Is C as fast or faster than hand-coded assembler code?

C code would be just as fast as assembly if you

• Write small interface procedures with C calling convention.
• Put local variables on the stack

Assembler code is usually much faster if you do like this:

• Pass parameters in registers
• Use special assembler only instructions
• Jump tables
• Put local variables in registers, and save registers when required

Is C code more portable that assembler code?

C code is usually more portable than assembler if any of this holds:

• You port to another processor
• You don't use vendor-specific extensions

Why use assembler in an OS project?

It will be both faster and smaller. This is significant for all parts of an OS. Some say you don't need to optimize the file-system since the disk-drives are slow anyway. Well, that's not true, since many of the requests for files will be read from buffers. Many portable, C based OSes have a very slow file-system, and it's not because the disk-drives are slow, it's because the C code and all the layering. File-system performance is actually one of the most critical parameters in an OS!