CPU Basics: What a CPU Is and How It Executes Instructions

1. What a CPU Is

The CPU is the core component of a computer, comparable to a brain, implemented as a small chip on the motherboard and constructed from billions of microscopic transistors. It determines the computer's computational capability because it performs the calculations required by programs stored in system memory.

2. What the CPU Actually Does

The CPU’s work revolves around obtaining instructions from RAM, decoding their meaning, and carrying out the required computation. This process consists of three key phases: fetch, decode, and execute.

3. Internal Structure

Generally, a CPU is divided into two major parts: the control unit , which fetches instructions from memory and decodes them, and the arithmetic‑logic unit (ALU) , which performs arithmetic and logical operations.

4. Registers and the Program Counter

Beyond the control unit and ALU, a CPU contains many registers that hold intermediate data. The program counter (PC) stores the address of the next instruction to be executed. During sequential execution the PC is incremented by 1, but conditional branches or loops cause the PC to jump to a different address, as illustrated by the example where the PC moves from address 0100 to 0102, evaluates a condition at 0106, and then jumps to 0104.

5. Flag Register

The flag register records the result of the most recent arithmetic operation using three bits: one for a positive result, one for zero, and one for a negative result. These bits are consulted by jump (branch) instructions to decide whether a conditional branch should be taken.

6. Instruction Execution Pipeline

Most von Neumann‑style CPUs execute instructions in five stages:

Fetch: read the instruction from memory into a CPU register.

Decode: split the instruction according to its format and identify operands.

Execute: perform the operation specified by the instruction.

Memory access: retrieve or store operands in main memory as needed.

Write‑back: write the result back to a register for use by subsequent instructions.

7. Function Calls and Array Addressing

Function calls differ from simple jumps: the CPU saves the return address, sets the PC to the function’s entry point, and after the function finishes, restores the PC to continue execution. Array‑like access is achieved with a base register and an index register , which together form an address that can be incremented to step through contiguous memory locations.

8. Visual Illustrations