8-bit

Bit
12481216182426313236486064128256512
Application
8163264
Binary floating-point precision
1632406480128256
×½×1×2×4×8
Decimal floating-point precision
3264128

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.

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 or popular 8-bit processors (incomplete)
Manufacturer Processor Year Comment
Intel80081972Datapoint 2200 compatible
Signetics26501973
Intel808019748008 source compatible
Motorola68001974
FairchildF81975
MOS65021975Similar to 6800, but incompatible
MicrochipPIC1975Harvard architecture microcontroller
Electronic ArraysEA900219768-bit data, 12-bit addressing
RCA18021976
ZilogZ8019768080 binary compatible
Intel808519778080 binary compatible
Motorola680919786800 source compatible
ZilogZ81978Harvard architecture microcontroller
Intel80511980Harvard architecture microcontroller
MOS65101982Enhanced 6502 custom-made for use in the Commodore 64
Ricoh2A0319826502 clone minus BCD instructions for the Nintendo Entertainment System
ZilogZ1801985Z80 binary compatible
Motorola68HC111985
AtmelAVR1996
ZilogEZ801999Z80 binary compatible
InfineonXC8002005
Freescale68HC08
HudsonHuC6280
Motorola6803
NEC78K0[2]

References

  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
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