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A wait state is a delay experienced by a computer processor
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
 when accessing external memory
Computer storage

Computer data storage, often called storage or memory, refers to computer components, devices, and recording medium that retain digital data used for computing for some interval of time....
 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
Athlon 64 X2

The Athlon 64 X2 is the first multi-core desktop computer Central processing unit designed by AMD. It is essentially a processor consisting of two Athlon 64 cores joined together on one Die with additional control logic....
 run with a clock of several GHz
GHZ

GHZ or GHz may refer to:# Hertz .# Greenberger-Horne-Zeilinger state - a quantum entanglement of three particles.# Habitable zone - the region of a galaxy that is favorable to the formation of life....
, while the main memory clock generally ranges from 667 to 1333 MHz.






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Encyclopedia


A wait state is a delay experienced by a computer processor
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
 when accessing external memory
Computer storage

Computer data storage, often called storage or memory, refers to computer components, devices, and recording medium that retain digital data used for computing for some interval of time....
 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
Athlon 64 X2

The Athlon 64 X2 is the first multi-core desktop computer Central processing unit designed by AMD. It is essentially a processor consisting of two Athlon 64 cores joined together on one Die with additional control logic....
 run with a clock of several GHz
GHZ

GHZ or GHz may refer to:# Hertz .# Greenberger-Horne-Zeilinger state - a quantum entanglement of three particles.# Habitable zone - the region of a galaxy that is favorable to the formation of life....
, while the main memory clock generally ranges from 667 to 1333 MHz. Some second-level CPU cache
CPU cache

A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access computer storage. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations....
s 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
Address bus

An address bus is a computer bus that is used to specify a memory address. When a central processing unit or direct memory access-enabled device needs to read or write to a memory location, it specifies that memory location 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 cache
CPU cache

A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access computer storage. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations....
s, instruction pipeline
Instruction pipeline

File:5 Stage Pipeline.svgAn instruction pipeline is a technique used in the design of computers and other digital electronic devices to increase their instruction throughput ....
s, instruction prefetch
Instruction prefetch

In computer architecture, instruction prefetch is a technique used in microprocessors to speed up the execution of a program by reducing wait states....
, branch prediction, simultaneous multithreading
Simultaneous multithreading

Simultaneous multithreading, often abbreviated as SMT, is a technique for improving the overall efficiency of superscalar Central processing unit with 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.

See also

  • Bubble (computing)
    Bubble (computing)

    In computing, a bubble is a "hiccup" in execution of an instruction set in an instruction pipeline.The following is two executions of the same four instruction through a 4-stage pipeline but, for whatever reason, a delay in fetching of the purple instruction in cycle #2 leads to a bubble being created delaying all instructions after it as w...