Hi,
Dom, here is the simplest question: have you read C51 manual once? It is basic question you may do answer yourself if try think logically.
1) C-function may return a value (remember those which are return(x)? As Erik says: make return value (int/long etc) from registers and return it back.
2) calling any function (as well as assembly one) you may pass to it the address where result should be placed, like: do_it_with_ASM(data, put_result_here); where put_result_here is a pointer to memory location or even the address of variable (oops, you need to know how has it located in memory, I mean Big/Little Endian form).

Good days!