Wait state
Encyclopedia
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 2011, 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 cache
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
. 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
s, instruction pipeline
s, instruction prefetch
, branch prediction, simultaneous multithreading
and others. No single technique is 100% successful, but together they can significantly reduce the problem.
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
when accessing external memory
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
or another device that is slow to respond.
As of late 2011, 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 dual-core desktop CPU designed by AMD. It was designed from scratch as native dual-core by using an already multi-CPU enabled Athlon 64, joining it with another functional core on one die, and connecting both via a shared dual-channel memory controller/north bridge and...
run with a clock of several GHz
GHZ
GHZ or GHz may refer to:# Gigahertz .# Greenberger-Horne-Zeilinger state — a quantum entanglement of three particles.# Galactic 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 memory. 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 physical address. When a processor or DMA-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 memory. 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
An 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 CPUs with hardware multithreading...
and others. No single technique is 100% successful, but together they can significantly reduce the problem.