OCR GCSE Computer Science

## What the CPU does The central processing unit (**CPU**) is the part of a computer that runs programs. A program is a list of instructions held in main memory, and the CPU repeatedly fetches an instruction, works out what it means and carries it out. Everything a computer does, from drawing a web page to playing a game, is done by the CPU carrying out very large numbers of these small instructions one after another. ## Components of the CPU The **arithmetic logic unit** (ALU) does the calculations and the logical comparisons. Adding two numbers, checking whether one value is greater than another and combining values with AND, OR and NOT all happen in the ALU. The **control unit** (CU) directs the rest of the machine. It decodes each instruction, then sends signals that tell the ALU, the registers and memory what to do and when, and it controls the flow of data around the CPU. **Cache** is a small amount of very fast memory inside or very close to the CPU. It holds copies of the instructions and data that are used most often, so the CPU does not have to wait for the much slower main memory. **Registers** are tiny storage locations inside the CPU, and they are the fastest memory in the computer. Each one holds a single item that the CPU is working on at that moment, and each has a fixed purpose. ### The registers in the Von Neumann architecture The **Von Neumann architecture** describes a computer in which program instructions and the data they use are stored together in…