Where can I find the ABI for gcc-ia16 described?

[ecm-pushbx]

I only found the "Submodel Options" → "IA-16 Options" in the gcc.info file and that links to "Function Attributes" → "IA-16 Function Attributes". This lists the cdecl, stdcall, and regparmcall attributes, but it does not fully describe the ABI.

I want to convert some old programs to using NASM-assembled subfunctions instead of inline assembly. For that I want to know the exact ABI. Here's an example of what I am currently playing with:

$ cat test.asm

%include "lmacros3.mac"

    numdef FARCALL, 0
%if _FARCALL
 %define frametype far
%else
 %define frametype near
%endif

bits 16

global testfunction
testfunction:
    lframe frametype
    lframe_needonlyregistered
    lenter
    mov ax, 12345
    lleave
    lret
$ cat test3.c

#include <stdio.h>

extern unsigned int testfunction(void);

int main(int argc, char** argv) {
  int volatile a = testfunction();
  printf("Hello world! a=%04Xh a=%u\n", a, a);
  return 0;
}
$ model="-mcmodel=medium"; farcall="-D_FARCALL"; nasm -I ../lmacros/ test.asm $farcall -f elf -o test.o && ia16-elf-gcc test3.c test.o $model -Os -o test3.exe && dosemu -K "$PWD" -dumb -quiet -E "c:ldebugu test3.exe"
About to Execute : c:ldebugu test3.exe
-g
Hello world! a=3039h a=12345

Program terminated normally (0000)
-q
$ 

[tkchia]

Hello @ecm-pushbx,

The default calling convention, if you do not specify any options, is cdecl --- sorry I did not make that clear.

However, if you use a -mrtd or -mregparmcall option during both compilation and linking, you can effectively switch your whole program to a different calling convention. I have actually arranged to build different versions of the C library and libgcc for the different calling conventions (cdecl, stdcall, and regparmcall).

Thank you!

[ecm-pushbx]

My question is which registers are call-preserved and which are call-clobbered. This does not seem to be documented anywhere.

[tkchia]

Hello @ecm-pushbx,

OK... yes, unfortunately gcc-ia16 does not yet have a spiffy ABI document. But in brief:

• %ax, %cx, %dx, and %bx are call-clobbered. Even for the regparmcall calling convention.
• %ds is a bit special --- it is assumed to be the same as %ss, and must be restored on exit. A function can mess with %ds without really saving it, but if so, it should do a pushw %ss; popw %ds just before returning.
• All other registers are callee-saved --- %bp, %si, %di, and (!) %es.

I will try to find a place in the info file to stash this information.

Thank you!

[ecm-pushbx]

Ok, thanks! I suggest you should close this issue once you have things documented somewhere.

[tkchia]

Thanks @ecm-pushbx!