A Central Processing Unit (CPU) or processor is an
electronic circuit that can execute computer programs, which are actually sets
of instructions. This term has been in use in the computer industry at least
since the early 1960s (Weik 2007). The form, design and implementation of CPUs
have changed dramatically since the earliest examples, but their fundamental
operation remains much the same.
Early CPUs were custom-designed as a part of a larger,
sometimes one-of-a-kind, computer. However, this costly method 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
mainframes and minicomputers and has rapidly accelerated with the popularization
of the integrated circuit (IC). The IC has allowed increasingly complex CPUs to
be designed and manufactured to tolerances on the order of nanometers. Both the
miniaturization and standardization of CPUs have increased the presence of these
digital devices in modern life far beyond the limited application of dedicated
computing machines. Modern microprocessors appear in everything from automobiles
to cell phones and children's toys.
History of Central Processing Unit
Before the advent of machines that resemble today's CPUs,
computers such as the ENIAC had to be physically rewired in order to perform
different tasks. These machines are often referred to as "fixed-program
computers," since they had to be physically reconfigured in order to run a
different program. 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 idea 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 the machine could be finished sooner. On June 30, 1945,
before ENIAC was even completed, mathematician John von Neumann distributed the
paper entitled "First Draft of a Report on the EDVAC." It outlined the design of
a stored-program computer that would eventually be completed in August 1949 (von
Neumann 1945). 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 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 computer's
memory.[1]
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 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.
As a digital device, a CPU is limited to a set of discrete
states, and requires some kind of switching elements to differentiate between
and change states. Prior to commercial development of the transistor, electrical
relays and vacuum tubes (thermionic valves) were commonly used as switching
elements. Although these had distinct speed advantages over earlier, purely
mechanical designs, they were unreliable for various reasons. For example,
building direct current sequential logic circuits out of relays requires
additional hardware to cope with the problem of contact bounce. While vacuum
tubes do not suffer from contact bounce, they must heat up before becoming fully
operational, and they eventually cease to function due to slow contamination of
their cathodes that occurs in the course of normal operation. If a tube's vacuum
seal leaks, as sometimes happens, cathode contamination is accelerated. See
vacuum tube. Usually, when a tube failed, the CPU would have to be diagnosed to
locate the failed component so it could be replaced. Therefore, early electronic
(vacuum tube based) computers were generally faster but less reliable than
electromechanical (relay based) computers.
Tube computers like EDVAC tended to average eight hours
between failures, whereas relay computers like the (slower, but earlier) Harvard
Mark I failed very rarely (Weik 1961:238). In the end, tube based CPUs became
dominant because the significant speed advantages afforded generally outweighed
the reliability problems. Most of these early synchronous CPUs ran at low clock
rates compared to modern microelectronic designs (see below for a discussion of
clock rate). Clock signal frequencies ranging from 100 kHz to 4 MHz were very
common at this time, limited largely by the speed of the switching devices they
were built with.
Discrete transistor and Integrated Circuit CPUs
The design complexity of CPUs increased as various
technologies facilitated building smaller and more reliable electronic devices.
The first such improvement came with the advent of the transistor.
Transistorized CPUs during the 1950s and 1960s no longer had to be built out of
bulky, unreliable, and fragile switching elements like vacuum tubes and
electrical relays. With this improvement more complex and reliable CPUs were
built onto one or several printed circuit boards containing discrete
(individual) components.
During this period, a method of manufacturing many
transistors in a compact space gained popularity. The integrated circuit (IC)
allowed a large number of transistors to be manufactured on a single
semiconductor-based die, or "chip." At first only very basic non-specialized
digital circuits such as NOR gates were miniaturized into ICs. CPUs based upon
these "building block" ICs are generally referred to as "small-scale
integration" (SSI) devices. SSI ICs, such as the ones used in the Apollo
guidance computer, usually contained transistor counts numbering in multiples of
ten. To build an entire CPU out of SSI ICs required thousands of individual
chips, but still consumed much less space and power than earlier discrete
transistor designs. As microelectronic technology advanced, an increasing number
of transistors were placed on ICs, thus decreasing the quantity of individual
ICs needed for a complete CPU. MSI and LSI (medium- and large-scale integration)
ICs increased transistor counts to hundreds, and then thousands.
