The Central Processing Unit (CPU)

The central processing unit (CPU) is the part of the computer that actually carries out the processing — it interprets and executes instructions and controls the working of all the other units. Because it does the "thinking" and directs the whole machine, the CPU is called the brain of the computer. In a modern computer the CPU is a single tiny chip called a microprocessor.

The CPU has three main components:

1. Arithmetic and Logic Unit (ALU). The ALU is the part that does the actual calculations and comparisons. It performs:

• Arithmetic operations — addition, subtraction, multiplication, and division.
• Logical operations — comparisons such as equal to, greater than, less than, and decisions based on conditions (true/false, yes/no).

All the real "work" of computing a result happens in the ALU.

2. Control Unit (CU). The control unit is the part that manages and coordinates the whole system. It does not perform calculations itself; instead it:

• Fetches instructions from memory one by one.
• Decodes (interprets) each instruction to find out what is to be done.
• Sends the necessary control signals to the ALU, memory, and input/output units so that the instruction is carried out.

Because it directs the flow of data and instructions inside the computer, the control unit is sometimes called the "nervous system" of the computer.

3. Registers. Registers are very small, very fast storage locations inside the CPU that temporarily hold the data and instructions the CPU is working on right now. Because they are inside the CPU, they are the fastest storage in the whole computer, though they can hold only a tiny amount of data at a time. Examples include the accumulator (which holds results of the ALU) and the instruction register (which holds the instruction being executed).

How the CPU works — the machine cycle. The CPU repeats a simple cycle over and over, millions of times a second: Fetch an instruction from memory, Decode it in the control unit, Execute it in the ALU, and Store the result back in memory or a register. This repeated fetch–decode–execute–store sequence is called the machine cycle (or instruction cycle). …