How do you do a SWAP instruction in C?
If your compiler is smart, it will interpret your C code to use SWAP, if not you will get something else.

If you decide to use C (often a good decision) you are at the mercy of the compiler, do not expect the compiler to use specific instructions.

Very often (when using a quality compiler) C generates efficient code, however in some special cases (4 bit interface is a good example) can it be very beneficial to write it in assembler and call it from your C code.

I have found that writing assembler usually isn't worth it except in two cases: complex I/O and loops.

Erik