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Problem 2—A NOP (no operation) instruction is included in most of processors. Is just idles the CPU hardware for one instruction cycle and does nothing useful. Why is it needed?

Answer

The NOP instruction has several important applications, including:

• Modern pipelined processors sometimes have a "branch delay" slot following each branch instruction. An instruction placed there will be executed whether or not the branch is taken. If the compiler doesn't have anything that needs to be done at that point, it must fill the slot with a NOP.
• When doing physical I/O (especially in embedded applications), it is sometimes necessary to introduce tiny time delays between actions. A sequence or loop of NOP instructions is one way to do this.
• When hand-patching binary code, NOP instructions can be used to blot out an unwanted instruction, or to pad a patch that is shorter than the code it is replacing.

Contributor: Naveen P N

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