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. [https://www.righto.com/2023/08/datapoint-to-8086.html 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. The die shot of the 4004 shows reveals the initials FF of the chip designer, Federico Faggin. The die shot of the 8008 shows reveals the initials HF of the chip designer, Hal Feeney.
See: [https://www.righto.com/2016/12/die-photos-and-analysis-of_24.html Die photos and analysis] [https://www.righto.com/2017/02/reverse-engineering-surprisingly.html The ALU] [https://www.righto.com/2017/03/analyzing-vintage-8008-processor-from.html The counters] [https://www.righto.com/2020/11/reverse-engineering-carry-lookahead.html The carry-lookahead circuit] 8008 microprocessor reverse engineered