Central processing unit: Difference between revisions

Revision as of 18:03, 10 August 2011

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|caption2 = An Intel 80486DX2 from below}}

The '''central processing unit''' ('''CPU''') is the portion of a [[computer]] system that carries out the instructions of a [[computer program]], and is the primary element carrying out the functions of the computer or other processing device. It carries out each [[Instruction (computer science)|instruction]]s of thea [[computer program in sequence]], to perform the basic arithmetical, logical, and input/output operations of the system. In other words, theThe CPU plays a role somewhat analogous to the [[Human brain|brain]] in the computer. The term has been in use in the computer industry at least since the early 1960s.<ref name="weik1961">{{cite paper | author = Weik, Martin H. | title = A Third Survey of Domestic Electronic Digital Computing Systems | publisher = [[Ballistics Research Laboratory|Ballistic Research Laboratories]] | url = http://ed-thelen.org/comp-hist/BRL61.html | date = 1961 }}</ref> The form, design and implementation of CPUs have changed dramatically since the earliest examples, but their fundamental operation remains much the same.

On large machines, CPUs require one or more printed circuit boards. On personal computers and small workstations, the CPU is housed in a single chip called a microprocessor. Since the 1970's the microprocessor class of CPUs has almost completely overtaken all other CPU implementations. Modern CPUs are large scale [[integrated circuit]]s in small, rectangular packages, with multiple conecting pins.

The CPU itself is an internal component of the computer. Modern CPUs are small, rectangular, and contain multiple metallic connectors or pins on the underside. The CPU is inserted directly into a CPU socket, pin side down, on the [[motherboard]].

Each motherboard will support only a specific type or range of CPU so that one has to check the motherboard manufacturer's specifications before attempting to replace or upgrade a CPU. Modern CPUs also have an attached heat sink and small fan that go directly on top of the CPU to help dissipate heat.

Two typical components of a CPU are the arithmetic logic unit (ALU), which performs arithmetic and logical operations, and the control unit (CU), which extracts instructions from memory and decodes and executes them, calling on the ALU when necessary.

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Computers such as the [[ENIAC]] had to be physically rewired in order to perform different tasks, which caused these machines to be called "fixed-program computers." Since the term "CPU" is generally defined as a [[software]] (computer program) execution device, the earliest devices that could rightly be called CPUs came with the advent of the stored-program computer.

The structureidea of a stored-program computer was already present in the design of [[J. Presper Eckert]] and [[John William Mauchly]]'s ENIAC, but was initially omitted so that it could be finished sooner. On June&nbsp;30, 1945, before ENIAC was made, mathematician [[John von Neumann]] distributed the paper entitled ''[[First Draft of a Report on the EDVAC]]''. It was the designoutline of a stored-program computer that would eventually be completed in August 1949.<ref>{{cite paper | author = [[]] | title = First Draft of a Report on the EDVAC | publisher = Pennsylvania[[Moore School of Electrical Engineering]], [[University of Pennsylvania]] | url = http://www.virtualtravelog.net/entries/2003-08-TheFirstDraft.pdf | date = 1945 }}</ref> EDVAC was designed to perform a certain number of instructions (or operations) of various types. These instructions could be combined to create useful programs for the EDVAC to run. Significantly, the programs written for EDVAC were stored in high-speed [[Memory (computers)|computer memory]] rather than specified by the physical wiring of the computer. This overcame a severe limitation of ENIAC, which was the considerable time and effort required to reconfigure the computer to perform a new task. With von Neumann's design, the program, or software, that EDVAC ran could be changed simply by changing the contents of the memory.

Early CPUs were custom-designed as a part of a larger, sometimes one-of-a-kind, computer. However, this chupa titimethod of designing custom CPUs for a particular application has largely given way to the development of mass-produced processors that are made for one or many purposes. This standardization trend generally began in the era of discrete [[transistor]] [[Mainframe computer|mainframes]] and [[minicomputer]]s and has rapidly accelerated with the popularization of the [[integrated circuit]]&nbsp;(IC). The IC has allowed increasingly complex CPUs to be designed and manufactured to tolerances on the order of [[nanometer]]s. Both the miniaturization and standardization of CPUs have increased the presence of digital devices in modern life far beyond the limited application of dedicated computing machines. Modern microprocessors appear in everything from [[automobile]]s to [[cell phone]]s and children's toys.{{Clarify|date=June 2011|Does this refer to microprocessors in general or to the CPU, particularly?}}

While von Neumann is most often credited with the design of the stored-program computer because of his design of EDVAC, others before him, such as [[Konrad Zuse]], had suggested and implemented similar ideas. The so-called [[Harvard architecture]] of the [[Harvard Mark I]], which was completed before EDVAC, also utilized a stored-program design using [[Punched tape|punched paper tape]] rather than electronic memory. The key difference between the von Neumann and Harvard architectures is that the latter separates the storage and treatment of CPU instructions and data, while the former uses the same memory space for both. Most modern CPUs are primarily von Neumann in design, but elements of the Harvard architecture are commonly seen as well.