Q. Writing down your own Interrupt Service Routines?

Here are a few rules which should be kept in mind while writing down your own Interrupt Service Routines:

1.  Use Int 21h, function 35h to get the needed IVT entry from IVT. Save this entry for later use.

2.  Use Int 21h, function 25h to modify IVT. 

3.  If your program isn't going to stay resident then save the contents of the IVT and later restore them when your program exits.

4.  If your program is going to stay resident then use one of the terminate and stay resident functions to reserve proper amount of memory for your handler.