6502
The 6502 CPU is an 8-bit processor designed in the mid 70s. It has been used in various home computers and consoles at the time, amongst them the Apple II. The core (or a variation of it) is still in use in some Microcontrollers.
An excellent article exists on Wikipedia: https://en.wikipedia.org/wiki/MOS_Technology_6502
For those really interested in hardware design of a CPU, the Javascript simulator might be interesting http://www.visual6502.org/JSSim/index.html Another really interesting site is Monster 6502: They build a complete, working transistor-scale replica of the classic MOS 6502 microprocessor. https://monster6502.com/