View source for 8-bit computing

{{Short description|Computer architectures using an 8-bit word}}
{{Computer architecture bit widths}}
In [[Computer architecture|computer architecture]], '''8-bit''' [[Integer (computer science)|integer]]s or other [[Data (computing)|data]] units are those that are 8 [[Bit|bit]]s wide (1 [[Octet (computing)|octet]]). Also, 8-bit [[Central processing unit|central processing unit]] (CPU) and [[Arithmetic logic unit|arithmetic logic unit]] (ALU) architectures are those that are based on [[Processor register|register]]s or [[Engineering:Bus (computing)|data bus]]es of that size. [[Memory address]]es (and thus [[Engineering:Address bus|address bus]]es) for 8-bit CPUs are generally larger than 8-bit, usually 16-bit. 8-bit [[Microcomputer|microcomputer]]s are microcomputers that use 8-bit [[Engineering:Microprocessor|microprocessor]]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]], including the ISO/IEC 8859 series of national character sets{{snd}} 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.<ref>{{Cite journal|last1=Amdahl|first1=G. M.|last2=Blaauw|first2=G. A.|last3=Brooks|first3=F. P.|year=1964|title=Architecture of the IBM System/360|url=https://www.ece.ucdavis.edu/~vojin/CLASSES/EEC272/S2005/Papers/IBM360-Amdahl_april64.pdf|url-status=live|journal=IBM Journal of Research and Development|volume=8|issue=2|pages=87–101|doi=10.1147/rd.82.0087|archive-url=https://web.archive.org/web/20170810085620/https://www.ece.ucdavis.edu/~vojin/CLASSES/EEC272/S2005/Papers/IBM360-Amdahl_april64.pdf|archive-date=2017-08-10|}}</ref>

The first widely adopted 8-bit [[Engineering:Microprocessor|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|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 [[Engineering:Home computer|home computer]]s and second- and third-generation game consoles of the 1970s and 1980s. Many 8-bit CPUs or [[Engineering:Microcontroller|microcontroller]]s are the basis of today's ubiquitous embedded systems.

== 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.

==Details==
<!-- ==Range for storing integers== -->
An 8-bit register can store 2<sup>8</sup> different values. The [[Range (computer programming)|range]] of [[Integer|integer]] values that can be stored in 8 bits depends on the [[Integer (computer science)#Value and representation|integer representation]] used. With the two most common representations, the range is 0 through 255 (2<sup>8</sup> − 1) for representation as an ([[Signedness|unsigned]]) [[Binary number|binary number]], and −128 (−1 × 2<sup>7</sup>) through 127 (2<sup>7</sup> − 1) for representation as [[Two's complement|two's complement]].

8-bit CPUs use an [[Octet (computing)|8-bit]] [[Engineering:Bus (computing)|data bus]] and can therefore access 8 bits of data in a single machine instruction. The address bus is typically a double octet ([[16-bit computing|16 bit]]s) wide, due to practical and economical considerations. This implies a direct [[Address space|address space]] of 64&nbsp;[[Kilobyte|KB]] (65,536 bytes) on most 8-bit processors.

Most [[Engineering:Home computer|home computer]]s from the 8-bit era fully exploited the address space, such as the [[Engineering:BBC Micro|BBC Micro]] (Model B) with 32&nbsp;KB of [[Random-access memory|RAM]] plus 32&nbsp;KB of [[Read-only memory|ROM]]. Others like the very popular [[Engineering:Commodore 64|Commodore 64]] had full 64&nbsp;KB RAM, plus 20&nbsp;KB ROM, meaning with 16-bit addressing you could not use all of the RAM by default (e.g. from the included [[BASIC]] language interpreter in ROM);<ref>{{Cite web|title=Bank Switching - C64-Wiki|url=https://www.c64-wiki.com/wiki/Bank_Switching|access-date=2021-04-08|website=www.c64-wiki.com}}</ref> without exploiting [[Bank switching|bank switching]], which allows for breaking the 64&nbsp;KB (RAM) limit in some systems. Other computers would have as low as 1&nbsp;KB (plus 4&nbsp;KB ROM), such as the Sinclair [[Engineering:ZX80|ZX80]] (while the later very popular [[Engineering:ZX Spectrum|ZX Spectrum]] had more memory), or even only 128 bytes of RAM (plus [[Biology:Storage (memory)|storage]] from a [[ROM cartridge]]), as in an early game console [[Engineering:Atari 2600|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&nbsp;KB, i.e. 128&nbsp;KB RAM, also the [[Engineering:BBC Master|BBC Master]] with it expandable to 512&nbsp;KB of RAM.

