Questions tagged [no-op]

The great art of doing nothing

What is it, and how did it come to be?

No-OPeration (nop, no-op) is the proper instruction to instruct a processor to do nothing (but waste time).
Just about every instruction-set has at least some instructions which have no effect at all. They naturally occur because it pays to build a regular IS, which leads to curiosities like exchange A for A, assign A to A, bitwise-and/or A and A.

While some of those may be redefined for better instruction density, one will be chosen as intended nop.

Purpose/Uses

There are a great many uses for doing nothing. Some examples:

• Reserving space for adding patches

  Some Windows system binariesreserve space to allow live-patching.

• Neutralizing code

  An example is an implementation of a seldom changed flag in high-performance-code: Either there's an unconditional jump to additional code, or there's a single, efficient nop

• Aligning code for better caching

  If a function begins at the start of a cache-line (or even memory-page), there will often be fewer cache-misses

• Wasting time

  Counting instructions and busy-Loops for delaying are getting less common in high-end systems, though they are still alive and well on slower machines.

• (Attacking) Making a bigger target for a (semi-)blind jump by building a nop-slide.

References

• Wikipedia article