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In computer architecture, 8-bit integers, memory addresses, or other data units are those that are 8 bits (1 octet) wide. Also, 8-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. '8-bit' is also a generation of microcomputers in which 8-bit microprocessors were the norm.
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, 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 was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system; it had 8-bit data words and 16-bit addresses. The Zilog Z80 (compatible with the 8080) and the Motorola 6800 were also used in similar computers. The Z80 and the MOS Technology 6502 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 microcontrollers are the basis of today's ubiquitous embedded systems.
There are 28 (256) different possible values for 8 bits. When unsigned, it has possible values ranging from 0 to 255; when signed, it has -128 to 127.
Eight-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 wide (i.e. 16-bit), due to practical and economical considerations. This implies a direct address space of only 64 kB on most 8-bit processors.
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 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; it, and variants of it, were used in a number of personal computers such as the Apple I and Apple II, the Atari 8-bit family, the BBC Micro, and the Commodore PET and Commodore VIC-20, and in a number of video game consoles such as the Atari 2600 and the Nintendo Entertainment System.
|Intel||8008||1972||Datapoint 2200 compatible|
|Intel||8080||1974||8008 source compatible|
|MOS||6502||1975||Similar to 6800, but incompatible|
|Microchip||PIC||1975||Harvard architecture microcontroller|
|Electronic Arrays||EA9002||1976||8-bit data, 12-bit addressing|
|Zilog||Z80||1976||8080 binary compatible|
|Intel||8085||1977||8080 binary compatible|
|Zilog||Z8||1978||Harvard architecture microcontroller|
|Motorola||6809||1978||6800 source compatible|
|Intel||8051||1980||Harvard architecture microcontroller|
|Motorola||68008||1982||32-bit registers, 20-bit or 22-bit addressing, three 16-bit ALUs, 8-bit data bus; Motorola 68000 software-compatible, 6809 hardware-compatible|
|MOS||6510||1982||Enhanced 6502 custom-made for use in the Commodore 64|
|Ricoh||2A03||1982||6502 clone minus BCD instructions for the Nintendo Entertainment System|
|Zilog||Z180||1985||Z80 binary compatible|
|Zilog||EZ80||1999||Z80 binary compatible|
|Hudson||HuC6280||65C02 binary compatible|
- Amdahl, G. M.; Blaauw, G. A.; Brooks, F. P. (1964). "Architecture of the IBM System/360" (PDF). IBM Journal of Research and Development. 8 (2): 87–101. doi:10.1147/rd.82.0087. Archived (PDF) from the original on 2017-08-10.
- "NEC 78K0". NEC. Archived from the original on 2008-10-28. Retrieved 2009-02-10.