ivt_backup:
  push ds
  push es
  xor ax, ax
  mov ds, ax
  mov es, ax
  mov si, ax
  mov di, 0x0100 ; int 0x40 address
  mov cx, 0x0040 ; 0x20 interrupts * 2 words
  repe movsw
  pop es
  pop ds
  ret

; set item in interrupt vector table
; in:	bx	interrupt number
;	cs:dx	new handler address
ivt_set:
  push ax
  push es

  xor ax,ax
  mov es,ax

  sal bx,1
  sal bx,1

  mov [es:bx], dx
  mov [es:bx+2], cs

  pop es
  pop ax
  ret