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In
computer architecture In computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunn ...
, 8-bit
integer An integer (from the Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to ...
s or other
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...
units are those that are 8
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algo ...
s wide (1
octet Octet may refer to: Music * Octet (music) In music, an octet is a musical ensemble consisting of eight Musical instrument, instruments or voices, or a musical composition written for such an ensemble. Octets in classical music Octets in Europea ...
). Also, 8-bit
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of ...

CPU
and ALU architectures are those that are based on
register A register is an authoritative list of one kind of information. Register or registration may refer to: Arts entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Regis ...
s or
data bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a machine that can be programmed to carry ...
es of that size.
Memory address In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
es (and thus
address bus In computer architecture In computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other ...
es) for 8-bit CPUs are generally larger than 8-bit, usually 16-bit, while they could in theory be 8-bit, and in some situations 8-bit addresses are also used with 16-bit addresses mainly used. '8-bit' is also a generation of
microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) circuitry mounted on a single printed ci ...
s in which 8-bit
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip ...

microprocessor
s were the norm. The term '8-bit' is also applied to the
character set Character encoding is the process of assigning numbers to Graphics, graphical character (computing), characters, especially the written characters of Language, human language, allowing them to be Data storage, stored, Data communication, transmit ...
s that could be used on computers with 8-bit bytes, the best known being various forms of
extended ASCII Extended ASCII (EASCII or high ASCII) character encoding Character encoding is the process of assigning numbers to Graphics, graphical character (computing), characters, especially the written characters of Language, human language, allowing th ...
, including the
ISO/IEC 8859 ISO/IEC 8859 is a joint ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for ...
series of national character sets especially
Latin 1 ISO/IEC 8859-1:1998, ''Information technology — 8-bit computing, 8-bit single-byte coded graphic character (computing), character sets — Part 1: Latin alphabet No. 1'', is part of the ISO/IEC 8859 series of ASCII-based standard character ...
for English and Western European languages. The
IBM System/360 The IBM System/360 (S/360) is a family of mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or ...
introduced byte-addressable memory with 8-bit bytes, as opposed to bit-addressable or decimal digit-addressable or word-addressable memory, although its
general-purpose registers General-purpose may refer to: * General-purpose technology General-purpose technologies (GPTs) are technologies that can affect an entire Economic system, economy (usually at a national or global level). GPTs have the potential to drastically alter ...
were 32 bits wide, and addresses were contained in the lower 24 bits of those addresses. Different models of System/360 had different internal data path widths; the
IBM System/360 Model 30 The IBM System/360 Model 30 was a low-end member of the IBM System/360 family. It was announced on April 7, 1964, shipped in 1965, and withdrawn on October 7, 1977. The Model 30 was designed by IBM's General Systems Division in Endicott, New York, ...
(1965) implemented the 32-bit System/360 architecture, but had an 8-bit native path width, and performed 32-bit arithmetic 8 bits at a time. The first widely adopted 8-bit
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip ...

microprocessor
was the
Intel 8080 The Intel 8080 (''"eighty-eighty"'') is the second 8-bit microprocessor designed and manufactured by Intel. It first appeared in April 1974 and is an extended and enhanced variant of the earlier Intel 8008, 8008 design, although without binary ...
, being used in many hobbyist computers of the late 1970s and early 1980s, often running the
CP/M CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system An operating system (OS) is system software System software is software designed to provide a platform ...
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
; it had 8-bit data words and 16-bit addresses. The
Zilog Z80 The Z80 is an 8-bit In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. Some definitions of archite ...

Zilog Z80
(compatible with the 8080) and the
Motorola 6800 The 6800 ("''sixty-eight hundred''") is an 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the M6800 Microcomputer System that also included serial and parallel interface integrated c ...

Motorola 6800
were also used in similar computers. The Z80 and the
MOS Technology 6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit In computer architecture In computer ...
8-bit CPUs were widely used in
home computer Home computers were a class of microcomputers that entered the market in 1977 and became common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a s ...

home computer
s and second- and third-generation game consoles of the 1970s and 1980s. Many 8-bit CPUs or
microcontroller A microcontroller (MCU for ''microcontroller unit'') is a small computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets o ...

microcontroller
s are the basis of today's ubiquitous
embedded systems An embedded system is a computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Com ...
.


