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 2<sup>4</sup> = 16 possible values, with a range of 0 to 15.

4-bit computation is largely obsolete, i.e. CPUs supporting 4-bit as the maximum size, or 4-bit data bus are no longer available, but 4-bit microcontrollers, such as the EM Microelectronic EM6580, are still available .

<!-- However, 4-bit integers (or smaller), and 4-bit floating point is gaining ground for AI, large-language models. -->

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. By the 1990s, most such uses had been replaced by general purpose binary designs.

History

thumb|20-pin PSOP – NEC D63GS: a 4-bit microcontroller for [[infrared remote control transmission]]

thumb|left|16-pin DIP – Intel C4004

thumb|Olympia CD700 Desktop Calculator using the National Semiconductor MAPS MM570X [[bit-serial 4-bit microcontroller]]

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. 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.

The 4-bit processors were programmed in assembly language or Forth, e.g. "MARC4 Family of 4 bit Forth CPU" sometimes called "nybble-serial".

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. 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. Operations were performed nybble-serial.<!-- Its instructions were 10 bits wide.--><!-- The previous statement is factually inaccurate and not supported by the cited sources --><!-- --><!-- 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. -->

Uses

thumb|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".

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

  • Mattel's Funtronics Jacks, Red Light Green Light, Tag, Plus One and Dalla$.
  • Milton Bradley Lightfight and Electronic Battleship 1982.
  • Coleco Head to Head Basketball.
  • National Semiconductor Quiz Kid Racer.
  • Entex Space Invader.
  • Texas Instruments My Little Computer.

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

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.

The Apple Lisa utilizes a 4-bit microcontroller to control the keyboard, mouse, RTC, and soft power switch.

Details

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

thumb|upright|National Semiconductor MM5700CA/D [[bit-serial 4-bit microcontroller]]

  • Intel 4004 (first 4-bit microprocessor and widely regarded as the first commercially available microprocessor from 1971, discontinued 1981)
  • Intel 4040 (discontinued 1981)
  • TMS 1000 (the first high-volume commercial microcontroller, from 1974, after Intel 4004; now discontinued<!-- for sure, and when?-->)
  • American Microsystems S2000
  • Atmel MARC4 core
  • National Semiconductor MAPS MM570X
  • Rockwell PPS-4
  • Sharp SM590/SM591/SM595

</references>

  • Saturn CPU
  • Considerations for 4-bit processing