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4-bit computing

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Computer architecture bit widths
Bit
Application
Binary floating-pointprecision
Decimal floating-pointprecision

4-bit computing is the use of computer architectures in which integers and other data units are 4 bits wide. 4-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers or 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4-bit computation is obsolete, i.e. CPUs supporting 4-bit as the maximum size.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.[1]

4-bit processors were widely used in electronic calculators and other roles where decimal math was used, like electronic cash registers, 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 circuits 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 By the 1990s, most such uses had been replaced by general purpose binary designs.

History

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20-pin PSOP – NEC D63GS: a 4-bit microcontroller for infrared remote control transmission
16-pin DIP – Intel C4004
Olympia CD700 Desktop Calculator using the National Semiconductor MAPS MM570X bit-serial 4-bit microcontroller
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.[2]

Some of the first microprocessors had a 4-bit word length and were developed around 1970. The first commercial microprocessor was the binary-coded decimal (BCD-based) Intel 4004,[3][4] 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 InstrumentsTMS 1000 (1974).[2] It contained a 4-bit 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.[5]

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.[6]

The 4-bit processors were programmed in assembly language or Forth, e.g. "MARC4 Family of 4 bit Forth CPU"[7] (which is now discontinued) because of the extreme size constraint on programs and because common programming languages (for microcontrollers, 8-bit and larger), such as the C programming language, do not support 4-bit data types (C, and C++, and more languages require that the size of the char data type be at least 8 bits,[8] and that all data types other than bitfields have a size that is a multiple of the character size[9][10][11]).

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 games 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 Am2901 or 74181.

The Zilog Z80 (discontinued in 2024), although it is an 8-bit microprocessor, has a 4-bit ALU.[12][13]

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,[14] sometimes called "nybble-serial".[15]

The HP Saturn processors, used in many Hewlett-Packard calculators between 1984 and 2003 (including the HP 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.[16][17][18]

In addition, some early calculators – such as the 1967 Casio AL-1000, the 1972 Sinclair Executive, and the aforementioned 1984 HP Saturn – had 4-bit datapaths that accessed their registers 4 bits (one BCD digit) at a time.[19]

Uses

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National Semiconductor COP410L, a low-end 4-bit microcontroller. 512 bytes of ROM in upper left corner, 128 bits of RAM in upper right corner. Click to zoom.

One bicycle computer specifies that it uses a "4 bit, 1-chip microcomputer".[20] Other typical uses include coffee makers, infrared remote controls,[21] and security alarms.[22]

The processor in Barbie typewriters that can encrypt is a 4-bit microcontroller.[23]

Several manufacturers used 4-bit microcontrollers in their early electronic games:[24]

Western Digital used a 4-bit microcontroller as the basis for their WD2412 time-of-day clock.[26]

The Grundy Newbrain computer uses a 4-bit microcontroller to manage its keyboard, tape I/O, and its built-in 16 character VF alphanumeric display.[27]

Details

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Main article: Nibble

With 4 bits, it is possible to create 16 different values. All single-digit hexadecimal numbers can be written with four bits.

Binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by four bits.

List of 4-bit processors

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National Semiconductor MM5700CA/D bit-serial 4-bit microcontroller