{{Further|Zero page}}

While in general 8-bit CPUs have 16-bit addressing, in some architectures you have both, such as in the [[Company:MOS Technology|MOS Technology]] 6502 CPU, where the [[Zero page|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. Commonly [[Index register|index register]]s are 8-bit (while other "8-bit" CPUs, such as 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==
{{Main|Microprocessor chronology}}

The first commercial 8-bit processor was the Intel 8008 (1972) which was originally intended for the [[Engineering:Datapoint 2200|Datapoint 2200]] intelligent terminal. Most competitors to [[Company:Intel|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 [[Engineering:Apple I|Apple I]] and the Apple II series, the [[Engineering:Atari 8-bit family|Atari 8-bit family]], the [[Engineering:BBC Micro|BBC Micro]], and the Commodore PET and VIC-20, and in a number of video game consoles, such as the [[Engineering:Atari 2600|Atari 2600]] and the [[Engineering:Nintendo Entertainment System|Nintendo Entertainment System]].

{|class="wikitable sortable"
|+ Early or popular 8-bit processors (incomplete)
! Manufacturer
! Processor
! Year
! Comment
|-
| Intel || 8008 || 1972 || [[Engineering:Datapoint 2200|Datapoint 2200]] compatible
|-
|[[Company:Signetics|Signetics]]||[[Engineering:Signetics 2650|2650]] ||1973 ||
|-
|Intel || 8080 || 1974 || 8008 source compatible
|-
|[[Company:Motorola|Motorola]]||6800 ||1974 ||
|- 
|[[Company:Fairchild Semiconductor|Fairchild]]||[[Engineering:Fairchild F8|F8]] || 1975 ||
|-
|[[Company:MOS Technology|MOS]]||6502 || 1975 || Similar to 6800, but incompatible
|-
|[[Company:Microchip Technology|Microchip]]||PIC ||1975 ||[[Harvard architecture]] microcontroller
|-
|[[Company:Electronic Arrays|Electronic Arrays]] || EA9002 || 1976 || 8-bit data, 12-bit addressing
|-
|[[Company:RCA|RCA]]||1802 ||1976 ||
|-
|Zilog||Z80 ||1976 || 8080 binary compatible
|-
|Intel || 8085 || 1977 || 8080 binary compatible
|-
|Zilog || [[Engineering:Zilog Z8|Z8]] ||1978 ||Harvard architecture microcontroller
|-
|Motorola || 6809 ||1978 || 6800 source compatible
|-
|Intel || [[Engineering:Intel 8051|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; [[Engineering:Motorola 68000|Motorola 68000]] software-compatible, 6809 hardware-compatible
|-
|MOS ||6510 || 1982 || Enhanced 6502 custom-made for use in the [[Engineering:Commodore 64|Commodore 64]]
|-
|[[Company:Ricoh|Ricoh]]||[[Engineering:Ricoh 2A03|2A03]] || 1982 || 6502 clone minus BCD instructions for the [[Engineering:Nintendo Entertainment System|Nintendo Entertainment System]]
|-
| Zilog||Z180 ||1985 || Z80 binary compatible
|-
| Motorola || [[Engineering:Freescale 68HC11|68HC11]] ||1985 || 
|-
|Hudson||HuC6280 || 1987 || 65C02 binary compatible
|-
|Atmel||AVR || 1996 ||
|-
|Zilog || EZ80  ||1999  ||Z80 binary compatible
|-
| Infineon || [[Engineering:XC800 family|XC800]]  || 2005 ||
|-
| [[Company:Freescale Semiconductor|Freescale]]  || [[Engineering:Freescale 68HC08|68HC08]] || {{dunno}} ||
|-
| Motorola||6803 || {{dunno}} ||
|-
|[[Company:NEC|NEC]]||78K0<ref>{{Cite web|url=http://www.am.necel.com/micro/product/all_8_general.html/|title=NEC 78K0|publisher=[[Company:NEC|NEC]]|archive-url=https://web.archive.org/web/20081028210428/http://www.am.necel.com/micro/product/all_8_general.html/|archive-date=2008-10-28|url-status=dead|access-date=2009-02-10}}</ref>|| {{dunno}} ||
|}

==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 [[Engineering:7400-series integrated circuits|7400-series integrated circuits]] on a [[Engineering:Breadboard|breadboard]].<ref>{{cite web|first=Daniel|last=Oberhaus|title= This Guy Designed and Built an 8-bit CPU from Scratch|website=Motherboard|date=February 9, 2019|url=https://www.vice.com/en/article/a34ege/diy-8-bit-cpu|access-date=November 4, 2021}}</ref><ref>{{cite AV media|url=https://www.youtube.com/watch?v=g_ZaioqF1B0|title=Homebuilt 8-bit CPU + Computer with graphics and sound made from scratch using 74HC Logic|first=Paulo|last=Constantino}}</ref> 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==
* [[Engineering:Kenbak-1|Kenbak-1]]

==References==
{{Reflist}}

{{CPU technologies}}

[[Category:Data unit]]

{{Sourceattribution|8-bit computing}}