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In
computer architecture In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, t ...
, 8-bit
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
s or other
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...
units are those that are 8
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
s wide (1
octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compos ...
). Also, 8-bit
central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, an ...
(CPU) and
arithmetic logic unit In computing, an arithmetic logic unit (ALU) is a Combinational logic, combinational digital circuit that performs arithmetic and bitwise operations on integer binary numbers. This is in contrast to a floating-point unit (FPU), which operates on ...
(ALU) architectures are those that are based on
register 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. * ''Register'', a 2017 album by Travis Miller * Registration (organ), the ...
s or
data bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
es of that size.
Memory address In computing, a memory address is a reference to a specific memory location used at various levels by software and hardware. Memory addresses are fixed-length sequences of digits conventionally displayed and manipulated as unsigned integers. Su ...
es (and thus
address bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
es) for 8-bit CPUs are generally larger than 8-bit, usually 16-bit. 8-bit
microcomputer A microcomputer is a small, relatively inexpensive computer having a central processing unit (CPU) made out of a microprocessor. The computer also includes memory and input/output (I/O) circuitry together mounted on a printed circuit board (PC ...
s are microcomputers that use 8-bit
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
s. The term '8-bit' is also applied to the
character set Character encoding is the process of assigning numbers to graphical characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using digital computers. The numerical values that ...
s that could be used on computers with 8-bit bytes, the best known being various forms of
extended ASCII Extended ASCII is a repertoire of character encodings that include (most of) the original 96 ASCII character set, plus up to 128 additional characters. There is no formal definition of "extended ASCII", and even use of the term is sometimes critic ...
, including the
ISO/IEC 8859 ISO/IEC 8859 is a joint ISO and IEC series of standards for 8-bit character encodings. The series of standards consists of numbered parts, such as ISO/IEC 8859-1, ISO/IEC 8859-2, etc. There are 15 parts, excluding the abandoned ISO/IEC 8859-12. ...
series of national character sets especially Latin 1 for English and Western European languages. The
IBM System/360 The IBM System/360 (S/360) is a family of mainframe computer systems that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applica ...
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 A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only. ...
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 Yor ...
(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, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
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 8008 design, although without binary compatibil ...
, 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 created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initial ...
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
; it had 8-bit data words and 16-bit addresses. The
Zilog Z80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples wer ...
(compatible with the 8080) and the
Motorola 6800 The 6800 ("''sixty-eight hundred''") is an 8-bit computing, 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the Motorola 6800 family, M6800 Microcomputer System (latter dubbed ''68xx' ...
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 microprocessor that was designed by a small t ...
8-bit CPUs were widely used in home computers and second- and third-generation game consoles of the 1970s and 1980s. Many 8-bit CPUs or
microcontroller A microcontroller (MCU for ''microcontroller unit'', often also MC, UC, or μC) is a small computer on a single VLSI integrated circuit (IC) chip. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable i ...
s are the basis of today's ubiquitous
embedded systems An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...
.


