Cheap Computer CPU
How it Works

Part 2

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How does a CPU Work?

The basic operation of a CPU (microprocessor) is to execute a sequence of stored instructions known as a program.

A program is represented by a series of numbers that are retained in the computer memory.

To execute a program, the processor performs four (4) basic steps; namely fetch, decode, execute and writeback.

  1. Fetch - In this first step, instruction is retrieved from memory. Fetching the instruction from memory can be slow and causes the processor to stall while waiting for the instruction to return. This problem was obvious in older processors but in newer processors it was corrected by the incorporation of cache.

  2. Decode - In this step, instruction is divided up into portions that are significant to other parts of the microprocessor. The processor's instruction set architecture (ISA) interprets this numerical instruction value.

  3. Execute - During this step, various parts of the processor are connected to perform a required operation. For example, if an addition or a subtraction operation was required, the arithmetic logic unit (ALU) would be connected to a set of inputs and outputs. The inputs are the numbers that will be added or subtracted and the outputs would be the final sum.

  4. Writeback - The last step does exactly what it is called......writes back the results of the execute step to computer memory. Most times the results are written to the processor's internal register for quick access by the next set of instructions.

    After the instruction execution and data writeback, the entire process repeats.

Let's add a little more to the above information:

This is the process by which external data is received into the computer. This could either be running a program or getting keyboard responses. Common inputs include a keyboard, mouse, modem, scanner, etc.

The computer system analyzes the input data.

Process describes the converting of input into output and is generally guided by a program.

Output is the process by which the CPU sends data to devices such as the monitor, printer, disk drive, etc. Output takes the results of the processing and sends them to be stored in memory or printed or displayed.

Output is the final result of the processing of the data that was input and used by the computer system.

Data must be stored either temporarily or permanently. Therefore, this is where memory is required. The computer needs a way to hold onto data as the processing is being performed.

The computer stores data in memory and retrieves the data it needs from memory. There are two kinds of memory - ROM and RAM.

ROM (Read-Only Memory) is permanent and will be retained even when the computer is turned off - eg. BIOS.

RAM (Random Access Memory) is volatile and data stored in RAM will be erased when the computer is turned off. The processor uses RAM to store data and retrieves data from RAM as it's needed. The instructions of a program, for example, would be stored in RAM. RAM will be lost if the computer loses power.

Processor Limitations

Having described the computer processor and how it works, we now need to know its capabilities and limitations.

Processor Cooling

Very little power was consumed by the earlier processors but today's 'power-hungry' processors consume much more power. Some are built for energy efficiency.

Processors will crash or malfunction if overheated; therefore, it is of utmost importance that they have adequate cooling.

The two main methods of CPU cooling are liquid and air cooling of which the latter is more widely used. Liquid cooling is now provided by many PC manufacturers and is undoubtably the more efficient.

In both types, the heat sink or heat sink/fan assembly must be securely mounted to the processor after applying thermal compound to the core. Thermal compound assists in the fast dissipation of heat from the processor.

EXERCISE EXTREME CAUTION when installing the heat sink since improper installation can damage the processor or motherboard.

Substandard cooling can cause maximum operating temperatures to be exceeded. This may or may not damage the processor but could cause crashes in processors. This malfunction generally disappears when the processor cools down.

In the case of Intel, if it overheats, it will automatically reduce its speed thus avoiding damage. Sometimes this reduction in speed goes unnoticed.

If you operate the processor without a heat sink, some processors will generally be permanently damaged unless the motherboard is equipped with special protection circuitry.

Browse here for some CPU coolers CPU Coolers

For anyone considering the building of their own cheap computer using cheap computer parts, the AMD CPUs, although not competitive with Intel in terms of performance and speed, provides a better price-performance ratio.

Buying a boxed versus OEM CPU

  • The boxed version comes with a heat sink, fan assmebly and manufacturer's warranty.

  • OEM processors do not come with a heat sink fan assembly and enables you to purchase a CPU cooler of your choice.


I have given you a concise guide to cheap computer processorss. The choice is now yours.

One final advice......Don't be fooled by sales 'pitches' recommending an 'upgradeable' processor. This is an added cost which at most times is impractical.

Replacing the system would be more ideal but, if you are electronically inclined and like to 'tinker' and build cheap computers, you could upgrade the CPU. Otherwise, my recommendation to you is do not upgrade!

< < < < Click here to go to Part 1 of this Guide

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