Example of overriding BIOS interrupts in i386 assembly.

intthief.asm

[org 0x7c00]
[bits 16]
[map all thief.map]

;Stack init
mov bp, 0xffff
mov sp, bp

;Display initial interrupt address
mov bx, [intnum*4+2]
mov cx, [intnum*4]
mov si, mesg_origs
call puts
mov ax, bx
call puthexw
mov si, mesg_nl
call puts
mov si, mesg_origo
call puts
mov ax, cx
call puthexw
mov si, mesg_nl
call puts

mov cx, [intnum*4+2]
mov bx, [intnum*4]
mov es, cx
mov ax, 0
loop:
	inc bx
	inc ax
	cmp [es:bx], byte 0xcf
	je loop_end
	jmp loop
loop_end:
mov si, mesg_intlen
call puts
call putdec
mov al, 'b'
call putc
mov si, mesg_nl
call puts


intnum equ 0x13
mov ax, 0x07c0
mov word [intnum*4+2], ax
mov word [intnum*4], ( myint - $$ )

int 0x13
mov si, mesg_hello
call puts

jmp $

myint:
	mov si, mesg_nl
	push ax
	mov al, 'a'
	call putc
	pop ax
	call puthexw
	call puts
	mov al, 'b'
	call putc
	mov ax, bx
	call puthexw
	call puts
	mov al, 'c'
	call putc
	mov ax, cx
	call puthexw
	call puts
	mov al, 'd'
	call putc
	mov ax, dx
	call puthexw
	call puts
	iret

%include "stdio.asm"

mesg_origs: db "org int cs: ", 0
mesg_origo: db "org int ip: ", 0
mesg_intlen: db "org int len: ", 0
mesg_hello: db "hello!", 10, 13, 0
mesg_nl: db 10, 13, 0

times 510 - ( $ - $$ ) db 0
dw 0xaa55