Details

An 8-bit register can store 28 different values. The
range Range may refer to: Geography * Range (geographic), a chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra) ** Mountain range, a group of mountains bordered by lowlands * Range, a term used to i ...
of
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
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 c ...
)
binary number A binary number is a number expressed in the base-2 numeral system or binary numeral system, a method of mathematical expression which uses only two symbols: typically "0" (zero) and "1" ( one). The base-2 numeral system is a positional notatio ...
, and −128 (−1 × 27) through 127 (27 − 1) for representation as
two's complement Two's complement is a mathematical operation to reversibly convert a positive binary number into a negative binary number with equivalent (but negative) value, using the binary digit with the greatest place value (the leftmost bit in big- endian ...
. 8-bit CPUs use an
8-bit In computer architecture, 8-bit Integer (computer science), integers or other Data (computing), data units are those that are 8 bits wide (1 octet (computing), octet). Also, 8-bit central processing unit (CPU) and arithmetic logic unit (ALU) arc ...
data bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
and can therefore access 8 bits of data in a single
machine instruction In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
. The address bus is typically a double octet (
16 bit 16-bit microcomputers are microcomputers that use 16-bit microprocessors. A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two mos ...
s) wide, due to practical and economical considerations. This implies a direct
address space In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity. For software programs to save and retrieve st ...
of 64  KB (65,536 bytes) on most 8-bit processors. Most home computers 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 microcomputers and associated peripherals designed and built by Acorn Computers in the 1980s for the BBC Computer Literacy Project. Designed with an emphas ...
(Model B) with 32 KB of
RAM Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * ...
plus 32 KB of
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * R ...
. Others like the very popular
Commodore 64 The Commodore 64, also known as the C64, is an 8-bit home computer introduced in January 1982 by Commodore International (first shown at the Consumer Electronics Show, January 7–10, 1982, in Las Vegas). It has been listed in the Guinness ...
had full 64 KB RAM, plus 20 KB ROM, meaning with 16-bit addressing you could not 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 languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
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 Sinclair
ZX80 The Sinclair ZX80 is a home computer launched on 29 January 1980 by Science of Cambridge Ltd. (later to be better known as Sinclair Research). It is notable for being one of the first computers available in the United Kingdom for less than a h ...
(while the later very popular Sinclair
ZX Spectrum The ZX Spectrum () is an 8-bit computing, 8-bit home computer that was developed by Sinclair Research. It was released in the United Kingdom on 23 April 1982, and became Britain's best-selling microcomputer. Referred to during development as t ...
had more memory), or even only 128 bytes of RAM (plus storage from a
ROM cartridge A ROM 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 device such as a home computer, video game console or, to a lesser extent, electroni ...
), as in an early game console
Atari 2600 The Atari 2600, initially branded as the Atari Video Computer System (Atari VCS) from its release until November 1982, is a home video game console developed and produced by Atari, Inc. Released in September 1977, it popularized microprocessor- ...
and thus 8-bit addressing would have been enough for the RAM, if it wouldn't have needed to cover ROM too). The
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 home computer that was commercially released by Commodore Business Machines (CBM). Introduced in January 1985 at the ...
, 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 in early 1986. It was designed and built for the BBC, British Broadcasting Corporation (BBC) and was the successor to the BBC Micro, BBC Micro Model B. The Master 128 remained in produ ...
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), later known as CSG (Commodore Semiconductor Group) and GMT Microelectronics, was a semiconductor design and fabrication company based in Audubon, Pennsylvania. It is mo ...
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 microprocessor that was designed by a small te ...
CPU, where the
zero page The zero page or base page is the block of memory at the very beginning of a computer's address space; that is, the page whose starting address is zero. The size of a page depends on the context, and the significance of zero page memory versus h ...
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 is a processor register (or an assigned memory location) used for pointing to operand addresses during the run of a program. It is useful for stepping through strings and arrays. It can also be used for hol ...
s are 8-bit (while other "8-bit" CPUs, such as
Motorola 6800 The 6800 ("''sixty-eight hundred''") is an 8-bit computing, 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the Motorola 6800 family, M6800 Microcomputer System (latter dubbed ''68xx' ...
had 16-bit index registers), such as the 6502 CPU, and then the size of the arrays addressed using
indexed addressing Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture define how the machine language instructions i ...
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 designed by Computer Terminal Corporation (CTC), implemented and manufactured by Intel, and introduced in April 1972. It is an 8-bit C ...
(1972) which was originally intended for the
Datapoint 2200 The Datapoint 2200 was a mass-produced programmable computer terminal usable as a computer, designed by Computer Terminal Corporation (CTC) founders Phil Ray and Gus Roche and announced by CTC in June 1970 (with units shipping in 1971). It was i ...
intelligent terminal A computer terminal is an electronic or electromechanical hardware device that can be used for entering data into, and transcribing data from, a computer or a computing system. The teletype was an example of an early-day hard-copy terminal and ...
. Most competitors to
Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...
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 microprocessor that was designed by a small t ...
. 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 an 8-bit desktop computer released by the Apple Computer Company (now Apple Inc.) in 1976. It was designed by Steve Wozniak. The idea ...
and
Apple II The Apple II (stylized as ) is an 8-bit home computer and one of the world's first highly successful mass-produced microcomputer products. It was designed primarily by Steve Wozniak; Jerry Manock developed the design of Apple II's foam-m ...
, the
Atari 8-bit family The Atari 8-bit family is a series of 8-bit home computers introduced by Atari, Inc. in 1979 as the Atari 400 and Atari 800. The series was successively upgraded to Atari 1200XL , Atari 600XL, Atari 800XL, Atari 65XE, Atari 130XE, Atari 800XE, ...
, the
BBC Micro The British Broadcasting Corporation Microcomputer System, or BBC Micro, is a series of microcomputers and associated peripherals designed and built by Acorn Computers in the 1980s for the BBC Computer Literacy Project. Designed with an emphas ...
, and the
Commodore PET The Commodore PET is a line of personal computers produced starting in 1977 by Commodore International. A single all-in-one case combines a MOS Technology 6502 microprocessor, Commodore BASIC in read-only memory, keyboard, monochrome monitor, an ...
and
VIC-20 The VIC-20 (known as the VC-20 in Germany and the VIC-1001 in Japan) is an 8-bit home computer that was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the PE ...
, and in a number of video game consoles, such as the
Atari 2600 The Atari 2600, initially branded as the Atari Video Computer System (Atari VCS) from its release until November 1982, is a home video game console developed and produced by Atari, Inc. Released in September 1977, it popularized microprocessor- ...
and the
Nintendo Entertainment System The Nintendo Entertainment System (NES) is an 8-bit third-generation home video game console produced by Nintendo. It was first released in Japan in 1983 as the commonly known as the The NES, a redesigned version, was released in America ...
.


Use for training, prototyping, and general hardware education

8-bit processors continue to be designed today for general education about computer hardware, as well as for hobbyists' interests. One such CPU was designed and implemented using
7400-series integrated circuits The 7400 series of integrated circuits (ICs) are a popular logic family of transistor–transistor logic (TTL) logic chips. In 1964, Texas Instruments introduced the SN5400 series of logic chips, in a ceramic semiconductor package. A low-co ...
on a
breadboard A breadboard, solderless breadboard, or protoboard is a construction base used to build semi-permanent prototypes of electronic circuits. Unlike a perfboard or stripboard, breadboards do not require soldering or destruction of tracks and are ...
. Designing 8-bit CPU's and their respective assemblers is a common training exercise for engineering students, engineers, and hobbyists. FPGA's are used for this purpose.


See also

*
Kenbak-1 The Kenbak-1 is considered by the Computer History Museum, the Computer Museum of America and the American Computer Museum to be the world's first "personal computer", invented by John Blankenbaker (born 1929) of Kenbak Corporation in 1970, and fi ...


References

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