See also

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References

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  1. ^Liu, Shih-yang; Liu, Zechun; Huang, Xijie; Dong, Pingcheng; Cheng, Kwang-Ting (2023-10-25), LLM-FP4: 4-Bit Floating-Point Quantized Transformers, arXiv, doi:10.48550/arXiv.2310.16836, arXiv:2310.16836, retrieved 2025-03-30
  2. ^ abKen Shirriff. "Reverse engineering RAM storage in early Texas Instruments calculator chips".
  3. ^Mack, Pamela E. (2005-11-30). "The Microcomputer Revolution". Retrieved 2009-12-23.
  4. ^"History in the Computing Curriculum"(PDF). Archived from the original(PDF) on 2011-07-19. Retrieved 2017-06-22.
  5. ^TMS 1000 Series Data Manual(PDF). Texas Instruments. December 1976. Retrieved 2013-07-20.
  6. ^"Rockwell PPS-4".
  7. ^"Forth Chips". www.ultratechnology.com.
  8. ^ISO/IEC 9899:1999 specification. p. 20, § 5.2.4.2.1. Retrieved 2023-07-24.
  9. ^ISO/IEC 9899:1999 specification. p. 37, § 6.2.6.1 (4). Retrieved 2023-07-24.
  10. ^Cline, Marshall. "C++ FAQ: the rules about bytes, chars, and characters".
  11. ^"4-bit integer". cplusplus.com. Retrieved 2014-11-21.
  12. ^Shima, Masatoshi; Faggin, Federico; Ungermann, Ralph; Slater, Michael (2007-04-27). "Zilog Oral History Panel on the Founding of the Company and the Development of the Z80 Microprocessor"(PDF).
  13. ^Shirriff, Ken. "The Z-80 has a 4-bit ALU".
  14. ^Hendrie, Gardner (2002-11-22). "Oral History of Edson (Ed) D. de Castro"(PDF) (Interview). p. 44.
  15. ^"Nova 1200"
  16. ^"The Saturn Processor". Retrieved 2015-12-23.
  17. ^"Guide to the Saturn Processor". Retrieved 2014-01-14.
  18. ^"Introduction to Saturn Assembly Language". Retrieved 2014-01-14.
  19. ^"Desk Electronic Calculators: Casio AL-1000"
  20. ^"Cateye Commuter Manual"(PDF). Retrieved 2014-02-11.
  21. ^ ab"μPD67, 67A, 68, 68A, 69 4-bit single-chip microcontroller for infrared remote control transmission"(PDF). documentation.renesas.com. Archived from the original(PDF) on 2016-01-06.
  22. ^Haskell, Richard. "Introduction to Digital Logic and Microprocessors (Lecture 12.2)". Archived from the original on 2014-02-22. Retrieved 2014-02-11.
  23. ^Paul Reuvers and Marc Simons. Crypto Museum. "Barbie Typewriter", 2015
  24. ^"National Semiconductor COP400". Sean Riddle. Retrieved 2021-12-24.
  25. ^Woerner, Joerg. "Texas Instruments My Little Computer". Datamath Calculator Museum. Retrieved 2024-06-19.
  26. ^"1983 Components Catalog"(PDF). Western Digital. p. 621. Retrieved 2021-12-24.
  27. ^"COP420 4-Bit Processor - Newbrain". Retrieved 2021-12-30.
  28. ^"MARC4 4-bit Microcontrollers – Programmer's Guide"(PDF). Atmel. Archived from the original(PDF) on 2014-12-15. Retrieved 2014-01-14.
  29. ^"MARC4 4-Bit Architecture". Atmel. Archived from the original on 2009-05-31.
  30. ^"Product End-of-Life (EOL) Notification"(PDF). Atmel. 2014-03-07. Archived from the original(PDF) on 2016-08-07.
  31. ^"μPD6P9 4-bit single-chip microcontroller for infrared remote control transmission"(PDF). documentation.renesas.com. Archived from the original(PDF) on 2016-03-27.
  32. ^"μPD17240, 17241, 17242, 17243, 17244, 17245, 17246 4-bit single-chip microcontrollers for small general-purpose infrared remote control transmitters"(PDF). documentation.renesas.com. Archived from the original(PDF) on 2016-03-27.
  33. ^"Microcontrollers for Remote Controllers"(PDF). documentation.renesas.com. Archived from the original(PDF) on 2013-12-19.
  34. ^"Mask ROM/ROMless Products 4/8bit Remote Control". Archived from the original on 2008-10-28.
  35. ^Cravotta, Robert. "Embedded Processing Directory".
  36. ^"EM6580". Archived from the original on 2013-12-19. Retrieved 2013-05-12.
  37. ^"EM6580".
  38. ^"EM6682".
  39. ^Culver, John (2014-09-27). "National Semiconductor: The COP before the COPS". www.cpushack.com. Retrieved 2020-05-28.
  40. ^ abcdefghijklmnSharp Microcomputers Data Book(PDF). September 1990. Retrieved 2018-01-05.
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