4-bit computing: Difference between revisions

Revision as of 23:12, 22 December 2024

Revision as of 18:37, 30 March 2025

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{{Computer architecture bit widths}}

'''4-bit computing''' is the use of [[computer architecture]]s in which [[integer (computer science)|integers]] and other [[data (computer science)|data]] units are 4 [[bit]]s wide. 4-bit [[central processing unit]] (CPU) and [[arithmetic logic unit]] (ALU) architectures are those that are based on [[processor register|registers]] or [[Busbus (computing)|data buses]] of that size. A group of four bits is also called a [[nibble]] and has 2⁴ = 16 possible values, with a range of 0 to 15.

4-bit computation is obsolete, i.e. CPUs supporting 4-bit as the maximum size. However, 4-bit integers (or smaller), and 4-bit floating point is gaining ground for AI, large-language models.<ref>{{Citation |last=Liu |first=Shih-yang |title=LLM-FP4: 4-Bit Floating-Point Quantized Transformers |date=2023-10-25 |url=https://arxiv.org/abs/2310.16836 |access-date=2025-03-30 |publisher=arXiv |doi=10.48550/arXiv.2310.16836 |id=arXiv:2310.16836 |last2=Liu |first2=Zechun |last3=Huang |first3=Xijie |last4=Dong |first4=Pingcheng |last5=Cheng |first5=Kwang-Ting}}</ref>

4-bit processors were widely used in [[electronic calculator]]s and other roles where decimal math was used, like electronic [[cash register]]s, [[microwave oven]] timers, and so forth. This is because a 4-bit value holds a single [[binary coded decimal]] (BCD) digit, making it a natural size for directly processing decimal values. As a 4-bit value is generally too small to hold a [[memory address]] for real-world programs or data, the [[address bus]] of these systems was generally larger. For instance, the canonical 4-bit [[microprocessor]], the [[Intel 4004]], had a 12-bit address format.

4-bit processors were widely used in [[electronic calculator]]s and other roles where decimal math was used, like electronic [[cash register]]s, [[microwave oven]] timers, and so forth. This is because a 4-bit value holds a single [[binary -coded decimal]] (BCD) digit, making it a natural size for directly processing decimal values. As a 4-bit value is generally too small to hold a [[memory address]] for real-world programs or data, the [[address bus]] of these systems was generally larger. For instance, the canonical 4-bit [[microprocessor]], the [[Intel 4004]], had a 12-bit address format.

4-bit designs were used only for a short period when [[integrated circuit]]s were still expensive, and were found primarily in cost-sensitive roles. While 4-bit computing is mostly obsolete, 4-bit values are still used in the same decimal-centric roles they were developed for, and modern implementations are generally much wider and process multiple 4-bit values in parallel. An example of such a system is the [[HP Saturn]] design of the 1980s. By the 1990s, most such uses had been replaced by general purpose binary designs.

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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=httphttps://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" />).

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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>