A Cycle Accurate Simulator (CAS) is a computer program that simulates a microarchitecture cycle-accurate. 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 is 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 physical chip, and easily change design many times to meet expected plan.
Cycle Accurate Simulator, 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.