Details

An 8-bit register can store 28 different values. The
range Range may refer to: Geography * Range (geographic)A range, in geography, is a chain of hill A hill is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body. Landforms together ...
of
integer An integer (from the Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to ...
values that can be stored in 8 bits depends on the integer representation used. With the two most common representations, the range is 0 through 255 (28 − 1) for representation as an (
unsigned Unsigned can refer to: * An unsigned artist is a musical artist or group not attached or signed to a record label ** Unsigned Music Awards, ceremony noting achievements of unsigned artists ** Unsigned band web, online community * Similarly, the con ...
)
binary number In mathematics and digital electronics Digital electronics is a field of electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electr ...
, and −128 (−1 × 27) through 127 (27 − 1) for representation as
two's complement Two's complement is a mathematical operation In mathematics, an operation is a Function (mathematics), function which takes zero or more input values (called ''operands'') to a well-defined output value. The number of operands is the arity of the ...
. 8-bit CPUs use an
8-bit In computer architecture, 8-bit integer (computer science), integers or other data#Uses of data in computing, data units are those that are 8 bits wide (1 octet). Also, 8-bit central processing unit, CPU and arithmetic logic unit, ALU arch ...
data bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a machine that can be programmed to carry ...
and can therefore access 8 bits of data in a single
machine instruction In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
. The address bus is typically a double octet (
16 bit 16-bit microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a microprocessor, Computer memory, memory and minimal input/output (I/O) circuitry mounted ...
s) wide, due to practical and economical considerations. This implies a direct
address space In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
of 64  KB (65,536 bytes) on most 8-bit processors. Most
home computer Home computers were a class of microcomputers that entered the market in 1977 and became common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a s ...

home computer
s from the 8-bit era fully exploited the address space, such as the
BBC Micro The British Broadcasting Corporation Microcomputer System, or BBC Micro, is a series of microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a micr ...
(Model B) with 32 KB of
RAM Random-access memory (RAM; ) is a form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic proces ...
plus 32 KB of
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality The risk of mortality (ROM) provides a medical classification to estimate the likelihood of inhospital death for a patient. The ROM classes are minor, moderate, major, and ex ...
. Others like the very popular
Commodore 64 The Commodore 64, also known as the C64 or the CBM 64, is an 8-bit In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation ...

Commodore 64
had full 64 KB RAM, plus 20 KB ROM, meaning with 16-bit addressing you couldn't use all of the RAM by default (e.g. from the included
BASIC BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming language In computer science Computer science deals with the theoretical foundations of information, algorithms and the ar ...

BASIC
language interpreter in ROM); without exploiting
bank switching Bank switching is a technique used in computer design to increase the amount of usable memory beyond the amount directly addressable by the processor instructions. It can be used to configure a system differently at different times; for example ...
, which allows for breaking the 64 KB (RAM) limit in some systems. Other computers would have as low as 1 KB (plus 4 KB ROM), such as the Spectrum
ZX80 The Sinclair ZX80 is a home computer Home computers were a class of s that entered the market in 1977 and became common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were ...

ZX80
(while the later very popular Sinclair
ZX Spectrum The ZX Spectrum () is an 8-bit In computer architecture, 8-bit integer (computer science), integers or other data#Uses of data in computing, data units are those that are 8 bits wide (1 octet). Also, 8-bit central processing unit, CPU ...

ZX Spectrum
had more memory), or even only 128 bytes of RAM (plus storage from a
ROM cartridge A game cartridge, usually referred to in context simply as a cartridge, cart, or card, is a replaceable part designed to be connected to a consumer electronics Consumer electronics or home electronics are electronic Electronic may refer ...
), as in an early game console
Atari 2600 The Atari 2600, originally branded as the Atari Video Computer System (Atari VCS) until November 1982, is a home video game console A home video game console is a video game console that is designed to be connected to a display device, suc ...

