Have you read the section, "Interfacing C Programs to Assembler" in the C51 manual?

It sounds like you problem lies in linking your Project; not in calling assembler from 'C' per se.
You should fix the Linker warning first:
Examine the Map file to see where the overlap is occuring.
Then read the C51, A51 and Linker manuals to determine how to control the memory allocation.