How does asm("nop"); works?

Question

I red the definition from http://en.wikipedia.org/wiki/NOP but I still need a simpler definition.

I stumbled across a code and I don't know exactly what it does:

switch (Something)
    {

        case this_one:
            asm ("nop");
            break;
        case other_one:
            asm ("nop");
            break;
        default:
            asm ("nop");
            break;
    }

Answer 1

nop is an assembly instruction that does nothing--well as close to nothing as you can do and still execute a machine instruction, which means (probably) a REALLY tiny bit of time goes by (which can have limited value in certain realtime applications.

In this case, the statement asm("nop"); makes no semantic difference to the program. The only reason I can think that it might be present is to "force" the compiler to NOT collapse the code paths, making the machine structure of the switch statement visible if you look at the object code or disassemble the machine code or view it in a debugger.

Answer 2

Since nobody mentioned it, nop can also be useful to "yield" during a critical section, i.e. allow other interrupts to occur so as to reduce interrupt latency caused by the critical section.

This is typically useful in embedded applications without operating systems where you often have to poll variables and/or status registers during a critical section.

On many RISC processors (ARM, AVR), the instruction immediately following interrupt unmasking will still be masked, so if you just put sei/cli close together, you won't allow any interrupt to occur.

Each nop added between sei and cli allows one more interrupt to occur before the critical section resumes.