mov al, InterruptNumber ;interrupt vector number
mov ah, 35h ;Get Interrupt Vector
int 21h
mov word ptr [Handler], bx ;es:bx points to interrupt handler
mov word ptr [Handler+2], es
Get Interrupt Vector (Function 35h) returns the address of the routine that handles the specified interrupt.
InterruptNumber
Specifies the interrupt number that causes the interrupt routine to be called.
If the function is successful, the ES:BX registers point to the routine that handles the specified interrupt. The ES register contains the segment address of the interrupt-handling routine, and the BX register contains the offset.
Function 25h Set Interrupt Vector