Arithmetic logic unit (ALU)

Think before you reveal

Read carefully — only one of these is performed by the ALU itself.

A program runs the instruction: IF score > 100 THEN bonus = TRUE. Which part of the CPU carries out the comparison 'score > 100'?

How an ADD instruction flows through the CPU

Follow one instruction from memory through the CPU — each stage involves a different component

1. 1CU fetches the instructionThe control unit fetches the ADD instruction from main memory over a bus and decodes it to work out what to do.+
2. 2CU decodes and signalsThe CU decodes the binary instruction and sends a control signal to the ALU identifying 'perform addition'. The CU never does any arithmetic itself.+
3. 3Registers supply operandsThe two values to be added are loaded from registers into the ALU over a bus. Using registers rather than main memory keeps this stage fast.+
4. 4ALU performs the additionThe ALU carries out the arithmetic — adding the two operands together. This is an arithmetic operation, one of the two categories the ALU handles.+
5. 5Result placed in a registerThe sum is written into a register, where it is available for the next instruction or can be written back to main memory.+