Is there anybody who can help me for giving me source code for countdown timer in Assembly
Going to need a lot more detail. I've written such on MicroChip PIC, Atmel and other computers. This is very specific to your platform.
I'm assuming this is not for DOS, Linux or any x86 computer as that's been done and you would have done your research.
@Z, please add more detail. You could be writing for x86 to some hundred other computers. If it's x86 what you want is found a bit too easy.