Wait State

What Does Wait State Mean?

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.

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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.

Techopedia Explains Wait State

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.

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Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.