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A wait state is a situation in which the computer processor experiences a delay, mainly when accessing external memory or a device that is slow in its response. Therefore, wait states are considered wasteful in processor performance. However, modern-day designs try to either eliminate or minimize wait states. These include caches, instruction pre-fetch and pipelines, simultaneous multithreading and branch prediction. While all of these techniques cannot eliminate wait states entirely, they can significantly reduce the problem when working together.
Wait states are also used to reduce energy consumption, allowing the processor to slow down and pause if there is no work for the CPU.
When the processor requires access to the main memory, it starts by placing the address of the information requested into the address bus. Following this, the processor needs to wait for the response, which may come back several cycles later. Every one of these cycles is spent in a wait state. Microprocessors that power modern computers run extremely fast. However, the same cannot be said of the memory technology, which has not yet caught up to similar speeds. A typical AMD Athlon 64 X2 and the Intel Core run at speeds of several GHz, meaning a clock cycle is typically less than a nanosecond (0.3–0.5 ns). On the other hand, main memory has latency in the range of 15-30 ns. This mismatch results in a wait state for the microprocessor, as a result slowing the overall speed of operation.