CPU Register

A register is a very small, very fast storage location inside the CPU that holds the data and instructions the processor is working on right now. Registers are faster than cache and RAM, but there are only a few dozen of them.

Important registers include the Program Counter (address of the next instruction), the Instruction Register (the instruction being executed), the Accumulator (results of arithmetic), the Memory Address Register (MAR) and the Memory Data Register (MDR). A "64-bit" processor has general-purpose registers that are 64 bits wide.