Intel 8080

The 8080 is often said to be the "first truly usable microprocessor". Its predecessor, the first 8-bit microprocessor Intel 8008, had a rigid seven-level address call stack instead of a flexible Stack Pointer. The 8008 instruction set was itself based on the CPU board, built out of dozens of TTL chips, of the Datapoint 2200 computer. Source

Despite what their naming suggest, the Intel 8008 and Intel 4004 were separate projects with distinct architectures and purposes. The 8008 was not an upgrade or evolution of the 4004.

See: Die photos and analysis The ALU The counters The carry-lookahead circuit 8008 microprocessor reverse engineered

Fun fact: Before microprocessors were invented, there was already ALU chips, like the 74181 ALU chip, that were used in CPU boards Source. And creating a 16-bit ALU was just a matter of combining four 74181 ALU chips with a 74182 lookahead carry chip Source.

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.

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.

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.

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.)

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

• 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
• Intel 4004 Intel 8008 Intel 8080 Oral history panels