Editing 4-bit computing (section)

== History ==
[[File:Alps remote control BHR970001B - NEC D63GS-7525.jpg|thumb|20-pin PSOP – NEC D63GS: a 4-bit microcontroller for [[infrared remote control]] transmission]]
[[File:Intel C4004 greytraces CPU.jpg|thumb|left|16-pin DIP – Intel C4004]]
[[File:Olympia CD700 Desktop Calculator. 1971.Microprogrammable Arithmetic Processor System Devices (MAPS).jpg|thumb|Olympia CD700 Desktop Calculator using the National Semiconductor MAPS MM570X [[bit-serial]] 4-bit microcontroller]]
[[File:Alps remote control BHR970001B-7517.jpg|thumb|left|Infrared remote control PCB – an [[infrared remote control]] transmitter controlled by a NEC D63GS 4-bit microcontroller]]

A 4-bit processor may seem limited, but it is a good match for calculators, where each decimal digit fits into four bits.<ref name="Shirriff_TMS1000" />

Some of the first [[microprocessor]]s had a 4-bit word length and were developed around 1970. The first commercial microprocessor was the [[binary-coded decimal]] (BCD-based) [[Intel 4004]],<ref name="Mack_2005" /><ref name="Hofstra_History" /> developed for calculator applications in 1971; it had a 4-bit word length, but had 8-bit instructions and 12-bit addresses. It was succeeded by the [[Intel 4040]], which added [[interrupt]] support and a variety of other new features.

The first commercial single-chip computer was the 4-bit [[Texas Instruments]] [[TMS 1000]] (1974).<ref name="Shirriff_TMS1000">{{cite web |author=Ken Shirriff |url=https://www.righto.com/2020/11/reverse-engineering-ram-storage-in.html |title=Reverse engineering RAM storage in early Texas Instruments calculator chips}}</ref> It contained a 4-bit [[central processing unit|CPU]] with a [[Harvard architecture]] and 8-bit-wide instructions, an on-chip instruction ROM, and an on-chip data RAM with 4-bit words.<ref name="TI_1976_TMS1000" />

The [[Rockwell PPS-4]] was another early 4-bit processor, introduced in 1972, which had a long lifetime in handheld games and similar roles. It was steadily improved and by 1975 been combined with several support chips to make a one-chip computer.<ref>{{cite web |url=http://www.antiquetech.com/?page_id=796 |title=Rockwell PPS-4}}</ref>

The 4-bit processors were programmed in [[assembly language]] or [[Forth (programming language)|Forth]], e.g. "MARC4 Family of 4 bit Forth CPU"<ref name="UT_Forth" /> (which is now discontinued) because of the extreme size constraint on programs and because common programming languages (for [[microcontroller]]s, 8-bit and larger), such as the [[C (programming language)|C programming language]], do not support 4-bit data types (C, and [[C++]], and more languages require that the size of the [[character (computing)#char|<code>char</code>]] data type be at least 8 bits,<ref name="ISOIEC9899_1999_1" /> and that all data types other than bitfields have a size that is a multiple of the character size<ref name="ISOIEC9899_1999_2" /><ref name="Cline" /><ref name="CPP" />).

The 1970s saw the emergence of 4-bit software applications for mass markets like pocket calculators. During the 1980s, 4-bit microprocessors were used in [[handheld electronic game]]s to keep costs low.

In the 1970s and 1980s, a number of research and commercial computers used [[bit slicing]], in which the CPU's [[arithmetic logic unit]] (ALU) was built from multiple 4-bit-wide sections, each section including a chip such as an [[AMD Am2900|Am2901]] or [[74181]].

The [[Zilog Z80]] (discontinued in 2024), although it is an 8-bit microprocessor, has a 4-bit ALU.<ref name="Shima_Z80" /><ref name="Shirriff_Z80" />

Although the [[Data General Nova]] is a series of 16-bit minicomputers, the original Nova and the Nova 1200 internally processed numbers 4 bits at a time with a 4-bit ALU,<ref>{{ cite interview | first = Gardner | last = Hendrie | title = Oral History of Edson (Ed) D. de Castro | date = 22 November 2002 | url = http://archive.computerhistory.org/resources/access/text/2012/07/102702207-05-01-acc.pdf | pages = 44 }}</ref> sometimes called "nybble-serial".<ref>[https://rcsri.org/collection/nova-1200/ "Nova 1200"]</ref>

The [[HP Saturn]] processors, used in many [[Hewlett-Packard]] calculators between 1984<!-- intro of HP-71B --> and 2003<!-- when the HP49 was discontinued and replaced by an ARM based model developed by Kinpo --> <!-- EOL announcement of HP 50g --> (including the [[HP&nbsp;48 series]] of scientific calculators) are "4-bit" (or hybrid 64-/4-bit) machines; as the Intel 4004 did, they string multiple 4-bit words together, e.g. to form a 20-bit memory address, and most of the registers are 64 bits wide, storing 16 4-bit digits.<!-- Its instructions were 10 bits wide.--><!-- The previous statement is factually inaccurate and not supported by the cited sources --><!-- <ref name="HPM">{{cite web |url=http://www.hpmuseum.org/techcpu.htm |title=HP CPU and Programming |access-date=2014-01-14}}</ref> --><!-- This reference is invalid because it points to a page which describes the microarchitectures of the HP41 and older calculators which did not use the Saturn. The reference has been updated to point to the H.P. Saturn specific page. --><ref name="HPM_Saturn" /><ref name="Grack_Saturn" /><ref name="HPCalc_Saturn" /><!--{{Off-topic|date=December 2015|HP Saturn}}Since 2003, new Saturn-based HP calculators{{Dubious |date=December 2015| reason=These are not "Saturn-based" processors. The "Saturn+" isn't even a Saturn processor, but an emulation of one.}} (including the [[HP 49/50 series]]) use a 32-bit processor with an [[ARM920T]] core to emulate an extended Saturn processor architecture named [[HP Saturn+|Saturn+]] at a higher speed. -->

In addition, some early calculators{{snd}} such as the 1967 [[Casio AL-1000]], the 1972 [[Sinclair Executive]], and the aforementioned 1984 [[HP Saturn]]{{snd}} had 4-bit [[datapath]]s that accessed their registers 4 bits (one BCD digit) at a time.<ref>[http://www.vintagecalculators.com/html/casio_al-1000.html "Desk Electronic Calculators: Casio AL-1000"]</ref>