous PC sequence and so available via the $<CR>
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 version 41 has a new breakpoint facility - the ability to handle
unsolicited breakpoints (i.e., breakpoints that DDT did not itself
set). If control 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 contained 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 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.
DDT41 - DDT %41(260) Users Guide to New Features Page 13
Single-stepping the program
10.0 Single-stepping the program
The $X DDT command has been significantly modernized (and sped up in
general) with version 41 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
contents 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 (execu-
tive mode only) are not traced, only the contents of the register
specified in the AC field are displayed. Since the UNIBUS device reg-
isters 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
standard format (at least as far as DDT is concerned) the effective
address itself is not even displayed.
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 assem-
bly parameter SKPMAX (by default 3). If the PC changes more drastical-
ly than that (e.g., goes to a smaller address) DDT will type "<JUMP>
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-
DDT41 - DDT %41(260) Users Guide to New Features Page 14
Single-stepping the program
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
instruction does take the error jump then the ERCAL or ERJMP instruc-
tion 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 instruc-
tion itself caused the skip (as in a SKIPA) or if the PC merely "fell
through" the ERCAL or ERJMP instruction (as in a successful MOVE).
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
secondary 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
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 $X'ing from instr$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.
DDT41 - DDT %41(260) Users Guide to New Features Page 15
Single-stepping the program
10.11 $$X status
DDT will now respond to a ? character being typed during an $$X se-
quence by typing "Executing: " followed by the current user "pc" and
instruction being executed. Typing any other character terminates the
10.12 $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 version 41 handles searches are
simply bug fixes, not major changes in the logic of searching.
11.1 Non-existant pages
DDT version 41 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 Missed matches
The bug which caused TOPS-20 DDT to miss many valid matches is fixed
in DDT version 41.
11.3 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 version 41 will assume that each word it
examines is an instruction and perform an IFIW effective address cal-
culation. The final result must match in all 30 bits (actually
internally DDT will do a full 36-bit compare so the address being
searched for had better not contain anything in bits 0 to 5).
11.4 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
DDT41 - DDT %41(260) Users Guide to New Features Page 16
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.
11.5 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.
11.6 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-
DDT version 41 allows the user to "watch" a location, waiting for it
to change. Although primarily useful for FILDDT'ing the running moni-
tor, 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 dis-
played 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
DDT41 - DDT %41(260) Users Guide to New Features Page 17
typing "Watching: " followed by the current location and contents
being watched. Typing any other character terminates the $V immediate-
13.0 Zeroing memory
The algorithm used by DDT previous to version 41 has only limited
usefulness in today's modern virtual world (especially on TOPS-20).
However, to avoid "breaking" already extant control or MIC files which
may use the $$Z command it remains unchanged. A new command has been
implemented - lwr<upr>exp$z where lwr is the lowest (starting) ad-
dress, 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.
A special note: The creation of zeroed pages (which formerly were non-
existent) by the $Z and $$Z commands is under the control of the
automatic page create flag (i.e, the $1W and $$1W commands - see sec-
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 41 (it actually started with DDT version 40) has several
new "masks" (for lack of a better name and/or command) of interest to
the user. None of these masks are currently displayable (e.g., "$3M/")
in FILDDT although they may be set normally.
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.
14.2 $1M - TTY control mask
This mask controls special TTY behavior (primarily TOPS-10 and exec
14.2.1 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>.
14.2.2 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
DDT41 - DDT %41(260) Users Guide to New Features Page 18
manually set tab simulation in exec mode EDDT.
14.2.3 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 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).
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
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
DDT41 - DDT %41(260) Users Guide to New Features Page 19
17.0 Obsolete commands
The executive mode paper tape facilities (^R, $J, and $L DDT commands)
are no longer supported. The code is left in the source file for
reference purposes but will soon be removed.
18.0 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 41 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.
18.2.1 /D command
The /D command or function switch instructs FILDDT that the file
specified 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.
18.2.2 /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-
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
18.2.3 /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
DDT41 - DDT %41(260) Users Guide to New Features Page 20
FILDDT startup and commands
18.2.4 /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
18.2.5 /M command
The /M command or function switch instructs FILDDT to "DDT" the cur-
rently running monitor and physical memory address space (controlled
by use of the $U and $$U commands). This is a privileged command.
18.2.6 /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.
18.2.7 /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.
18.2.8 /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
With DDT version 41, 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" before going into DDT mode.
18.3.1 DRIVE command
The format of the DRIVE command is:
DRIVE (FOR PHYSICAL I/O IS ON CHANNEL) c (UNIT) u
The DRIVE command allows examination of the disk unit u on system
channel c 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.
DDT41 - DDT %41(260) Users Guide to New Features Page 21
FILDDT startup and commands
18.3.2 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.
18.3.3 ENABLE PATCHING command
The ENABLE PATCHING command instructs FILDDT to enable any subsequent-
ly specified address space for patching (writing). This command is
ignored when looking at the running monitor since there is no monitor
call to "poke" the running monitor.
18.3.4 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
18.3.5 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:
126.96.36.199 /DATA - The /DATA switch is equivilent to a previous ENABLE
188.8.131.52 /PATCH - The /PATCH switch is equivilent to a previous ENABLE
184.108.40.206 /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.
18.3.6 HELP command
The HELP command instructs FILDDT to type out a short summary of the
available FILDDT commands.
18.3.7 LOAD command
The format of the LOAD command is:
LOAD (SYMBOLS FROM) filespec
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.
18.3.8 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
DDT41 - DDT %41(260) Users Guide to New Features Page 22
FILDDT startup and commands
available. This is a privileged command.
18.3.9 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.
18.5 Other FILDDT-specific commands
Following are the commands which are unique (or different) to FILDDT.
18.5.1 ^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.
18.5.2 ^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
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).
18.5.3 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
DDT41 - DDT %41(260) Users Guide to New Features Page 23
FILDDT startup and commands
operation clears the error condition - if the physical disk surface
itself is at fault, then it is probably hopeless).
[End of DDT41 Users Guide]