OCR A-Level Computer Science

## What the processor does A processor runs a stored program. The program and the data it works on sit in main memory as binary patterns, and the processor repeatedly takes the next instruction from memory, works out what it means and carries it out. Everything a computer does is built from that one loop, and the parts of the processor are best understood by asking which step of the loop each of them serves. ## The arithmetic logic unit, the control unit and the registers The **arithmetic logic unit** (ALU) carries out arithmetic (addition, subtraction), logical operations (AND, OR, NOT, XOR), comparisons and bit shifts. It has no idea which instruction it is serving; it is given operands and told which operation to perform, and it returns a result together with some status information, such as whether the result was zero or negative. The **control unit** (CU) decodes each instruction and then sends control signals to the other parts of the computer to make it happen: it tells the ALU which operation to perform, tells memory whether to read or write, and routes data along the right paths. It also keeps the steps in time with the clock, a regular electronic pulse that sets the pace of the whole processor. A **register** is a tiny store built into the processor itself. It is far faster to reach than main memory, which is why the processor does all its work on values held in registers. Five registers matter in the model of a processor used here. | Register | Holds | |---|---|…