| Computer architecture bit widths |
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| Decimal floating-pointprecision |
4-bit computing is the use ofcomputer architectures in whichintegers and otherdata units are 4bits wide. 4-bitcentral processing unit (CPU) andarithmetic logic unit (ALU) architectures are those that are based onregisters ordata buses of that size. A group of four bits is also called anibble and has 24 = 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; 4-bit microcontrollers can though still be bought as of 2025[update].[1]
4-bit processors were widely used inelectronic calculators and other roles where decimal math was used, like electroniccash registers,microwave oven timers, and so forth. This is because a 4-bit value holds a singlebinary-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 amemory address for real-world programs or data, theaddress bus of these systems was generally larger. For instance, the canonical 4-bitmicroprocessor, theIntel 4004, had a 12-bit address format.
4-bit designs were used only for a short period whenintegrated 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 By the 1990s, most such uses had been replaced by general purpose binary designs.
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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 firstmicroprocessors had a 4-bit word length and were developed around 1970. The first commercial microprocessor was thebinary-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 theIntel 4040, which addedinterrupt support and a variety of other new features.
The first commercial single-chip computer was the 4-bitTexas InstrumentsTMS 1000 (1974).[2] It contained a 4-bitCPU with aHarvard architecture and 8-bit-wide instructions, an on-chip instruction ROM, and an on-chip data RAM with 4-bit words.[5]
TheRockwell 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 inassembly language orForth, 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 (formicrocontrollers, 8-bit and larger), such as theC programming language, do not support 4-bit data types (C, andC++, and more languages require that the size of thechar 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 inhandheld electronic games to keep costs low.
In the 1970s and 1980s, a number of research and commercial computers usedbit slicing, in which the CPU'sarithmetic logic unit (ALU) was built from multiple 4-bit-wide sections, each section including a chip such as anAm2901 or74181.
TheZilog Z80 (discontinued in 2024), although it is an 8-bit microprocessor, has a 4-bit ALU.[12][13]
Although theData 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]
TheHP Saturn processors, used in manyHewlett-Packard calculators between 1984 and 2003 (including theHP 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.[16][17][18]
In addition, some early calculators – such as the 1967Casio AL-1000, the 1972Sinclair Executive, and the aforementioned 1984HP Saturn – had 4-bitdatapaths that accessed their registers 4 bits (one BCD digit) at a time.[19]
One bicycle computer specifies that it uses a "4 bit, 1-chip microcomputer".[20] Other typical uses includecoffee makers,infrared remote controls,[21] andsecurity 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]
TheGrundy Newbrain computer uses a 4-bit microcontroller to manage its keyboard, tape I/O, and its built-in 16 characterVF alphanumeric display.[27]
TheApple Lisa utilizes a 4-bit microcontroller to control the keyboard, mouse, RTC, and soft power switch.[28]
With 4 bits, it is possible to create 16 different values. All single-digithexadecimal 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.
RAM 80 × 4 bits