Intel 8080

The Intel 8080 is a seminal CPU introduced in 1974 that gave rise to the personal computer/home computer/microcomputer revolution.

The 8080 is often said to be the "first truly usable microprocessor".

It was integrated in early home computers like the Altair 8800, the Interact Home Computer, the DAI Personal Computer, and arcade systems like the Taito 8080 and Midway 8080.

Technology

The 8008 was built with PMOS circuitry, while the 8080 was built with NMOS, which provided much superior performance.

NMOS became the standard microprocessor technology until the rise of CMOS in the 1980s, combining NMOS and PMOS to dramatically reduce power consumption.

Compatibility

The 8080 is not binary-compatible with the 8008 because almost all the instructions were shifted to different opcodes.

One important but subtle change was that the 8 register/memory codes were reordered to start with B instead of A. The motivation is that this gave registers in a 16-bit register pair (BC, DE, or HL) codes that differ only in the low bit. This makes it easier to specify a register pair with a two-bit code.

The 8080 ultimately led to the machine code-compatible, but more powerful clone Zilog Z80, which was of course also used in the Amstrad CPC and Sinclair ZX Spectrum among others. (Note that 8080 assembly language looks different from Z80 mnemonics; e.g. the Z80 uses LD whereas the 8080 has MOV and several other commands.)

CP/M required an 8080, 8085 or Z80 CPU and between 1976 and about 1983, microcomputers with a Z80 that ran CP/M were the norm before the market shifted to MS-DOS.

Evolution

Zilog produced the Z80, a more feature-rich and backward-compatible evolution of the 8080.

Meanwhile Intel produced the 8085, an improved, backward-compatible version of the 8080, with refinements to power consumption and integration.

See: Notes on PLA Inside the ALU Instruction set: the octal table Register file Flag circuits The ALU and its hidden registers Decimal adjust circuitry 8085 CPU reverse engineered

Links

• Intel 8080 at the English-language Wikipedia
• Intel 8080A datasheet
• 8080 assembly language programming manual
• 8080 Systems User's Manual Contains a detailed breakdown of machine cycles
• Mapping between i8080 and Z80 mnemonics
• 8080 opcode map
• 8080 CPU Exerciser
• Oral history collection: Intel 8080