.386
cseg segment use16
assume CS:cseg, DS:cseg, SS:cseg, ES:cseg
Org 100h

ENTRY:
	jmp main

exit proc
	jmp dword ptr cs:[interruptVector]
exit endp

removeHook proc
	push ds

	mov dx, word ptr cs:[interruptVector]
	mov ds, word ptr cs:[interruptVector+2]
	mov ax, 2521h
	int 21h


	mov ah, 49h
    int 21h

	pop ds
	ret
removeHook endp

interruptHook proc
	cmp ah, 2ah
	jne exit

	push ds
	push ax

	push cs
	pop ds

	pushf
	call dword ptr cs:[interruptVector]

	mov ah, 09h
	lea dx, mess
	int 21h

	call removeHook

	mov ah, 09h
    lea dx, remm
	int 21h

	mov cx, fakeYear
	mov dh, fakeMonth
	mov dl, fakeDay

	pop ax
	pop ds

	iret

interruptVector	dd ?
fakeYear dw 2048
fakeDay db 128
fakeMonth db 16

mess db "---",13,10,"Interruption 21h  AH = 2ah was intercepted",13,10,"Original registers was changed",13,10,"---",13,10,'$'
startm db "Start interrupt hook, good luck", 13, 10, "$"
remm db "---",13,10,"Remove hook from 21h  AH = 2ah, bye :*",13,10,"---",13,10,"$"
interruptHook endp

main:
	mov ah, 09h
	lea dx, startm
	int 21h

	xor dx, dx
	xor ax, ax

	mov	ax,3521h
	int	21h

	mov	word ptr interruptVector,bx
	mov	word ptr interruptVector+2,es

	mov	ax,2521h
	mov	dx,offset interruptHook
	int	21h

mov	ax,4c00h
    int	21h
cseg ends
end ENTRY