4-bit

Bit
1 4 8 12 16 18 24 26 31 32 36 48 60 64 128 256 512
Application
16 32 64
Floating point precision
×½ ×1 ×2 ×4 ×8
Floating point decimal precision
32 64 128

In computer architecture, 4-bit integers, memory addresses, or other data units are those that are at most 4 bits wide. Also, 4-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. A group of four bits is also called a nibble.

Some of the first microprocessors had a 4-bit word length and were developed around 1970. The TMS 1000, the world's first single-chip microprocessor, was a 4-bit CPU; it had a Harvard architecture, with an on-chip instruction ROM with 8-bit-wide instructions and an on-chip data RAM with 4-bit words. The first commercial microprocessor was the binary coded decimal (BCD-based) Intel 4004, developed for calculator applications in 1971; it had a 4-bit word length, but had 8-bit instructions and 12-bit addresses.

The HP Saturn processors, used in many Hewlett-Packard calculators between 1984 and 2015 (including the HP 48 series of scientific calculators) 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 its registers are 64 bits, storing 16 4-bit digits. Since 2003, new Saturn-based HP calculators (including the HP 49/50 series) utilize a 32-bit processor with an ARM920T core to emulate an extended Saturn processor architecture named Saturn+ at a higher speed.

The 4-bit processors were programmed in assembly language because of the extreme size constraint on programs and common programming languages (for microcontrollers, 8-bit and larger), such as C programming language, do not support 4-bit (C requires that the size of the char data type be at least 8 bits, and that that all data types other than bitfields have a size that is a multiple of the character size). While larger than 4-bit values can be used by combining more than one manually, the language has to support the smaller values used in the combining. If not, assembly is the only option.

The 1970s saw the emergence of 4-bit software applications for mass markets like pocket calculators.

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.

Modern uses

While 32- and 64-bit processors are more prominent in modern consumer electronics, 4-bit CPUs continue to be used (usually as part of a microcontroller) in cost-sensitive applications which require minimal computing power. For example, one popular bicycle computer specifies that it uses a "4 bit 1-chip microcomputer". Other typical uses include coffee makers, infrared remote controls, and security alarms.

Use of 4-bit processors has declined relative to 8- or even 32-bit processors as they are hard to find cheaper in general computer supplier's stores. The simplest kinds are not available in any of them and others are "Non-stock" and more expensive (a few expensive ones can be found, as of 2014, on eBay).

Electronics store still carry, as of 2014, non-CPU/MCU 4-bit chips, such as counters.

As of 2015, most PC motherboards, especially laptop motherboards, use a 4-bit LPC bus (introduced in 1998) to connect the southbridge to the motherboard firmware flash ROM (UEFI or BIOS) and the Super I/O chip.

Details

Main article: Nibble

With four 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.

Binary	Octal	Decimal	Hexadecimal
0000	0	0	0
0001	1	1	1
0010	2	2	2
0011	3	3	3
0100	4	4	4
0101	5	5	5
0110	6	6	6
0111	7	7	7
1000	10	8	8
1001	11	9	9
1010	12	10	A
1011	13	11	B
1100	14	12	C
1101	15	13	D
1110	16	14	E
1111	17	15	F

List of 4-bit processors

• TMS 1000
• Intel 4004
• Intel 4040
• Atmel MARC4 core - Mature.
• Toshiba TLCS-47 series
• HP Saturn and Saturn+ series
• NEC μPD75X
• NEC (now Renesas) µPD612xA (discontinued), µPD613x, μPD6x and μPD1724x infrared remote control transmitter microcontrollers
• EM Microelectronic-Marin EM6600 family, EM6580, EM6682, etc.
• Epson S1C63 family

See also

• GMC-4
• Low Pin Count

External links

• Saturn CPU
• Epson - High Performance 4-bit Microcontrollers
• Considerations for 4-bit processing