There is no general answer to your question. The process required can and will vary substantually based upon which 8051 C compiler you intend to use. Once you have selected a C compiler then look in the users manual and see how they recommend the C to ASM interface works.

A common recommendation is to write a prototype shell of your intended assembler language subroutine in C. Then compile it to make sure you have the C syntax correct. Next you change the source code for this shell C code so that the compiler will convert the shell to assembly language source. This converted to assembler shell can then be edited by you to become an ASM source file in your project in place of the C version of the routine.

The way to make the compiler output the assembler source will also vary between various compilers. Sometimes it may be done with a #pragma statement at the top of the C source file while other setups may use a compiler command line option or a project file setting in your Integrated Development Envoronment. Once again it is necessary to cruize into the users manuals for your compiler to see how this is done.

Michael Karas