A cycle-accurate simulator (CAS) is a computer program that simulates a microarchitecture on a cycle-by-cycle basis. In contrast an instruction set simulator simulates an instruction set architecture usually faster but not cycle-accurate to a specific implementation of this architecture; they are often used when emulating older hardware, where time precisions are very important from legacy reasons. More often CAS is used when designing new microprocessors – they can be tested, and benchmarked accurately (including running full operating system, or compilers) without actually building a physical chip, and easily change design many times to meet expected plan.
Cycle-accurate simulators must ensure that all operations are executed in the proper virtual (or real if it is possible) time – branch prediction, cache misses, fetches, pipeline stalls, thread context switching, and many other subtle aspects of microprocessors.