Central Processing Unit (CPU)

If the computer system has a brain, it is the CPU. The Central Processing Unit is the electronic circuitry of a computer that carries out the actual processing, and for this reason it is usually referred to as the brain of the computer. It is also commonly called the processor, and — because of how it is manufactured today — it is popularly known as the microprocessor as well. Three names, one component.

What the CPU is physically

Physically, a CPU is placed on one or more microchips called integrated circuits (ICs). These ICs are made from semiconductor materials. So the "brain" is not an abstract idea — it is real circuitry etched onto small chips.

What the CPU does

The CPU never invents work for itself; it is given its instructions and data through programs. Its working cycle follows a clear pattern:

• it fetches the program and the data from the memory,
• it performs arithmetic and logical operations on that data, exactly as the instructions direct, and
• it stores the result back into memory.

This fetch–process–store rhythm repeats continuously while the computer runs.

Registers — the CPU's own local memory

While processing, the CPU does not reach out to main memory for every small step. It keeps the data and the instructions it is currently working on in its own local memory, called registers. Three points about registers are worth fixing in mind:

• they are part of the CPU chip itself, not a separate component;
• they are limited in both size and number; and
• different registers serve different purposes — some hold data, some hold instructions, and some hold intermediate results produced midway through a calculation.

Because registers sit inside the CPU, they are the closest storage the processor has, holding exactly what is needed at the moment of processing.

The two main components: ALU and CU

Apart from its registers, the CPU has two main functional components:

• Arithmetic Logic Unit (ALU) — performs all the arithmetic and logic operations that a program's instructions require. Every calculation and every comparison the computer makes ultimately happens here. …