The processor is the part of a computer that follows a program. This is a simplified picture, because a real computer has more parts. A computer is built from a few main parts, including a CPU and memory. A program is a list of instructions for a computer. The processor in a computer follows them, and a programmer writes them. A computer's processor has two parts. One is the arithmetic-logic unit, a simple calculator. The other is the control unit that follows the program's instructions. The CPU of a computer can add two numbers. The parts of a computer are connected by a set of wires called the bus.

The CPU runs a program by repeating a cycle: fetch an instruction, then execute it. A machine code instruction is extremely simple, such as adding two numbers or testing whether a number is zero. Modern CPU chips have several cores, and each is a semi-independent CPU. Four of them are not four times as fast as one. A GPU is like a CPU but specialized for images, and computer games use it heavily.

A bit is the smallest piece of computer storage, and it holds a 0 or a 1. Binary is a number system that uses only two digits, 0 and 1. A byte is a group of 8 bits that can store one letter or other character, such as A or x. Each extra bit doubles the number of different patterns that bits can make. Bits are used to store text, numbers, images and video. A picture on a computer screen is made of many tiny squares called pixels. A picture on a computer is stored as a long list of numbers. Inside a computer, things such as pictures are stored with only 0s and 1s.

RAM is fast memory that holds the programs and files a computer is using right now. RAM loses what it holds when the power is turned off. A computer copies a program from storage into RAM before it runs. A hard drive stores a computer's bits as magnetic patterns on a spinning disk. Files that are saved on a computer's drive are still there when the power goes off. A hard drive can be far slower than RAM. One college course says at least a million times slower.

What was inside computers long ago? Charles Babbage designed several mechanical calculators in the 1800s. He also designed a computer called the Analytical Engine. Relays are mechanical switches that are opened or closed with electromagnets. A vacuum tube is an electronic valve that lets a small signal control a larger one, and tubes could also be used as switches. ENIAC was built between 1943 and 1945. It was the first large-scale computer to run at electronic speed with no slowing mechanical parts.

In 1947, John Bardeen and Walter Brattain built the first semiconductor amplifier, which was named the transistor. A transistor is a solid state switch with no moving parts, turned on and off by an electric signal. Transistors can work as switches in computers, and they replaced vacuum tubes in computers. Microscopic transistors are etched onto a fingernail-sized piece of silicon to make a chip. One computer chip can hold billions of them. So the processor is built from a great many tiny switches.

Words to know

• processor: follows the instructions
• transistor: solid state switch
• bus: set of wires
• cores: semi-independent CPUs