In 1964 IBM introduced its System/360 computer architecture
which was used in a series of computers that could run the same programs with
different speed and performance. This was significant at a time when most
electronic computers were incompatible with one another, even those made by the
same manufacturer. To facilitate this improvement, IBM utilized the concept of a
microprogram (often called "microcode"), which still sees widespread usage in
modern CPUs (Amdahl et al. 1964). The System/360 architecture was so popular
that it dominated the mainframe computer market for the decades and left a
legacy that is still continued by similar modern computers like the IBM zSeries.
In the same year (1964), Digital Equipment Corporation (DEC) introduced another
influential computer aimed at the scientific and research markets, the PDP-8.
DEC would later introduce the extremely popular PDP-11 line that originally was
built with SSI ICs but was eventually implemented with LSI components once these
became practical. In stark contrast with its SSI and MSI predecessors, the first
LSI implementation of the PDP-11 contained a CPU composed of only four LSI
integrated circuits (Digital Equipment Corporation 1975).
Transistor-based computers had several distinct advantages
over their predecessors. Aside from facilitating increased reliability and lower
power consumption, transistors also allowed CPUs to operate at much higher
speeds because of the short switching time of a transistor in comparison to a
tube or relay. Thanks to both the increased reliability as well as the
dramatically increased speed of the switching elements (which were almost
exclusively transistors by this time), CPU clock rates in the tens of megahertz
were obtained during this period. Additionally while discrete transistor and IC
CPUs were in heavy usage, new high-performance designs like SIMD (Single
Instruction Multiple Data) vector processors began to appear. These early
experimental designs later gave rise to the era of specialized supercomputers
like those made by Cray Inc.
Microprocessors
The introduction of the microprocessor in the 1970s
significantly affected the design and implementation of CPUs. Since the
introduction of the first microprocessor (the Intel 4004) in 1970 and the first
widely used microprocessor (the Intel 8080) in 1974, this class of CPUs has
almost completely overtaken all other central processing unit implementation
methods. Mainframe and minicomputer manufacturers of the time launched
proprietary IC development programs to upgrade their older computer
architectures, and eventually produced instruction set compatible
microprocessors that were backward-compatible with their older hardware and
software. Combined with the advent and eventual vast success of the now
ubiquitous personal computer, the term "CPU" is now applied almost exclusively
to microprocessors.
Previous generations of CPUs were implemented as discrete
components and numerous small integrated circuits (ICs) on one or more circuit
boards. Microprocessors, on the other hand, are CPUs manufactured on a very
small number of ICs; usually just one. The overall smaller CPU size as a result
of being implemented on a single die means faster switching time because of
physical factors like decreased gate parasitic capacitance. This has allowed
synchronous microprocessors to have clock rates ranging from tens of megahertz
to several gigahertz. Additionally, as the ability to construct exceedingly
small transistors on an IC has increased, the complexity and number of
transistors in a single CPU has increased dramatically. This widely observed
trend is described by Moore's law, which has proven to be a fairly accurate
predictor of the growth of CPU (and other IC) complexity to date.
While the complexity, size, construction, and general form
of CPUs have changed drastically over the past sixty years, it is notable that
the basic design and function has not changed much at all. Almost all common
CPUs today can be very accurately described as von Neumann stored-program
machines. As the aforementioned Moore's law continues to hold true, concerns
have arisen about the limits of integrated circuit transistor technology.
Extreme miniaturization of electronic gates is causing the effects of phenomena
like electromigration and subthreshold leakage to become much more significant.
These newer concerns are among the many factors causing researchers to
investigate new methods of computing such as the quantum computer, as well as to
expand the usage of parallelism and other methods that extend the usefulness of
the classical von Neumann model.
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