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Eight-bit CPUs normally use an 8-bit data bus and a 16-bit
16-bit

16-bit architectureThe HP 2100#Descendants and variants , introduced in 1975, was the world's first 16-bit microprocessor.Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816....
 address bus which means that their address space
Address space

In computing, an address space defines a range of discrete addresses, each of which may correspond to a physical or virtual memory register, a Node , peripheral device, disk sector or other logical or physical entity....
 is limited to 64 KB
KB

The abbreviation KB or kb can refer to:*Kilobit , a unit of information used, for example, to quantify computer memory or storage capacity...
s. This is not a "natural law", however, so there are exceptions.

The first widely adopted 8-bit microprocessor
Microprocessor

A microprocessor incorporates most or all of the functions of a central processing unit on a single integrated circuit . The first microprocessors emerged in the early 1970s and were used for electronic calculators, using Binary-coded decimal arithmetic on 4-bit Word ....
 was the Intel 8080
Intel 8080

The Intel 8080 was an early microprocessor designed and manufactured by Intel. The 8-bit microprocessor was released in April 1974 running at 2 megahertz , and is generally considered to be the first truly usable microprocessor....
, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M
CP/M

CP/M is an operating system originally created for Intel 8080/Intel 8085 based microcomputers by Gary Kildall of Digital Research. Initially confined to single tasking on 8-bit processors and no more than 64 kilobytes of memory, later versions of CP/M added multi-user variations, and were migrated to 16-bit processors....
 operating system
Operating system

An operating system is an interface between hardware and applications; it is responsible for the management and coordination of activities and the sharing of the limited resources of the computer....
. The Zilog Z80
Zilog Z80

The Zilog Z80 is an 8-bit microprocessor designed and sold by Zilog from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes....
 (compatible with the 8080) and the Motorola 6800
Motorola 6800

The 6800 is an 8-bit microprocessor produced by Motorola and released shortly after the Intel 8080 in late 1974. It had 78 instructions, including the famous, undocumented Halt and Catch Fire bus test instruction....
 were also used in similar computers.






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Eight-bit CPUs normally use an 8-bit data bus and a 16-bit
16-bit

16-bit architectureThe HP 2100#Descendants and variants , introduced in 1975, was the world's first 16-bit microprocessor.Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816....
 address bus which means that their address space
Address space

In computing, an address space defines a range of discrete addresses, each of which may correspond to a physical or virtual memory register, a Node , peripheral device, disk sector or other logical or physical entity....
 is limited to 64 KB
KB

The abbreviation KB or kb can refer to:*Kilobit , a unit of information used, for example, to quantify computer memory or storage capacity...
s. This is not a "natural law", however, so there are exceptions.

The first widely adopted 8-bit microprocessor
Microprocessor

A microprocessor incorporates most or all of the functions of a central processing unit on a single integrated circuit . The first microprocessors emerged in the early 1970s and were used for electronic calculators, using Binary-coded decimal arithmetic on 4-bit Word ....
 was the Intel 8080
Intel 8080

The Intel 8080 was an early microprocessor designed and manufactured by Intel. The 8-bit microprocessor was released in April 1974 running at 2 megahertz , and is generally considered to be the first truly usable microprocessor....
, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M
CP/M

CP/M is an operating system originally created for Intel 8080/Intel 8085 based microcomputers by Gary Kildall of Digital Research. Initially confined to single tasking on 8-bit processors and no more than 64 kilobytes of memory, later versions of CP/M added multi-user variations, and were migrated to 16-bit processors....
 operating system
Operating system

An operating system is an interface between hardware and applications; it is responsible for the management and coordination of activities and the sharing of the limited resources of the computer....
. The Zilog Z80
Zilog Z80

The Zilog Z80 is an 8-bit microprocessor designed and sold by Zilog from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes....
 (compatible with the 8080) and the Motorola 6800
Motorola 6800

The 6800 is an 8-bit microprocessor produced by Motorola and released shortly after the Intel 8080 in late 1974. It had 78 instructions, including the famous, undocumented Halt and Catch Fire bus test instruction....
 were also used in similar computers. The Z80 and the MOS Technology 6502
MOS Technology 6502

The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured central processing unit on the market by a considerable margin, costing less than one-sixth the price of competing designs from larger companies such...
 8-bit CPUs were widely used in home computer
Home computer

A home computer was a class of personal computer entering the market in 1977 and becoming common during the 1980s. They were marketed to consumers as accessible personal computers, more capable than video game consoles....
s and game consoles of the 70s and 80s. Many 8-bit CPUs or microcontroller
Microcontroller

A microcontroller is a small computer on a single integrated circuit consisting of a relatively simple CPU combined with support functions such as a crystal oscillator, timers, watchdog, serial and analog I/O etc....
s are the basis of today's ubiquitous embedded systems.

There are 28 (256) possible values for 8 bits.

Why 8 bits?

