From Wikipedia, the free encyclopedia
Jump to: navigation, search
1 2 4 8 12 16 18 24 26 31 32 36 48 60 64 128 256 512
8 16 32 64
Binary floating point precision
16 32 40 64 80 128 256
×½ ×1 ×2 ×4 ×8
Decimal floating point precision
32 64 128

In computer architecture, 8-bit integers, memory addresses, or other data units are those that are 8 bits (1 octet) wide. Also, 8-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 8-bit is also a generation of microcomputers in which 8-bit microprocessors were the norm.

The IBM System/360 introduced byte-addressable memory with 8-bit bytes, as opposed to bit-addressable or decimal digit-addressable or word-addressable memory, although its general purpose registers were 32 bits wide, and addresses were contained in the lower 24 bits of those addresses. Different models of System/360 had different internal data path widths; the IBM System/360 Model 30 (1965) implemented the 32-bit System/360 architecture, but had an 8 bit native path width, and performed 32-bit arithmetic 8 bits at a time.[1]

The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system; it had 8-bit data words and 16-bit addresses. The Zilog Z80 (compatible with the 8080) and the Motorola 6800 were also used in similar computers. The Z80 and the MOS Technology 6502 8-bit CPUs were widely used in home computers and second- and third-generation game consoles of the 1970s and 1980s. Many 8-bit CPUs or microcontrollers are the basis of today's ubiquitous embedded systems.


There are 28 (256) different possible values for 8 bits. When unsigned, it has possible values ranging from 0 to 255, when signed, it has -128 to 127.

Eight-bit CPUs use an 8-bit data bus and can therefore access 8 bits of data in a single machine instruction. The address bus is typically a double octet wide (i.e. 16-bit), due to practical and economical considerations. This implies a direct address space of only 64 kB on most 8-bit processors.

Notable 8-bit CPUs[edit]

The first commercial 8-bit processor was the Intel 8008 (1972) which was originally intended for the Datapoint 2200 intelligent terminal. Most competitors to Intel started off with such character oriented 8-bit microprocessors. Modernized variants of these 8-bit machines are still one of the most common types of processor in embedded systems.

Another notable 8-bit CPU is the MOS Technology 6502; it, and variants of it, were used in a number of personal computers such as the Apple I and Apple II, the Atari 8-bit family, the BBC Micro, and the Commodore PET and Commodore VIC-20, and in a number of video game consoles such as the Atari 2600 and the Nintendo Entertainment System.

Early or popular 8-bit processors (incomplete)
Manufacturer Processor Year Comment
Intel 8008 1972 Datapoint 2200 compatible
Signetics 2650 1973
Intel 8080 1974 8008 source compatible
Motorola 6800 1974
Fairchild F8 1975
MOS 6502 1975 Similar to 6800, but incompatible
Microchip PIC 1975 Harvard architecture microcontroller
RCA 1802 1976
Zilog Z80 1976 8080 binary compatible
Intel 8085 1977 8080 binary compatible
Motorola 6809 1978 6800 source compatible
Zilog Z8 1978 Harvard architecture microcontroller
Intel 8051 1980 Harvard architecture microcontroller
MOS 6510 1982 Enhanced 6502 custom-made for use in the Commodore 64
Ricoh 2A03 1982 6502 clone minus BCD instructions for the Nintendo Entertainment System
Zilog Z180 1985 Z80 binary compatible
Motorola 68HC11 1985
Atmel AVR 1996
Zilog EZ80 1999 Z80 binary compatible
Infineon XC800 2005
Freescale 68HC08
Hudson HuC6280
Motorola 6803
NEC 78K0[2]

See also[edit]

  • Vasm, a free assembler


  1. ^ Amdahl, G. M.; Blaauw, G. A.; Brooks, F. P. (1964). "Architecture of the IBM System/360". IBM Journal of Research and Development. 8 (2): 87–101. doi:10.1147/rd.82.0087. 
  2. ^ NEC 78K0