Instruction Register (IR): The instruction register holds the instruction currently being executed.
Memory Data Register (MDR): The memory data register (also known as the memory buffer register or data buffer) holds the piece of data that has been fetched from memory.
Memory Address Register (MAR): The memory address register holds the address of the next piece of memory to be fetched.
Program Counter (PC): The program counter holds the location of the next instruction to be fetched from memory. It is automatically incremented between supplying the address of the next instruction and the instruction being executed.
Accumulator: The accumulator is an internal CPU register used as the default location to store any calculations performed by the arithmetic and logic unit.
current instruction register (CIR)
control unit (CN)
central processing unit (CPU)