Date:	Fri, 10 Mar 1995 12:20:46
From:	jbarrett AT dallas DOT geco-prakla DOT slb DOT com (John Barrett)
To:	djgpp AT sun DOT soe DOT clarkson DOT edu
Subject:	system()/interrupt (phase 2)

    The original problem: To somehow make a protected mode driver callable
via software interrupts from a real mode program...

    1st attempt: Set up interrupt traps and used system() to execute real
mode application (command.com, then a short test program)... found out that
system() swaps the protected mode application out of memory.

    As per suggestions from DJ... I used stubedit to reserve memory for my
program (1024k virtual, 640k Conventional), also found right way to set up a
real mode interrupt (0x66) that calls a protected mode handler
(_go32_dpmi_alloc_real_mode_callback_iret)

    "TSR" compiles, runs, and exits gracefully, so long as no software
interrupts occur... my test application does a "geninterrupt(0x66)" POOF
the system hangs, requiring a hard reset

    I also tried writing my own version of the system() function, which
works using Int21 Func4B... this also worked until I tested the interrupt

    So the question is... Am I still getting swapped out??? or is there a
glitch in my interrupt handler setup.... or should I get a copy of V2 before
proceeding ?????  Source to Interrupt Handler follows:

   
#include <stdio.h>
#include <pc.h>
#include <sys/types.h>
#include <dpmi.h>
#include <conio.h>
#include <dos.h>

volatile int                            new_tc;

_go32_dpmi_registers            regs;

union REGS                                      inp,out;
struct SREGS                            sregs;

void handler(_go32_dpmi_registers *regs)
{
        new_tc++;
}

typedef struct
{
        char    fname[32];
        short   envaddr;
        long    cmdline;
        long    fcb1ptr;
        long    fcb2ptr;
} execdata;

int main()
{
        _go32_dpmi_seginfo      old_handler, new_handler, info;

        execdata                                        edat;

        printf("grabbing interrupt\n");
        _go32_dpmi_get_real_mode_interrupt_vector(0x66, &old_handler);
  
        new_handler.pm_offset = (int)handler;
        _go32_dpmi_allocate_real_mode_callback_iret(&new_handler,&regs);
        _go32_dpmi_set_real_mode_interrupt_vector(0x66, &new_handler);

        printf("Trapping interrupts, type EXIT to stop.\n");

        strcpy(edat.fname,"command");
        edat.envaddr = 0;
        edat.cmdline = 0;
        edat.fcb1ptr = 0;
        edat.fcb2ptr = 0;

        info.size = 4;
        _go32_dpmi_allocate_dos_memory(&info);

        _dos_memput(&edat,64,&info);

        inp.x.ax = 0x4300;
        inp.x.bx = 32;
        inp.x.dx = 0;
        sregs.ds = info.rm_segment;
        sregs.es = info.rm_segment;

        intdosx(&inp,&out,&sregs);

        _go32_dpmi_free_dos_memory(&info);

        printf("releasing interrupt\n");

        _go32_dpmi_set_real_mode_interrupt_vector(0x66, &old_handler);
        _go32_dpmi_free_real_mode_callback(&new_handler);

        printf("Test count = %i\n",new_tc);

        return 0;
}

Any suggestions???

    John Barrett
    jbarrett AT dallas DOT sgp DOT slb DOT com

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