Question: can a __interrupt routine set ax or al register on return

[LowLevelMahn]

for a reverse engineering project i need to implement two interrupt routines that return values through ax and al register
is that possible with __interrupt or do i need to use inline-asm/#pragma aux? INTPACK seems only useable for input

im not able to change the code that calls the interrupt - its still the original code i want to reverse

uint16_t feature_flags = ...;
uint8_t dos_version = ...;

void __interrupt __far interrupt_97h()
{
  //ax = feature_flags;
}

void __interrupt __far interrupt_24h()
{
  //al = dos_version;
}

[jmalak]

OW uses two forms of interrupt routines. First one which you use just now and second form which enable manipulate with entry/return registers. It is form with parameter union INTPACK.
Below is sample code with second form.

#include <i86.h>

void __interrupt my_interrupt( union INTPACK r )
{
    int x;
    char y;

    x = r.w.ax;
    y = r.h.bh;
    ...
    r.h.al = y + 1;
}

[LowLevelMahn]

_dos_setvect does not work with the INPACK parameter - can't find something that works in the reference

[jmalak]

what _dos_setvect has with interrupt routines and its argument?
_dos_setvect only changes cpu interrupt table nothing more it has parameter address of interrupt routine.

[LowLevelMahn]

what _dos_setvect has with interrupt routines and its argument?

_dos_setvect wants a __interrupt __far (*handler)()

but my interrupt_24h has one parameter - should i just cast?

void __interrupt __far interrupt_24h(INTPACK regs_)
{
  regs_.h.al = dos_version;
}

_dos_setvect(0x24, interrupt_24h); // fails
_dos_setvect(0x24, (__interrupt __far (*handler)())interrupt_24h); // just cast?

or does that not work
i have no idea how the Watcom Runtime handles the INTPACK parameter when
the vector gets called from outside the program (there is spawned TSR com program and game-code exe that calling the interrupt_24h isr)

[jmalak]

You must use correct type union INTPACK not undefined type INTPACK.

below is fully compilable sample, no errors

#include <dos.h>

void __interrupt __far interrupt_24h(union INTPACK regs_)
{
  char dos_version = 5;

  regs_.h.al = dos_version;
}

int main( void )
{
    _dos_setvect(0x24, interrupt_24h); // fails
//    _dos_setvect(0x24, (__interrupt __far (*handler)())interrupt_24h); // just cast?
    return( 0 );
}

and disassembled code

.387
                PUBLIC  interrupt_24h_
                PUBLIC  main_
                EXTRN   _dos_setvect_:BYTE
                EXTRN   _small_code_:BYTE
                EXTRN   _cstart_:BYTE
DGROUP          GROUP   CONST,CONST2,_DATA
_TEXT           SEGMENT BYTE PUBLIC USE16 'CODE'
                ASSUME CS:_TEXT, DS:DGROUP, SS:DGROUP
interrupt_24h_:
    push        ax 
    push        cx 
    push        dx 
    push        bx 
    push        sp 
    push        bp 
    push        si 
    push        di 
    push        ds 
    push        es 
    push        ax 
    push        ax 
    mov         bp,sp 
    cld         
    mov         ax,DGROUP:CONST 
    mov         ds,ax 
    mov         byte ptr 16H[bp],5 
    pop         ax 
    pop         ax 
    pop         es 
    pop         ds 
    pop         di 
    pop         si 
    pop         bp 
    pop         bx 
    pop         bx 
    pop         dx 
    pop         cx 
    pop         ax 
    iret        
main_:
    push        bx 
    push        cx 
    mov         bx,offset interrupt_24h_ 
    mov         cx,seg interrupt_24h_ 
    mov         ax,24H 
    call        near ptr _dos_setvect_ 
    xor         ax,ax 
    pop         cx 
    pop         bx 
    ret         
_TEXT           ENDS
CONST           SEGMENT WORD PUBLIC USE16 'DATA'
CONST           ENDS
CONST2          SEGMENT WORD PUBLIC USE16 'DATA'
CONST2          ENDS
_DATA           SEGMENT WORD PUBLIC USE16 'DATA'
_DATA           ENDS
                END

[LowLevelMahn]

sorry i forgot to write that im using "wcl.exe -bt=dos starter.cpp" to build so its the C, C++ difference here

your sample build with wcl.exe -bt=dos test.cpp

test.cpp(12): Error! E473: col(17) function argument(s) do not match those in prototype
test.cpp(12): Note! N392: col(17) definition: 'void __watcall _dos_setvect( unsigned, void __interrupt (__far * )( void ))'
test.cpp(12): Note! N643: col(17) cannot convert argument 2 defined in: test.cpp(12) (col 24)
test.cpp(12): Note! N630: col(17) source conversion type is 'void __interrupt __declspec(farss) (__far )( INTPACK ) (lvalue)'
test.cpp(12): Note! N631: col(17) target conversion type is 'void __interrupt (__far * )( void )'

[jmalak]

it fixes simple cast
_dos_setvect(0x24, (void __interrupt (__far *)( void ))interrupt_24h); // fails