; 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

; Register a interrupt handler
; in:	AL	interrupt number
;	ES:DI	far ptr to routine/data
intr_register:
  ; backup original values
  push ax
  push es
  push di

  ; pop as ax later
  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
  pop ax
  stosw

  ; store segment
  pop ax
  stosw

  pop di
  pop es
  pop ax
  ret

; Get address for a interrupt vector
; in	AL	interrupt number
; out	DS:SI	far ptr to routine/data
intr_load:
  push ax

  ; SI = AL * 4
  mov ah, 4
  mul ah
  mov si, ax

  ; DS = 0
  xor ax, ax
  mov ds, ax

  ; load segment and offset in one go
  lds si, [si]

  pop ax
  ret