Well, DOS transfers to segment:100h after loading the binary and setting up it's data/stack segments (although for .COM executables only. .EXE have it in their file headers, which are dynamically allocated).

In UniPCemu, you can specify a breakpoint with just an offset, making it ignore the segment part of the breakpoint.
Just add O to the breakpoint segment:offset.

Just use the command "debug prog.exe"?

superfury wrote on 2023-02-19, 09:53:

Well, DOS transfers to segment:100h after loading the binary and setting up it's data/stack segments (although for .COM executables only. .EXE have it in their file headers, which are dynamically allocated).

In UniPCemu, you can specify a breakpoint with just an offset, making it ignore the segment part of the breakpoint.
Just add O to the breakpoint segment:offset.

That's an elegant solution for COM files. Don't know why I didn't think of it. Shame it won't work for EXE files, but at least that will help me debug MUSHROOM.COM.

its not clear what you are trying to reach?

debugging of DOS exe or com programs can be done using:
-(very archaic) using the debug command from a real MS DOS: https://en.wikipedia.org/wiki/Debug_(command)
-using the dosbox internal debugger (much better then the real DOS debugger): Guide to the DOSBox debugger
-Turbo Debugger from Borland or WD Debugger from Open Watcom V2: https://en.wikipedia.org/wiki/Borland_Turbo_Debugger, https://github.com/Baron-von-Riedesel/HX-OpenWatcom-Debugger
-serveral other DOS-able debuggers - but i normaly only using the dosbox debugger for my reversing projects

maybe you don't know that there are so many working debugger solutions for DOS or i didn't understand you questions

but for understanding a (maybe small) COM program like MUSHROOM.COM the best option is a disassembler like IDA Pro or Ghidra, im always using my licensed version of IDA for reversing and re-assembling of DOS programs

Official IDA Pro 5 Freeware: https://www.scummvm.org/news/20180331/ - the only Freeware Version that supports DOS EXE/COM

but at least that will help me debug MUSHROOM.COM.

the song playing thing?

llm wrote on 2023-03-03, 07:04:

its not clear what you are trying to reach?
...
maybe you don't know that there are so many working debugger solutions for DOS or i didn't understand you questions

I'm writing my own emulator. I would like to be able to drop into my emulator's native debugger when a program starts. I am aware I can debug inside the emulated machine, but that's not exactly what I'm looking for.

Write your own .com that calls int21.4b with al=1 to load the program to be debugged, then jump to the new ip with the trap flag set (or some other method to notify your emulator to break).

jmarsh wrote on 2023-03-05, 00:02:

Write your own .com that calls int21.4b with al=1 to load the program to be debugged, then jump to the new ip with the trap flag set (or some other method to notify your emulator to break).

jmarsh idea is the best i think - you can't activate a debugger by just hooking some interrupts because you're then too early in the process

but some sort of "preapre for debug" program that that loads (and not directly executes - as jmars explains) with int 21h/4b/al=1 combined with some control-handover to your emulation (using interrupts, ports, whatever to let your emulation know that it should start the internal debugger) is i think the only good or always working solution

jmarsh wrote on 2023-03-05, 00:02:

Write your own .com that calls int21.4b with al=1 to load the program to be debugged, then jump to the new ip with the trap flag set (or some other method to notify your emulator to break).

Thanks, I went with this method and it works pretty well. Now I'm wondering if I couldn't intercept calls to 4bh and actually change the parameter of AH to 1, then intercept the write to the parameter block.

1execute:
2    mov   ax, ds
3    mov   word [pb_cmdline_seg], ax     ; Fill out data segment for command line param
4    mov   ax, word [cmdline]
5    mov   word [pb_cmdline_offset], ax  ; Fill out offset for command line param
6    mov   ah, 4bh                       ; EXEC/Load and Execute Program
7    mov   al, 01h                       ; Create program segment prefix but don't execute
8    mov   dx, fn_buf                    ; Point to filename
9    mov   bx, pb_env_seg                ; ES:BX to parameter block
10    clc
11    int   21h                           ; Execute    
12    jnc   success
13    jmp   fail
14
15success:
16    mov   ah, 09h
17    mov   dx, execute_ok
18    int   21h
19    mov   ah, 01h               ; Emulator service 01h = debug program
20    mov   bx, [pb_cs]
21    mov   cx, [pb_ip]
22    int   0fch                  ; Emulator service interrupt
23    jmp   exit

Appreciate everyone's suggestions!