Dos Interrupt Handler Locks Up Program

Question

I'm trying to make a simple game engine/library for creating my own DOS games. I'm currently working on getting input working correctly. The problem I'm having is that my custom ISR that handles int 9 (keyboard input) locks up my program.

NOTE: I got it working by using my compiler's _dos_getvect() and _dos_setvect(); however, I don't know why my own custom getInterruptVector() and setInterruptVector() do not work. I would be most appreciative if somebody could explain why.

Look at "interrupt_hooker.c" for the code in question.

main.c

#include "input.h"
#include "stdio.h"

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

    unsigned char i = 0;

    while(1)
    {
        printf("%i", 1);
        //i = getKey(SCAN_ESC);
        //printf("%i", i);
    }

    unhookKeyboardISR();
    return 0;
}

input.h

#ifndef INPUT_H_INCLUDED
#define INPUT_H_INCLUDED

#include "scan_codes.h"

void            hookKeyboardISR();
void            unhookKeyboardISR();

unsigned char   getKey(unsigned char scan_code);

unsigned char   isExtended();

#endif // INPUT_H_INCLUDED

input.c

#include "input.h"
#include "interrupt_hooker.h"

#ifndef NULL
#define NULL ((void*)0)
#endif // NULL

unsigned char key_array[MAX_SCAN_CODES] = {0};
unsigned char extended_key = 0;

void __interrupt (*oldKeyboardISR)() = NULL;


unsigned char getKey(unsigned char scan_code)
{
    return key_array[scan_code];
}

unsigned char isExtended()
{
    return extended_key;
}

void __interrupt keyboardISR()
{
    unsigned char scan_code = 0;

    unsigned char temp = 0;
    scan_code = inp(0x60);
    outp(0x61,(temp = inp(0x61)) | 0x80);
    outp(0x61,temp);
    /*asm
    {
        in  al, 60h         //get scan code from keyboard controller
        mov scan_code, al   //put it in scan_code

        in  al, 61h
        or  al, 80h
        out 61h, al
        xor al, 80h
        out 61h, al
    }*/

    switch(scan_code)
    {
    //if extended code
    case 0xe0:
        extended_key = 1;
        break;

    case 0xe1:
        break;

    default:
        //check if break code
        if(scan_code & 0x80){
            scan_code &= 0x7f;

            //set key to released
            key_array[scan_code] = 0;

        }
        else{
            //set key to pressed
            key_array[scan_code] = 1;
        }

        extended_key = 0;
        break;
    }

    //send EOI
    outp(0x20, 0x20);
    /*asm
    {
        sti
        mov al, 20h
        out 20h, al
    }*/
}

void hookKeyboardISR()
{
    if(oldKeyboardISR == NULL){
        getInterruptVector(0x09, oldKeyboardISR);
        setInterruptVector(0x09, keyboardISR);
    }
}

void unhookKeyboardISR()
{
    if(oldKeyboardISR != NULL){
        setInterruptVector(0x09, oldKeyboardISR);
        oldKeyboardISR = NULL;
    }
}

interrupt_hooker.h

#ifndef INTERRUPT_HOOKER_H_INCLUDED
#define INTERRUPT_HOOKER_H_INCLUDED

void getInterruptVector(unsigned char int_number, void __interrupt (*isr)());

void setInterruptVector(unsigned char int_number, void __interrupt (*isr)());

#endif // INTERRUPT_HOOKER_H_INCLUDED

interrupt_hooker.c

void getInterruptVector(unsigned char int_number, void __interrupt (*isr)())
{
    asm
    {
        cli
        mov al, int_number
        mov ah, 35h
        int 21h

        mov word ptr [isr], bx
        mov word ptr [isr+2], es
        sti
    }
}

void setInterruptVector(unsigned char int_number, void __interrupt (*isr)())
{
    asm
    {
        cli
        mov dx, word ptr [isr+2]
        mov ds, dx
        mov dx, word ptr [isr]

        mov al, int_number
        mov ah, 25h
        int 21h
        sti
    }
}

Answer 1

mov dx, word ptr [isr] might try to use ds that you just clobbered unless isr gets resolved to bp+offset in which case it will use ss and that should be good. Also, I guess the compiler won't be happy that you destroyed ds.

A safer code could be:

cli
push ds
mov al, int_number
mov ah, 25h
mov dx, word ptr [isr]
mov ds, word ptr [isr+2]
int 21h
pop ds
sti

Your getInterruptVector is broken because it's missing a level of indirection - you are just modifying the parameter which is pass-by-value in C. You'd have to invoke it as getInterruptVector(0x09, &oldKeyboardISR); (notice the &) and of course dereference it in your function, something like:

    mov al, int_number
    mov ah, 35h
    int 21h

    mov di, word ptr [isr]
    mov [di], bx
    mov [di+2], es

(That's assuming a 16 bit near pointer, not sure what your memory model is.)