How It Works: CPUs

The CPU Process

A CPU processes information stored in bytes of memory. That information can be data or instructions. Data is a binary representation of a letter, a number, or a color, for example. An instruction tells the CPU what to do with the data: to add, subtract, or move it, for example.

The CPU performs three basic operations on data: It reads data, it manipulates that data, and often it writes that data to memory. At the simplest level, the CPU needs only four elements to perform its data operations: instructions, an instruction pointer, some registers, and the arithmetic logic unit.

The instruction pointer tells the CPU where in memory the instruction it needs to run is located.

Registers are temporary storage places on the CPU. A register holds data waiting to be processed by an instruction, or data that has already been processed (say, the result of adding two numbers together).

The arithmetic logic unit, or ALU, acts as the CPU's calculator, performing the math and logic functions dictated by the instructions.

The CPU consists of some additional parts that help the basic parts do their jobs:

The instruction fetch grabs instructions from RAM or an area of memory located on the CPU.

The instruction decoder takes the instruction from the fetch and translates it so the CPU understands. It then determines what steps are necessary to accomplish that instruction.

The control unit manages and coordinates all the operations of the chip. It lets the ALU know when to calculate, tells the fetch when to grab a value, and tells the decoder when to translate the value into an instruction.

Tracing an Instruction

Here's how a CPU does its thing: The instruction pointer directs the instruction fetch to a spot in memory that holds an instruction. The fetch grabs the instruction and hands it to the instruction decoder, which examines the instruction and determines what steps are necessary to accomplish that instruction. (An instruction can comprise numerous steps that need to be accomplished in a specific order.)

The ALU then performs the work the instruction calls for: It adds, subtracts, or otherwise manipulates the data. After the CPU interprets and performs an instruction, the control unit tells the fetch to grab the next instruction in memory. This process continues--instruction after instruction, at a dizzying pace--to create the results you see on your monitor. That's what a program such as a word processor is: a series of instructions and data.

To make sure everything happens on time, the parts require a clock generator. The clock generator regulates each of the actions the CPU performs. Like a metronome, the clock generator sends out pulses that set the pace for the CPU. Pulses get measured in millions of cycles per second, or megahertz, which you'll recognize as the familiar measure of a CPU's raw speed. The more pulses, the faster the CPU. If all things were equal, a 700-MHz CPU would always be faster than a 600-MHz CPU, but additions to and variations on the basic CPU make those numbers less meaningful.