A wait state is a delay experienced by a computer processor when accessing external

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...

or another device that is slow to respond. Computer microprocessors generally run much faster than the computer's other subsystems, which hold the data the CPU reads and writes. Even memory, the fastest of these, cannot supply data as fast as the CPU could process it. In an example from 2011, typical PC processors like the

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...

Core 2 and the

AMD Advanced Micro Devices, Inc. (AMD) is an American multinational corporation and technology company headquartered in Santa Clara, California and maintains significant operations in Austin, Texas. AMD is a hardware and fabless company that de ...

Athlon 64 X2 run with a clock of several

GHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...

, which means that one

clock cycle In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') is an electronic logic signal (voltage or current) which oscillates between a high and a low state at a constant frequency and ...

is less than 1

nanosecond A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or seconds. The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit (e ...

(typically about 0.3 ns to 0.5 ns on modern desktop CPUs), while main memory has a latency of about 15–30 ns. Some second-level

CPU cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, whi ...

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 In computer architecture, a bus (historically also called a data highway or databus) is a communication system that transfers data between components inside a computer or between computers. It encompasses both hardware (e.g., wires, optical ...

. 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 of a processor's performance. Modern designs try to eliminate or hide them using a variety of techniques:

instruction pipeline In computer engineering, instruction pipelining is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming Mac ...

s, instruction prefetch,

branch prediction In computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch (e.g., an if–then–else structure) will go before this is known definitively. The purpose of the branch predictor is to improve the flow ...

simultaneous multithreading Simultaneous multithreading (SMT) is a technique for improving the overall efficiency of superscalar CPUs with hardware multithreading. SMT permits multiple independent threads of execution to better use the resources provided by modern proces ...

and others. No single technique is 100% successful, but together 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.

Alternative meaning on IBM mainframes

IBM mainframes IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, IBM dominated the computer market with the IBM 700/7000 series, 7000 series and the later System/360, followed by the System/370. Current mainfram ...

, the term ''wait state'' is used with a different meaning. A wait state refers to a CPU being halted, possibly due to some kind of serious error condition (such as an unrecoverable error during operating system to IPL). A wait state is indicated by bit 14 of the PSW being set to 1, with other bits of the PSW providing a wait state code giving a reason for the wait. In

z/Architecture z/Architecture, initially and briefly called ESA Modal Extensions (ESAME), is IBM's 64-bit complex instruction set computer (CISC) instruction set architecture, implemented by its mainframe computers. IBM introduced its first z/Architecture ...

mode, the wait state code is found in bits 116-127.

References

{{DEFAULTSORT:Wait State Central processing unit IBM mainframe technology

Energy conservation

Alternative meaning on IBM mainframes

See also

References