Wait state

Wait state

A wait state is a delay experienced by a computer processor when accessing external memory or another device that is slow to respond.

As of late 2007, computer microprocessors run at very high speeds, while memory technology does not seem to be able to catch up: typical PC processors like the Intel Core 2 and the AMD Athlon 64 X2 run with a clock of several GHz, while the main memory clock generally ranges from 667 to 1333 MHz. Some second-level CPU caches run slower than the processor core.

When the processor needs to access external memory, it starts placing the address of the requested information on the address bus. It then must wait for the answer, that may come back tens if not hundreds of cycles later. Each of the cycles spent waiting is called a wait state.

Wait states are a pure waste for a processor's performance. Modern designs try to eliminate or hide them using a variety of techniques: CPU caches, instruction pipelines, instruction prefetch, branch prediction, simultaneous multithreading and others. No single technique is 100% successful, but together they can significantly reduce the problem.

Energy Conservation

Wait states can be used to reduce the energy consumption of a processor, by allowing the main processor clock to either slow down or temporarily pause during the wait state if the CPU has no other work to do. Rather than spinning uselessly in a tight loop waiting for data, sporadically reducing the clock speed in this manner helps to keep the processor core cool and to extend battery life in portable computing devices.

ee also

* Bubble (computing)

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Look at other dictionaries:

  • wait state — noun (computing) The period of time that a microprocessor must wait while fetching data from a slower memory • • • Main Entry: ↑wait * * * wait state, a condition in which a computer processor must stop functioning between instructions: »A wait… …   Useful english dictionary

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  • wait state — noun Any of several instances in which a computers processor cannot execute instructions (either for the entire computer, or just for a specific task) until an I/O operation completes, or until an interrupt is resolved With memory that slow, the… …   Wiktionary

  • wait state — ● ►en /ouaiyt staiyt/ loc. m. ►ARCHI Voir la version française: état d attente …   Dictionnaire d'informatique francophone

  • wait state —    Time interval between an initiation of a call for data and the actual transmission of the data …   IT glossary of terms, acronyms and abbreviations

  • Zero wait state — is a feature of a processor or computer architecture in which the processor does not have to wait to perform memory access. Non zero wait state describes the situation when a processor operates at a higher frequency than the memory, it has a wait …   Wikipedia

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  • Wait (operating system) — In modern computer operating systems, a process (or task) may wait on another process to complete its execution. In most systems, a parent process can create an independently executing child process. The parent process may then issue a wait… …   Wikipedia

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