They are located inside the processor. They are the quickest memory and are then located at the top of the memory hierarchy.
Zoom of CPU register of the Z80 processor (Source)
One standard function of a register is to define the input and output operand of an instruction. See Device interface via Register
- string instructions use the contents of the general purpose: ECX, ESI, and EDI registers as operands.
A processor have:
- a number of general purpose registers
- and a smaller number of dedicated registers.
Register type hierarchy:
- Data registers,
- and Index registers.
Documentation / Reference
- Vol1 - 3.4 Basic Program Execution Registers - https://software.intel.com/en-us/articles/intel-sdm