From: ams AT ludd DOT luth DOT se (Martin Str|mberg)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Some assembly questions
Date: 18 Mar 1999 17:42:24 GMT
Organization: University of Lulea, Sweden
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References: <7cm980$f69$1 AT news DOT luth DOT se> <+n+tMKAodQ82Ewk+@xemu.demon.co.uk>
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Dave Bird (dave AT xemu DOT demon DOT co DOT uk) wrote:
: In article <7cm980$f69$1 AT news DOT luth DOT se>,
:  Martin Str|mberg <ams AT ludd DOT luth DOT se> writes:
: >How do I inhibit the gcc generated pre- and postlude to c functions?
: 
:  I think you are very confused and this will not achieve
:  what you are trying to do.

Confused? Yes. Very? I don't think so.

[Klippa, klapp, kluppit nice description of c -> assembly mapping.]

:  Basically I think you have started out all wrong,
:  and should do it like this instead:
: 
:  int eax=0;
: 
:  void some_c_stuff(void){
:     ........
:  };
: 
:  asm(
:      _my_interrupt_handler:
:         pushal;
:         mov    %eax, _eax;
:         call   _some_c_stuff;
:         popal;
:         reti;
:         :/*none*/:/*none*/:/*none*/
:  );

Yes, I think I understand now. The problem is depending on contents in
%eax, I have to either adjust registers or call the old interrupt
handler.

I have switched to a pure assembly handler now.

Trying things out I surely generated a lot GPFs. So I simplyfied my
code to the bare minimum, but it still won't work.

Perhaps somebody can tell me what I'm doing wrong in the following
code. It just chains the previous handler. When I get this to work
I'll not always jump to the previous one.

What happens for me is it says "About to insert me...", I press
<space> or <Enter>, then no matter how long I wait (at least 2
minutes) nothing more happens. I try hitting <space> or some other
keys: nothing happens. When I get bored, I hit C-C, and is rewarded
with "This program has performed an illegal instruction..." 
"Fault location 0028:C0001FE7
Faulting component: VMM(01) + 00000FE7
Interrupts in service: 1"


Shostakovich, String Quartet No. 15,

							MartinS

----- Code starts. -----
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <dpmi.h>
#include <go32.h>
#include <pc.h>
#include <keys.h>

#define MAX_BUF (16)

extern void handler(void);
extern void handler_end(void);

_go32_dpmi_seginfo old_isr, new_isr;

asm("
.text
.p2align 2

.globl _handler
_handler:

/* The two following lines (pushw %cs; popw %ds) doesn't help; same result.
pushw %cs
popw %ds*/
jmp *_old_isr

_handler_end:
nop

");


int main(int argc, char *argv[])
{

  printf("About to insert me...\n");
  getkey();

  _go32_dpmi_lock_code(&handler, &handler_end - &handler);

  _go32_dpmi_lock_data(&old_isr, sizeof(old_isr));


  _go32_dpmi_get_protected_mode_interrupt_vector(0x31, &old_isr);

  new_isr.pm_offset = (int)&handler;
  new_isr.pm_selector = _go32_my_cs();

  _go32_dpmi_set_protected_mode_interrupt_vector(0x31, &new_isr);

  /*  system("bash"); */
  sleep(5);

  printf("About to remove...\n");

  _go32_dpmi_set_protected_mode_interrupt_vector(0x31, &old_isr);


  return 0;

}
----- Code ends. -----