IDA Pro: __GETDS breaks decompiler output

Question

when trying to decompile some interrupt service routines done with Watcom, the __GETDS call at the beginning of the functions will break the decompiler output completely. Here's the disassembly:

And here the decompiler output:

What could be the reason for this? Is there a way to fix this? The only thing that worked for me was creating a separate function for the code below __GETDS, but that's not really satisfying. Thanks in advance!

Answer 1

This happens because __GETDS is placed just after the entrypoint code and is initially detected by IDA as part of the start function:

cseg01:0001AAD1                 push    eax
cseg01:0001AAD2                 mov     eax, 0
cseg01:0001AAD7                 mov     edx, 0Fh
cseg01:0001AADC                 call    __FiniRtns
cseg01:0001AAE1                 pop     eax
cseg01:0001AAE2                 mov     ah, 4Ch ; 'L'
cseg01:0001AAE4                 int     21h             ; DOS - 2+ - QUIT WITH EXIT CODE (EXIT)
cseg01:0001AAE4                                         ; AL = exit code
cseg01:0001AAE6                 mov     eax, eax
cseg01:0001AAE8
cseg01:0001AAE8 __GETDS:                                ; CODE XREF: __int23_handler+A↓p
cseg01:0001AAE8                                         ; __int_ctrl_break_handler+A↓p ...
cseg01:0001AAE8 __GETDSStart_:
cseg01:0001AAE8                 mov     ds, cs:word_1AAF1
cseg01:0001AAF0                 retn
cseg01:0001AAF0 start           endp

Since the call is to a middle of an existing function, IDA considers it to be non-returning and stops the code flow. The solution is to break start after the int 21h call (Set function end, or 'E' key) and create a proper function for __GETDS itself. Then you will need to reanalyze all call sites (and maybe recreate the functions) so that code flow is properly updated.