A group of four bits is also called a nibble and has 24 = 16 possible values. Some of the first microprocessors had a 4-bit word length and were developed around 1970. The first commercial microprocessor was the binary-coded decimalIntel 4004, 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. The Texas InstrumentsTMS 1000 was a 4-bit CPU; it had a Harvard architecture, with an on-chip instruction ROM, 8-bit-wide instructions 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" because of the extreme size constraint on programs and because common programming languages, such as the C programming language, do not support 4-bit data types. The 1970s saw the emergence of 4-bit software applications for mass markets like pocket calculators. During the 1980s 4-bit microprocessor 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 was built from multiple 4-bit-wide sections, each section including a chip such as an Am2901 or 74181 chip. The Zilog Z80, although it is an 8-bit microprocessor, has a 4-bit ALU. 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, sometimes called "nybble-serial". The HP Saturn processors, used in many Hewlett-Packard calculators between 1984 and 2003 are "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. 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 at a time.
Uses
While 32- and 64-bit processors are more prominent in modern consumer electronics, 4-bit CPUs can as of 2020 be bought online at down to $0.18, however 20 non-obsolete 8-bit CPUs can be bought for $1.80, a fraction of the 4-bit price, and even a single modern 32-bit microcontroller can be bought for $0.24 so it's unclear if 4-bit CPUs are still used for anything else than for replacement parts. For example, one bicycle computer specifies that it uses a "4 bit 1-chip microcomputer". Other typical uses include coffee makers, infrared remote controls, and security alarms. , most PC motherboards, especially laptop motherboards, use a 4-bit LPC bus to connect the southbridge to the motherboard firmware flash ROM and the Super I/O chip.
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.