4-bit
4-bit

The Intel 4004, the world's first commercially available single-integrated circuit microprocessor, was a 4-bit central processing unit. The F-14 Tomcat's Central Air Data Computer was created a year before the 4004, but its existence was Classified information by the United States Navy until 1997....
 microprocessors were developed in the early 1970s starting with the Intel 4004
Intel 4004

The Intel 4004 is a 4-bit central processing unit released by Intel Corporation in 1971. The 4004 is the first complete CPU on one chip, the first commercially available microprocessor, a feat made possible by the use of the new silicon gate technology allowing the integration of a higher number of transistors and a faster speed than was pos...
. Intel swiftly followed with 8-bit processors, and most competitors to Intel started with 8-bits. Performance and memory limitations meant that 4-bit processors fell out of more demanding applications quickly (4-bit processors are still mass-produced however).

List of 8-bit CPUs

A CPU can be classified on the basis of the data
DATA

Debt, AIDS, Trade in Africa is a multinational Non-governmental organization founded in January 2002 in London by U2's Bono along with Robert Sargent Shriver III and activists from the Jubilee 2000 Drop the Debt campaign....
 it can access in a single operation
Instruction (computer science)

In computer science, an instruction is a single operation of a central processing unit defined by an instruction set architecture. In a broader sense, an "instruction" may be any representation of an element of an executable program, such as a bytecode....
. An 8-bit processor can access 8 bits of data in a single operation, as opposed to a 16-bit
16-bit

16-bit architectureThe HP 2100#Descendants and variants , introduced in 1975, was the world's first 16-bit microprocessor.Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816....
 processor, which can access 16 bits of data in a single operation.

Examples of 8-bit processors (very incomplete):
  • Intel 8008
    Intel 8008

    The Intel 8008 was an early byte-oriented microprocessor designed and manufactured by Intel and introduced in April 1972. Originally known as the 1201, the chip was commissioned by Computer Terminal Corporation to implement an instruction set designed for their Datapoint 2200 programmable terminal....
  • Intel 8080
    Intel 8080

    The Intel 8080 was an early microprocessor designed and manufactured by Intel. The 8-bit microprocessor was released in April 1974 running at 2 megahertz , and is generally considered to be the first truly usable microprocessor....
     (8008 source compatible)
  • Intel 8085
    Intel 8085

    The Intel 8085 is an 8-bit microprocessor introduced by Intel in 1977. It was binary-compatible with the more-famous Intel 8080 but required less supporting hardware, thus allowing simpler and less expensive microcomputer systems to be built....
     (8080 binary compatible)
  • Intel 8051
    Intel 8051

    The Intel 8051 is a Harvard architecture, single chip microcontroller which was developed by Intel in 1980 for use in embedded systems. Intel's original versions were popular in the 1980s and early 1990s, but has largely been superseded by a vast range of faster and/or functionally enhanced 8051-compatible devices manufactured by more th...
     (Harvard architecture)
  • Zilog Z80
    Zilog Z80

    The Zilog Z80 is an 8-bit microprocessor designed and sold by Zilog from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes....
      (8080 binary compatible)
  • Zilog Z180
    Zilog Z180

    The Zilog Z180 8-bit processor is a successor of the Zilog Z80 CPU. It is compatible with the large base of software written for the Z80. The Z180 family adds higher performance and integrated peripheral functions like clock generator, 16 bit counters/timers, interrupt controller, wait-state generators, serial ports and a DMA controller....
      (Z80 binary compatible)
  • Zilog Z8
    Zilog Z8

    The Zilog Z8 is a microcontroller architecture, originally introduced in 1979, which today also includes the eZ8 Encore!, eZ8 Encore! XP, and eZ8 Encore! MC families....
  • Zilog eZ80
    Zilog eZ80

    The ZiLOG eZ80 is an 8-bit microprocessor which is essentially an updated version of the company's earlier Zilog Z80 8-bit microprocessor.The eZ80 is binary compatible with Z80 and Z180, but almost four times as fast as the old Z80 chip at the same core frequency....
      (Z80 binary compatible)
  • Motorola 6800
    Motorola 6800

    The 6800 is an 8-bit microprocessor produced by Motorola and released shortly after the Intel 8080 in late 1974. It had 78 instructions, including the famous, undocumented Halt and Catch Fire bus test instruction....
  • Motorola 6803
  • Motorola 6809
    Motorola 6809

    The Motorola 6809 is an 8-bit microprocessor central processing unit from Motorola, introduced circa 1977-78. It was a major advance over both its predecessor, the Motorola 6800, and the related, MOS Technology 6502....
     (partially 6800 compatible)
  • MOS Technology 6502
    MOS Technology 6502

    The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured central processing unit on the market by a considerable margin, costing less than one-sixth the price of competing designs from larger companies such...
  • Microchip PIC10
    PIC microcontroller

    PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1640 originally developed by General Instrument's Microelectronics Division....
  • Microchip PIC12
    PIC microcontroller

    PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1640 originally developed by General Instrument's Microelectronics Division....
  • Microchip PIC16
    PIC microcontroller

    PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1640 originally developed by General Instrument's Microelectronics Division....
  • Microchip PIC18
    PIC microcontroller

    PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1640 originally developed by General Instrument's Microelectronics Division....
  • Atmel AVR
    Atmel AVR

    The AVR is a Modified Harvard architecture 8-bit Reduced instruction set computer single chip microcontroller which was developed by Atmel in 1996....
     family microcontrollers
  • series microcontrollers