Question

I’m trying to learn more about how systems really work underneath all the pretty graphics on top. So I’m currently playing with memory in the 512bytes that the BIOS loads on startup, can’t really call it a bootloader at the moment I guess. Anyway, I’m replacing an interrupt vector but having issues with it. After replacing interupt 09h (the keyboard) it functions correctly, on keypress ‘Memory messing’ is outputted. But then nothing. Every keypress afterwards does nothing. I don’t know if the system is crashing or if I’m missing something from my handler, here’s the code:

jmp start
times 100 db 0 ; Cleared space for stack

start:
xor ax, ax
mov ax, start
sub ax, 80
mov sp, ax 

mov al, 0x09   ; Interupt number
mov bl, 4
mul bl
mov bx, ax
xor ax, ax
mov es, ax
mov [es:bx], word prints ; My interupt handler
add bx, 2
mov [es:bx], word 0x00

bloader:
    jmp bloader

prints:
    cli
    push ax
    push bx
    push si
    mov si, msg       ; Message to print
    mov bl, 0x07
    mov bh, 0x00
    printnb:
        lodsb     ; Grab byte from message
        cmp al, 0 ; End of message
        je printf
        mov ah, 0x0E
        int 0x10  ; Print byte
        jmp printnb
    printf:
        mov al, 0x20
        out 0x20, al ; Inform interupt controller interupt has been handled
        pop si
        pop bx
        pop ax
        sti
        iret ; Interupt return

msg db "Memory messing",0

times 510 - ($ - $$) db 0
dw 0xAA55

Answer 1

It’s been a long while since I worked on dealing with keyboard interrupts, but I think what you’re missing is actually handling the keyboard hardware so it’ll clear the interrupt condition and be ready to generate another interrupt.

Something like:

         in     al, 60h     ; Read input buffer 

might be all it takes.

If I get a chance I’ll see if I have any old notes or code lying around from way back when I wrote keyboard drivers.