8-bit

Bit
1 4 8 12 16 18 24 26 31 32 36 48 60 64 128 256 512
Application
16 32 64
Floating point precision
×½ ×1 ×2 ×4
Floating point decimal precision
32 64 128

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.

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^[2] series
• Infineon XC800 family microcontrollers
• Ricoh 2A03

See also

• Vasm, a free assembler

References

CPU technologies Architecture Harvard (Modified Harvard) von Neumann Dataflow TTA Comparison Instruction set ASIP CISC EDGE EPIC MISC OISC RISC VLIW NISC ZISC TRIPS Word size 1-bit 4-bit 8-bit 9-bit 10-bit 12-bit 15-bit 16-bit 18-bit 22-bit 24-bit 25-bit 26-bit 27-bit 31-bit 32-bit 33-bit 34-bit 36-bit 39-bit 40-bit 48-bit 50-bit 60-bit 64-bit 128-bit 256-bit 512-bit variable Execution Instruction pipelining Bubble Operand forwarding Out-of-order execution Register renaming Speculative execution Branch predictor Memory dependence prediction Hazards Parallel level Bit Bit-serial Word Instruction Scalar Superscalar Data Vector Memory Task Thread Multithreading Temporal multithreading Simultaneous multithreading Hyper-threading Flynn's taxonomy SISD SIMD MISD MIMD SPMD Addressing mode Types Digital signal processor (DSP) GPGPU Microcontroller Physics processing unit System on a chip (SoC) Cellular Components Address generation unit (AGU) Arithmetic logic unit (ALU) Barrel shifter Floating-point unit (FPU) Back-side bus (Multiplexer, Demultiplexer) Registers Memory management unit (MMU) Translation lookaside buffer (TLB) Cache Register file Microcode Control unit Clock rate Power management APM ACPI Dynamic frequency scaling Dynamic voltage scaling Clock gating CPU hardware security NX bit Hardware restriction (firmware) Trusted Execution Technology Secure cryptoprocessor Hardware security module Hengzhi chip