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.[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.
Details
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
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 and/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 | 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
References