Register

A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only. In computer architecture, registers are typically addressed by mechanisms other than main memory, but may in some cases be assigned a memory address e.g. DEC PDP-10, ICT 1900.

An address register is a special or general purpose register that may be loaded with an address to access memory.

A Program counter (PC) is a special register that points into the instruction address space.

Some address register also support an index register that applies an offset to the register address before accessing memory.