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Incomputer architecture,8-bitintegers or otherdata units are those that are 8bits wide (1octet). Also, 8-bitcentral processing unit (CPU) andarithmetic logic unit (ALU) architectures are those that are based onregisters ordata buses of that size.Memory addresses (and thusaddress buses) for 8-bitCPUs are generally larger than 8-bit, usually16-bit. 8-bitmicrocomputers are microcomputers that use 8-bitmicroprocessors.
The term '8-bit' is also applied to thecharacter sets that could be used on computers with 8-bit bytes, the best known being various forms ofextended ASCII, including theISO/IEC 8859 series of national character sets – especiallyLatin 1 for English and Western European languages.
TheIBM System/360 introducedbyte-addressable memory with 8-bit bytes, as opposed to bit-addressable or decimal digit-addressable or word-addressable memory, although itsgeneral-purpose registers were32 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; theIBM 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.[1]
The first widely adopted 8-bitmicroprocessor was theIntel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running theCP/Moperating system; it had 8-bit data words and 16-bit addresses. TheZilog Z80 (compatible with the 8080) and theMotorola 6800 were also used in similar computers. The Z80 and theMOS Technology 6502 8-bit CPUs were widely used inhome computers andsecond- andthird-generation game consoles of the 1970s and 1980s. Many 8-bit CPUs ormicrocontrollers are the basis of today's ubiquitousembedded systems.
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 1976Zilog Z80, one of the most popular 8-bit CPUs (though with4-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.[2]
An 8-bit register can store 28 different values. Therange ofinteger values that can be stored in 8 bits depends on theinteger representation used. With the two most common representations, the range is 0 through 255(28 − 1) for representation as an (unsigned)binary number, and −128(−1 × 27) through 127(27 − 1) for representation astwo's complement.
8-bit CPUs use an8-bitdata bus and can therefore access 8 bits of data in a singlemachine instruction. The address bus is typically a double octet (16 bits) wide, due to practical and economical considerations. This implies a directaddress space of 64 KB (65,536 bytes) on most 8-bit processors.
Mosthome computers from the 8-bit era fully exploited the address space, such as theBBC Micro (Model B) with 32 KB ofRAM plus 32 KB ofROM. Others like the very popularCommodore 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 includedBASIC language interpreter in ROM);[3] without exploitingbank 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 SinclairZX80 (while the later very popularZX Spectrum had more memory), or even only 128 bytes of RAM (plusstorage from aROM cartridge), as in an early game consoleAtari 2600 and thus 8-bit addressing would have been enough for the RAM, if it would not have needed to cover ROM too). TheCommodore 128, and other 8-bit systems, meaning still with 16-bit addressing, could use more than 64 KB, i.e. 128 KB RAM, also theBBC 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 theMOS Technology6502 CPU, where thezero 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.
Someindex registers, such as the two in the 6502, are 8-bit. This limits the size of the arrays addressed usingindexed addressing instructions to objects of up to 256 bytes without requiring more complicated code. Other 8-bit CPUs, such as theMotorola 6800 andIntel 8080, have 16-bit index registers.
The first commercial 8-bit processor was theIntel 8008 (1972) which was originally intended for theDatapoint 2200intelligent terminal. Most competitors toIntel 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.
TheMOS Technology 6502, and variants of it, were used in personal computers, such as theApple I,Apple II,Atari 8-bit computers,BBC Micro,PET,VIC-20, and inhome video game consoles such as theAtari 2600 and theNintendo Entertainment System.
| Manufacturer | Processor | Year | Comment |
|---|---|---|---|
| Intel | 8008 | 1972 | Datapoint 2200 compatible |
| Intel | 8080 | 1974 | 8008 source compatible |
| Motorola | 6800 | 1974 | |
| Signetics | 2650 | 1975 | |
| Fairchild | F8 | 1975 | |
| MOS | 6502 | 1975 | Similar to 6800, but incompatible |
| Microchip | PIC | 1975 | Harvard architecture microcontroller |
| Electronic Arrays | EA9002 | 1976 | 8-bit data, 12-bit addressing |
| RCA | 1802 | 1976 | |
| Zilog | Z80 | 1976 | 8080 binary compatible |
| Intel | 8085 | 1976 | 8080 binary compatible |
| Zilog | Z8 | 1978 | Harvard architecture microcontroller |
| Motorola | 6809 | 1978 | 6800 source compatible |
| Intel | 8051 | 1980 | Harvard architecture microcontroller |
| MOS | 6510 | 1982 | Enhanced 6502 custom-made for use in theCommodore 64 |
| Ricoh | 2A03 | 1982 | 6502 clone minus BCD instructions for theNintendo Entertainment System |
| Zilog | Z180 | 1985 | Z80 binary compatible |
| Motorola | 68HC11 | 1985 | |
| Hudson | HuC6280 | 1987 | 65C02 binary compatible |
| Atmel | AVR | 1996 | |
| Zilog | eZ80 | 1999 | Z80 binary compatible |
| Infineon | XC800 | 2005 | |
| Freescale | 68HC08 | ? | |
| Motorola | 6803 | ? | |
| NEC | 78K0[4] | ? |
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 using7400-series integrated circuits on abreadboard.[5][6] 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.
