for example you can just type mul ..... even for micro-controllers that dont have and wired multiplier, but the compiler will generate the right code so you can do things like multiplications.
this depends on the compiler or assembler, and the particular processor involved. I'm pretty sure that if you type "mul ..." in an AVR assembly language program, and the processor you've selected doesn't include a multiplier, the assembler will just give you an error message...