; Register a interrupt handler (21h AH=25h)
; in:	AL	interrupt number
;	DS:DX	far ptr
isr_intr_register:
  ; backup previous stos ptr
  push es
  push di

  ; DI = AL * 4
  mov ah, 4
  mul ah
  mov di, ax

  ; ES = 0
  xor ax, ax
  mov es, ax

  ; store offset
  mov ax, dx
  stosw

  ; store segment
  mov ax, ds
  stosw

  ; restore previous stos ptr
  pop di
  pop es

  iret