I agree with Michael - if you need to use assembler, do it properly in assembler!

Because the Compiler just converts the whole file to source, and it is the assembler that generates the object, you lose all your 'C' source-level debug info!

The easiest way to start your C-callable assembler functions is to write "skeletons" in 'C', use SRC to generate the corresponding assmbler, then fill in the rest in assembler (you can throw the original 'C' "skeletons" away!)

There is a whole section in the C51 Manual about interfacing 'C' and assembler.