A wait state is a delay experienced by a computer
processor
Processor may refer to:
Computing Hardware
* Processor (computing)
**Central processing unit (CPU), the hardware within a computer that executes a program
*** Microprocessor, a central processing unit contained on a single integrated circuit (I ...
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 remember ...
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. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 ser ...
Core 2
Intel Core 2 is the processor family encompassing a range of Intel's consumer 64-bit x86-64 single-, dual-, and quad-core microprocessors based on the Core microarchitecture. The single- and dual-core models are single-die, whereas the quad-co ...
and the
AMD Athlon 64 X2
The Athlon 64 X2 is the first native dual-core desktop central processing unit (CPU) designed by Advanced Micro Devices (AMD). It was designed from scratch as native dual-core by using an already multi-CPU enabled Athlon 64, joining it with anot ...
run with a clock of several
GHz, which means that one
clock cycle is less than 1
nanosecond (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 (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This e ...
. 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:
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,
instruction pipeline
In computer engineering, instruction pipelining or ILP 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 inco ...
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
On
IBM mainframes, 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/Architect ...
mode, the wait state code is found in bits 116-127.
See also
*
Bubble (computing)
In the design of pipelined computer processors, a pipeline stall is a delay in execution of an instruction in order to resolve a hazard.
Details
In a standard five-stage pipeline, during the decoding stage, the control unit will determine whe ...
*
*
Consistency model
*
Cache miss
In computing, a cache ( ) is a hardware or software component that stores data so that future requests for that data can be served faster; the data stored in a cache might be the result of an earlier computation or a copy of data stored elsewher ...
**
Page fault
In computing, a page fault (sometimes called PF or hard fault) is an exception that the memory management unit (MMU) raises when a process accesses a memory page without proper preparations. Accessing the page requires a mapping to be added t ...
*
Multithreading (computer architecture)
In computer architecture, multithreading is the ability of a central processing unit (CPU) (or a single core in a multi-core processor) to provide multiple threads of execution concurrently, supported by the operating system. This approach d ...
References
{{DEFAULTSORT:Wait State
Central processing unit
IBM mainframe technology