What does the ?DEBUG directive do in TASM?

[noseherbalindo]

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Product Feedback

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Binary analysis has revealed that Turbo Assembler 5.3 (TASM) supports the undocumented ?DEBUG directive. How can I use this directive to add extra records to the OMF .obj output file?

The ?DEBUG directive is undocumented, none of the Turbo Assembler manuals mention it, and a web search didn't give me any useful results.

Here is a quick proof that the directive exists:

$ strings tasm32.exe | grep DEBUG.
?DEBUG illegal when /ZI or /ZD selected
My test source file t.asm:

.model tiny
.code
public myproc
myproc proc a:word,b:word
db 'HELLO'
?debug this_is_my_debug_message
?debug 12345
db 'WORLD'
myproc endp
end
When compiling in debug mode (using the /zi flag), it triggers the error message:

$ wine tasm32.exe /ml /m2 /zi t
...
Fatal t.ASM(6) ?DEBUG illegal when /ZI or /ZD selected
...
However, when compiling in non-debug mode (using the /zn flag), the presence of the ?DEBUG directives above doesn't make any difference in the output OMF object file t.obj.

[nerufuyo]

13

The ?DEBUG directive is used to communicate debugging information from higher-level languages; you can see it used when Turbo C & co. are asked to generate assembly language instead of binaries. This explains why it’s incompatible with /zd and /zi: if you’re processing a file with ?DEBUG directives, you want that debug information to end up in the object file, not the debug information produced by the assembler (pointing at the file it was given, which is presumably an intermediate form). So you can’t use ?DEBUG to add debug information to that produced by the assembler itself.

The directive is followed by a single letter, and then any further arguments. From what I can tell so far, the letters are used as follows:

V indicates some kind of version information
S points to a source file (with D and T variants)
L gives a line number (in a source file referred to by S)
B and E seem to indicate the beginning and end of a function
I also see C followed by hex digits but I’m not sure what they’re used for; the encoded bytes appear as-is in the object file. These can be wrapped over several lines, with a + at the end of continued lines.

Here are some actual examples showing the expected format:

    ?debug  V 301h
    ?debug  S "../cpucheck.c"
    ?debug  C E8010D2E2E2F637075636865636B2E63E6894821
    ?debug  L 108
    ?debug  B
    ?debug  C E604737461740602F8FF015B0100046374726C06+
    ?debug  C 02FCFF015B0100

The directive itself is defined as “do nothing” macro if ??version doesn’t exist, presumably so it ends up ignored by (macro) assemblers that don’t support it:

    ifndef  ??version

?debug macro
endm

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