4-bit computing

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 By the 1990s, most such uses had been replaced by general purpose binary designs.

A 4-bit processor may seem limited, but it is a good match for calculators, where each decimal digit fits into four bits.^[1]

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,^[2][3] 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).^[1] 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.^[4]

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.^[5]

The 4-bit processors were programmed in assembly language or Forth, e.g. "MARC4 Family of 4 bit Forth CPU"^[6] (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,^[7] and that all data types other than bitfields have a size that is a multiple of the character size^[8][9][10]).

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, although it is an 8-bit microprocessor, has a 4-bit ALU.^[11][12]

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,^[13] sometimes called "nybble-serial".^[14]

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.^[15][16][17]

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.^[18]

One bicycle computer specifies that it uses a "4 bit, 1-chip microcomputer".^[19] Other typical uses include coffee makers, infrared remote controls,^[20] and security alarms.^[21]

The processor in Barbie typewriters that can encrypt is a 4-bit microcontroller.^[22]

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.

• Intel 4004 (first 4-bit microprocessor from 1971, though Four-Phase Systems AL1 from 1969 is older, discontinued 1981)
• Intel 4040 (discontinued 1981)
• TMS 1000 (the first high-volume commercial microcontroller, from 1974, after Intel 4004; now discontinued)
• Atmel MARC4 core^[23][24] (discontinued because of Low demand. "Last ship date: 7 March 2015"^[25])
• Samsung S3C7 (KS57 Series) 4-bit microcontrollers (RAM: 512 to 5264 nibbles, 6 MHz clock)
• Toshiba TLCS-47 series
• HP Saturn
• NEC μPD75X
• NEC μCOM-4
• NEC (now Renesas) μPD612xA (discontinued), μPD613x, μPD6x^[20][26] and μPD1724x^[27] infrared remote control transmitter microcontrollers^[28][29]
• EM Microelectronic-Marin EM6600 family,^[30] EM6580,^[31][32] EM6682,^[33] etc.
• Epson S1C63 family
• National Semiconductor "COPS I" and "COPS II" ("COP400") 4-bit microcontroller families^[34]
• National Semiconductor MAPS MM570X
• Sharp SM590/SM591/SM595^{[35]: 26–34}
• Sharp SM550/SM551/SM552^{[35]: 36–48}
• Sharp SM578/SM579^{[35]: 49–64}
• Sharp SM5E4^{[35]: 65–74}
• Sharp LU5E4POP^{[35]: 75–82}
• Sharp SM5J5/SM5J6^{[35]: 83–99}
• Sharp SM530^{[35]: 100–109}
• Sharp SM531^{[35]: 110–118}
• Sharp SM500^{[35]: 119–127} (ROM 1197×8 bit, RAM 40×4 bit, a divider and 56-segment LCD driver circuit)
• Sharp SM5K1^{[35]: 128–140}
• Sharp SM4A^{[35]: 141–148}
• Sharp SM510^{[35]: 149–158} (ROM 2772×8 bit, RAM 128×4 bit, a divider and 132-segment LCD driver circuit)
• Sharp SM511/SM512^{[35]: 159–171} (ROM 4032×8 bit, RAM 128/142×4 bit, a divider and 136/200-segment LCD driver circuit)
• Sharp SM563^{[35]: 172–186}

• GMC-4
• Hitachi HD44780 LCD controller
• Intel's LPC (low-pin-count) bus/interface for 4-bit communication
  • Its successor for modern computers, Intel's Enhanced Serial Peripheral Interface (eSPI), allows 1-bit, 2-bit, and 4-bit communication

• Saturn CPU
• "Products: High Performance 4-bit Microcontrollers (S1C63 family)". Epson. Archived from the original on 2013-07-29.
• Considerations for 4-bit processing