1. Register : Registers are the smallest data holding elements that are built into the processor itself. These are the memory locations that are directly accessible by the processor. It may hold an instruction, a storage address or any kind of data such as a bit sequence or individual characters. For example, an instruction may specify that the contents of two defined registers be multiplied together and then placed in a specific register.

Example: Accumulator register, Program counter, Instruction register, Address register, etc.