Gil,
You should write a subroutine in Asm using
an function calling convention(parameters
passing and returned values in/from function) of THAT compiler and machine you
are going to use.
I do not remember the rules used in
Keil compiler, you should read Chapter 6 of
"C51 compiler user's Guide"(in case of Keil and 8052).
When you have a number of RELOCATABLE object
modules(from asm, C,..) you can link them in
usual way.
Regards,
Oleg.