The CPU and the Von Neumann architecture

Last time

Before we start: last time, in Encryption, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

The brain of the computer

Every app you open, every key you press, ends up in one small chip. The central processing unit, the CPU. It processes data and executes instructions, billions of times every second.

What the CPU does

The CPU, also called the processor, is the part of the computer that processes data and executes instructions. It fetches each instruction from memory, works out what it means, and carries it out. A microprocessor is a CPU on a single small chip, like the one in your phone.

Von Neumann architecture

Almost every computer today follows the Von Neumann architecture, described in 1945. Its key idea: data and instructions are stored together, in the same memory, in binary. Every memory location has its own unique address. And instructions are fetched and executed one at a time, in order.

The picture

Here is the Von Neumann model. Inside the CPU: the control unit, the arithmetic logic unit, and the registers. Outside: the main memory, joined by three buses.

The control unit

The control unit decodes each instruction, working out what it means. It sends control signals to the other parts, telling them what to do and when. It manages the flow of data through the CPU. It does not do any calculations itself.

The arithmetic logic unit

The arithmetic logic unit carries out the calculations: add, subtract, multiply and divide. It also carries out logical operations, such as comparing two numbers, and AND, OR and NOT. It does the logical shifts too, and its result is held in the accumulator.