|Floating point precision|
|Floating point decimal precision|
In computer architecture, 8-bit integers, memory addresses, or other data units are those that are at most 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 term given to 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.
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 '70s and '80s. Many 8-bit CPUs or microcontrollers are the basis of today's ubiquitous embedded systems.
There are 28 (256) possible values for 8 bits.
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.
Important 8-bit CPUs
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.
Early and/or popular 8-bit processors (incomplete):
- Intel 8008 (1972 Datapoint 2200 compatible)
- Signetics 2650 (1973)
- Fairchild F8 (1975)
- Motorola 6800 (1974)
- Intel 8080 (1974 8008 source compatible)
- MOS Technology 6502 (1975 similar to 6800, but incompatible)
- RCA 1802 (1976)
- Zilog Z80 (1976 8080 binary compatible)
- Intel 8085 (1977 8080 binary compatible)
- Zilog Z8 (1978 Harvard architecture microcontroller)
- Motorola 6809 (1978 6800 source compatible)
- Intel 8051 (1980 Harvard architecture microcontroller)
- MOS Technology 6510 (1982 enhanced 6502 custom-made for use in the Commodore 64)
- Zilog Z180 (1985 Z80 binary compatible)
- Zilog eZ80 (1999/2001 Z80 binary compatible)
- Motorola 6803
- Freescale 68HC08
- Freescale 68HC11
- Hudson Soft HuC6280
- PIC microcontroller (1975 Harvard architecture microcontroller)
- Atmel AVR family microcontrollers
- NEC 78K0 series
- Infineon XC800 family microcontrollers
- Ricoh 2A03
- Vasm, a free assembler