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In computer architecture, 8-bit
integer An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
s or other
data Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...
units are those that are 8 bits wide (1 octet). Also, 8-bit
central processing unit A central processing unit (CPU), also called a central processor, main processor, or just processor, is the primary Processor (computing), processor in a given computer. Its electronic circuitry executes Instruction (computing), instructions ...
(CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers or data buses of that size. Memory addresses (and thus address buses) for 8-bit CPUs are generally larger than 8-bit, usually 16-bit. 8-bit microcomputers are microcomputers that use 8-bit
microprocessor A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...
s. The term '8-bit' is also applied to the character sets 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 series of national character sets especially Latin 1 for English and Western European languages. The IBM System/360 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 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 (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 (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...
was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M
operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...
; it had 8-bit data words and 16-bit addresses. The
Zilog Z80 The Zilog Z80 is an 8-bit computing, 8-bit microprocessor designed by Zilog that played an important role in the evolution of early personal computing. Launched in 1976, it was designed to be Backward compatibility, software-compatible with the ...
(compatible with the 8080) and the 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 computing, 8-bit microprocessor that was desi ...
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 ...
s and second- and third-generation game consoles of the 1970s and 1980s. Many 8-bit CPUs or microcontrollers are the basis of today's ubiquitous embedded systems. As of 2025, 8-bit microcontrollers cost well under a dollar (with some exceptions), but the cheapest 16-bit microcontrollers cost less than other types even including cheapest 8-bit (and are more powerful, and easier to program generally), making 8-bit legacy, i.e. not worth it for new applications; 32-bit microcontrollers are also well under half a dollar, close to cheapest 16-bit, and cheapest than most 8-bit.


Historical context

8-bit microprocessors were the first widely used microprocessors in the computing industry, marking a major shift from mainframes and minicomputers to smaller, more affordable systems. The introduction of 8-bit processors in the 1970s enabled the production of personal computers, leading to the popularization of computing and setting the foundation for the modern computing landscape. The 1976
Zilog Z80 The Zilog Z80 is an 8-bit computing, 8-bit microprocessor designed by Zilog that played an important role in the evolution of early personal computing. Launched in 1976, it was designed to be Backward compatibility, software-compatible with the ...
, one of the most popular 8-bit CPUs (though with 4-bit ALU, at least in the original), was discontinued in 2024 (its product line Z84C00), with Last Time Buy (LTB) orders by 14 June 2024.


Details

An 8-bit register can store 28 different values. The range of
integer An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
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 for representation as an ( unsigned)
binary number A binary number is a number expressed in the Radix, base-2 numeral system or binary numeral system, a method for representing numbers that uses only two symbols for the natural numbers: typically "0" (zero) and "1" (one). A ''binary number'' may ...
, and −128 through 127 for representation as two's complement. 8-bit CPUs use an 8-bit data bus and can therefore access 8 bits of data in a single machine instruction. The address bus is typically a double octet ( 16 bits) wide, due to practical and economical considerations. This implies a direct address space 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 ...
s from the 8-bit era fully exploited the address space, such as the
BBC Micro The BBC Microcomputer System, or BBC Micro, is a family of microcomputers developed and manufactured by Acorn Computers in the early 1980s as part of the BBC's Computer Literacy Project. Launched in December 1981, it was showcased across severa ...
(Model B) with 32 KB of RAM plus 32 KB of ROM. Others like the very popular Commodore 64 had full 64 KB RAM, plus 20 KB ROM, meaning with 16-bit addressing not all of the RAM could be used by default (e.g. from the included
BASIC Basic or BASIC may refer to: Science and technology * BASIC, a computer programming language * Basic (chemistry), having the properties of a base * Basic access authentication, in HTTP Entertainment * Basic (film), ''Basic'' (film), a 2003 film ...
language interpreter in ROM); without exploiting bank switching, 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 (while the later very popular
ZX Spectrum The ZX Spectrum () is an 8-bit computing, 8-bit home computer developed and marketed by Sinclair Research. One of the most influential computers ever made and one of the all-time bestselling British computers, over five million units were sold. ...
had more memory), or even only 128 bytes of RAM (plus storage from a ROM cartridge), as in an early game console Atari 2600 and thus 8-bit addressing would have been enough for the RAM, if it would not have needed to cover ROM too). The 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 with it expandable to 512 KB of RAM. While in general 8-bit CPUs have 16-bit addressing, in some architectures both are available, such as in the MOS Technology 6502 CPU, where the zero page 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. Some
index register An index register in a computer's central processing unit, 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 String (computer science ...
s, such as the two in the 6502, are 8-bit. This limits the size of the arrays addressed using indexed addressing instructions to objects of up to 256 bytes without requiring more complicated code. Other 8-bit CPUs, such as the Motorola 6800 and Intel 8080, have 16-bit index registers.


Notable 8-bit CPUs

The first commercial 8-bit processor was the Intel 8008 (1972) which was originally intended for the Datapoint 2200 intelligent terminal. Most competitors to
Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
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. 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 computing, 8-bit microprocessor that was desi ...
, and variants of it, were used in personal computers, such as the Apple I, Apple II, Atari 8-bit computers,
BBC Micro The BBC Microcomputer System, or BBC Micro, is a family of microcomputers developed and manufactured by Acorn Computers in the early 1980s as part of the BBC's Computer Literacy Project. Launched in December 1981, it was showcased across severa ...
, PET, VIC-20, and in home video game consoles such as the Atari 2600 and the Nintendo Entertainment System.


Use for training, prototyping, and general hardware education

8-bit processors continue to be designed for general education about computer hardware, as well as for hobbyists' interests. One such CPU was designed and implemented using 7400-series integrated circuits on a breadboard. Designing 8-bit CPUs and their respective assemblers is a common training exercise for engineering students, engineers, and hobbyists. FPGAs are used for this purpose.


See also

* Kenbak-1


References

{{Authority control Data unit 8-bit computers