DOS Multikey.c

#include "MoreTypes.h"

static ubyte KeyTbl[255];
//static ushort oldHandler[2];
ushort oldHandlerAddr;
ushort oldHandlerSeg;

// http://www.intel-assembler.it/portale/5/keyboard-programming-in-asm/keyboard-programming-in-asm.asp

__declspec(naked) void __interrupt KeyboardHandler() {
    _asm {
        "PUSH AX"
        "PUSH BX"
        "PUSH CX"
        "PUSH ES"

        "MOV BX, SEG KeyTbl"  // Set ES to segment of key table
        "MOV ES, BX"
        "LEA BX, KeyTbl"      // Set BX to address of key table

        "XOR AX, AX"     // Clear AX to 0
        "IN AL, 60h"     // read key pressed/unpressed into AL
        "ADD BX, AX"     // adjust pointer based on key down

        "CMP AL, 0E0h"   // Signifies an extended key code
        "JE Int_Exit"    // Ignore extended keys

        "MOV CL, AL"     // check if significant bit is set
        "AND AL, 128"    // (0 = key down, 1 = key up)
        "JZ Key_Pressed"

        "Key_Released:"
        "AND [ES:BX], 0FF00h"
        "JMP Int_Exit"

        "Key_Pressed:"
        "OR [ES:BX], 00FFh"


        "Int_Exit:"

        // this block just echos the key to the screen
        /*"XOR BX, BX"
        "MOV AL, CL"
        "MOV AH, 0Eh"
        "INT 10h"*/

        "POP ES"
        "POP CX"
        "POP BX"
        "POP AX"

        // Call old keyboard handler now that we're done
        "PUSH [oldHandlerSeg]"
        "PUSH [oldHandlerAddr]"
        "RETF"
    }
}

extern void InitKeyboard();
#pragma aux InitKeyboard = \
    "PUSH AX" \
    "PUSH BX"\
    "PUSH DX"\
    "PUSH ES"\
    "PUSH DS"\
    \
    "MOV AH, 35h" /* BIOS function to get interrupt handler */ \
    "MOV AL, 9h"  /* Interrupt function INT 09h */ \
    "INT 21h"     /* Returns pointer in ES:BX */ \
    "MOV [oldHandlerSeg], ES" /* Save old pointer to restore after we're done */\
    "MOV [oldHandlerAddr], BX"\
    \
    "MOV AH, 25h" /* Function to set interrupt vector to DS:DX */\
    "MOV DX, CS" /* Move CS into DS */\
    "MOV DS, DX"\
    "LEA DX, KeyboardHandler" /* Get address of handler code */\
    "INT 21h"\
    \
    "POP DS"\
    "POP ES"\
    "POP DX"\
    "POP BX"\
    "POP AX"\

extern void CloseKeyboard();
#pragma aux CloseKeyboard = \
    "PUSH AX"\
    "PUSH DX"\
    "PUSH DS"\
    \
    "MOV AH, 25h" /* Function to set interrupt vector for INT 09h */\
    "MOV AL, 09h"\
    "MOV DX, [oldHandlerAddr]"\
    "MOV DS, [oldHandlerSeg]" /* Function gets pointer from DS:DX */\
    "INT 21h"\
    \
    "POP DS"\
    "POP DX"\
    "POP AX"\

void main() {
    int iX;
    InitKeyboard();

    for (iX = 0; iX <= 0xFF; iX++)
        KeyTbl[iX] = 0;

    while(!KeyTbl[0x1C]){
    }

    CloseKeyboard();
}