I never use the SRC directive, but I simply make an A51 source file since I know exactly how the parameters are passed from C, so I write the assembly function from the very beginning on.

This is a separate modul that is compiled with A51 assembler and then the object files from C and ASM are linked together, then the project recompilation for register optimization and that's it.

Then I check what registers my ASM routine has altered and tell this with the $REGUSE directive to the compiler to the register optimization can also be used not concerning this asm function as a worst case.

And the result, tighter and highly optimized code.

Franc