CPU Components: ALU, Control Unit, Registers and Buses

Accumulator (ACC)

Holds the results of calculations carried out by the ALU and data loaded from memory via the MDR. Its value is checked when a conditional branch is executed, and input and output pass through it.

Address Bus

Carries the address of the memory location to be read from or written to, from the processor (the MAR) to memory. It is unidirectional, and its width determines how many locations can be addressed.

Arithmetic and Logic Unit (ALU)

The part of the processor that performs arithmetic operations such as addition and subtraction, logical operations such as AND, OR and NOT, and comparisons. Its results are placed in the accumulator.

Buses

Sets of parallel wires that transfer data, addresses and control signals between the processor, memory and input/output devices. They only carry signals; they do not store, process or control anything.

Control Bus

Carries control signals, such as memory read, memory write, clock and interrupt signals, between the control unit and the other components to coordinate their operation. It is bidirectional.

Control Unit (CU)

Decodes instructions and coordinates the processor by sending control signals, such as memory read and write, along the control bus, directing the movement of data between registers, the ALU, memory and input/output devices.

Current Instruction Register (CIR)

Holds the instruction that has just been fetched, copied from the MDR, while it is decoded and executed. The instruction is split into an opcode (the operation) and an operand (the data or address it uses).

Data Bus

Carries data and instructions between the processor and memory, and to and from input/output devices. It is bidirectional, since data is both read into the processor and written back out.

Memory Address Register (MAR)

Holds the address of the memory location that is about to be read from or written to, whether it contains an instruction or data. This address is then sent to memory along the address bus.

Memory Data Register (MDR)

Temporarily holds the instruction or data that has just been read from memory, or the data that is about to be written to memory. Its contents travel to and from memory along the data bus.

Program Counter (PC)

Holds the address of the next instruction to be fetched. Its contents are copied to the MAR at the start of each cycle and it is then incremented; a branch instruction overwrites it with a new address.

Registers

Very fast temporary storage locations inside the processor, each used for a single specific purpose, holding one item of data, an instruction or an address. They are quicker to access than RAM or secondary storage.