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 '70s and '80s. 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