Atari 2600
and thus 8-bit addressing would have been enough for the RAM, if it wouldn't have needed to cover ROM too). The commercial failure,
Commodore 128 The Commodore 128, also known as the C128, C-128, C= 128,The "C=" represents the graphical part of the logo. is the last 8-bit In computer architecture, 8-bit integer (computer science), integers or other data#Uses of data in computing, d ...

Commodore 128
, and other 8-bit systems, meaning still with 16-bit addressing, could use more than 64 KB, i.e. 128 KB RAM, also the
BBC Master The BBC Master is a home computer released by Acorn Computers Acorn Computers Ltd. was a British computer company established in Cambridge, England, in 1978. The company produced a number of computers which were especially popular in the United ...
with it expandable to 512 KB of RAM. While in general 8-bit CPUs have 16-bit addressing, in some architectures you have both, such as in the
MOS Technology MOS Technology, Inc. ("MOS" being short for Metal Oxide Semiconductor The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), i ...
,
6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit In computer architecture In computer ...
CPU, where the
zero page The zero page or base page is the block of memory at the very beginning of a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform gen ...
is used extensively, saving one byte in the instructions accessing that page, and also having 16-bit addressing instructions that take 2 bytes for the address plus 1 for the opcode. Commonly
index register An index register in a computer's CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PC ...
s are 8-bit (while other "8-bit" CPUs, such as
Motorola 6800 The 6800 ("''sixty-eight hundred''") is an 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the M6800 Microcomputer System that also included serial and parallel interface integrated c ...

Motorola 6800
had 16-bit index registers), such as the 6502 CPU, and then the size of the arrays addressed using indexed addressing instructions are at most 256 bytes, without needing longer code, i.e. meaning 8-bit addressing to each individual array.


Notable 8-bit CPUs

The first commercial 8-bit processor was the
Intel 8008 The Intel 8008 ("''eight-thousand-eight''" or "''eighty-oh-eight''") is an early byte-oriented microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit ...

Intel 8008
(1972) which was originally intended for the
Datapoint 2200 The Datapoint 2200 was a mass-produced desktop personal computer A personal computer (PC) is a multi-purpose computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatica ...
intelligent terminal. Most competitors to
Intel Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Legal personalit ...

Intel
started off with such character oriented 8-bit microprocessors. Modernized variants of these 8-bit machines are still one of the most common types of processor in embedded systems. Another notable 8-bit CPU is the
MOS Technology 6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit In computer architecture In computer ...
. It, and variants of it, were used in a number of personal computers, such as the
Apple I The Apple Computer 1, originally released as the Apple Computer and known later as the Apple I, or Apple-1, is a desktop computer A desktop computer is a personal computer A personal computer (PC) is a multi-purpose computer whose siz ...

Apple I
and
Apple II The Apple II (stylized as apple ] '') is an 8-bit home computer">8-bit.html" ;"title="'') is an 8-bit">'') is an 8-bit home computer and one of the world's first highly successful mass-produced microcomputer products. It was designed primari ...

Apple II
, the
Atari 8-bit family The Atari 8-bit family is a series of 8-bit In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. Some ...
, the
BBC Micro The British Broadcasting Corporation Microcomputer System, or BBC Micro, is a series of microcomputer A microcomputer is a small, relatively inexpensive computer with a microprocessor as its central processing unit (CPU). It includes a micr ...
, and the
Commodore PET The Commodore PET is a line of personal computer A personal computer (PC) is a multi-purpose computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...

Commodore PET
and
Commodore VIC-20 The Commodore VIC-20 (known as the VC-20 in Germany and the VIC-1001 in Japan) is an 8-bit In computer architecture, 8-bit integer (computer science), integers or other data#Uses of data in computing, data units are those that are 8 bit ...
, and in a number of video game consoles, such as the
Atari 2600 The Atari 2600, originally branded as the Atari Video Computer System (Atari VCS) until November 1982, is a home video game console A home video game console is a video game console that is designed to be connected to a display device, suc ...

Atari 2600
and the
Nintendo Entertainment System The Nintendo Entertainment System (NES) is an 8-bit In computer architecture, 8-bit integer (computer science), integers or other data#Uses of data in computing, data units are those that are 8 bits wide (1 octet). Also, 8-bit centr ...

Nintendo Entertainment System
.


References

{{CPU technologies Data unit