How Do Computers Work?
The main purpose of a modern computer is to automatically process information. For many centuries the act of recording information was accomplished by using manual methods.

Ancient civilizations used stone or clay tablets to store information. Once written, they had to re-write everything on a new tablet to make changes. Even in more modern times this was still true.

Up until the 1980s, if we wanted to change the wording on a paper or the contents of a list, we had to retype the whole thing. With the advent of computers, changing written information became much easer.

The fact is that personal computers were developed to increase the ease and speed of processing information. This section is devoted to how information is processed in the modern PC

Processing Information Several separate actions can happen to information as its processed in a computer.

1.  Input ; Information is typed into the computer (it is input or added to a program).

  • let us say you need to distribute several copies of the list of all employees within your office or activity.

  • You could type their names into a database program or word processor.

2.  Making Changes Information is changed according to instructions given to the computer.

  • Let us say that you needed this list in a certain format such as alphabetical or by rank.

  • You would then give the computer instructions to sort the list. (This is called processing).

3.  Printing Information is printed in many different forms (this is the output).

  • Lets say you now needed 10 copies of the list of names to distribute to each employee.

  • Instructions would be given to the computer to provide the copies on your printer.

3.  Storing Information is saved by the computer or program for future use (this is storage).

  • Let say that now you would like to be able to update and print this list in the future as needed.

  • You would then give instructions to the computer to save this list in a file that could be accessed later. B>


The Absolute Basics
Computers operate using electrical current to represent "states" in the computer. The most basic state is "ON" or "OFF".

For simplicity's sake we will say:

  • The "ON" state is represented by a positive voltage. (usually 4.5 volts)

  • The "OFF" state is represented by a negative voltage (usually 0 volts or a voltage negative to the "ON" voltage.)

The "state" we are talking about is not whether the computer is on or off. Instead we are talking about whether a "bit" (a binary digit) is on or off.

This concept is much the same as an electric light.

  • If the switch is on, the light is receiving approximately 110 volts.

  • If the switch is off, the light is receiving 0 volts, and you are in the dark.

Most computers prior to the 1990's operated at voltages of +5 volts or greater for the "ON" state, and 0 volts to -5 volts for the "OFF" state.

Currently, processors are operating at 3.3, 2.7, and even 2.5 volts. This is great because the lower the voltage, the less heat is generated by the PC's processor and other components.

So, a binary bit has 2 values - a 1 or a 0. If we use the "extended ASCII" code, it takes 8 binary bits to make a "byte" . A byte can be used to represent a character, number, or symbol.

  • A byte might look like "10001001",

  • The ON/OFF representation it would be:

    "ON-OFF-OFF-OFF-ON-OFF-OFF-ON"
         1     0       0       0      1      0       0       1
Number Conversions: Actually, characters, numbers, and symbols are represented by a number of binary bits. In most PCs, we use the "extended ASCII" code and it requires 8 binary bits (a byte) to represent a character, number or symbol. This is the same as 8 contiguous light switches in a row.

Combinations of "ON" bits and "OFF" bits represent the characters, numbers, and symbols.

Need more basic on numbering systems?

  1. You and I think in decimal numbers. But computers think in "binary" or base 2 numbers.

  2. There are several numbering systems with bases other than decimal (base 10), e.g. octal (base 8) and hexadecimal (base 16).

  3. In the decimal numbering system, we count to 9 as a single digit, and when we add one to 9, we say 10. So a 9 + 1 = 10 (or one 10, and zero 1 s.) Eleven (11) is one "10" and one "1".

  4. Notice in the chart above: One character in the base 16 can be used to represent any number up to 15. In decimal one character can be used to represent a number up to 9.

Note: In binary a bit can represent a 1 or 0, in base 8 you can only represent up to the decimal number 7 without a carry.

So, computers use binary numbers to represent everything inside them. Computer instructions at the machine level are composed of binary numbers that direct the computer to perform actions.

The instruction must tell the computer what to do (add, move, decision, etc.), what to use (which data to use), where to put the results, etc. Don't stop here, DATA or information is also represented as binary 1's and 0's; but they are sets of specific bit combinations for representing a set of characters.

If you would like a bit more information on Number conversions, You can view the Number Conversion Tutorial here.

ASCII and EBCDIC code explanations.

How is a computer instruction executed? The most basic process is like this:

  1. Fetch Cycle - gets the next instruction to be executed into a register.
  2. Decode Cycle - break down the instruction to see what to do.
  3. Execute Cycle - do the thing the instruction is supposed to do.

You see an instruction as "ADD A to B giving X" . In computer instructions, this may take up to 20 simple instructions. Special computer programs called "compilers" take the "ADD A to B giving X", and translate them into "machine" language instructions which can be understood in your CPU!

So, all computer instructions are a set of binary 1's and 0's, arranged in a specific sequence to represent a machine or computer language instruction. A machine language instruction can be decoded and executed by the microprocessor (CPU).

Now, when a computer program is run on a computer, it is first loaded into a 640KB piece of memory. This 640 KB can be the conventional memory, or it can be a part of your extended memory (See Memory)

  1. Each instruction is moved from memory to a register in the CPU by the fetch cycle.

  2. The instruction is then decoded, it determines that it is an add instruction, then the data such as A and B in the example above are retrieved and put in registers.

  3. The data in A and in B are added together and stored in a register.

  4. The register storing the sum of A and B is moved to a location specified as X. X takes on the value of the sum.

  5. The next instruction is 'fetched' from memory and the cycle goes on.

  6. Actually the steps above may be many small instructions. We will worry about details later.

More sophisticated, modern PCs often use more complex procedures in executing a program's instructions. Some of the modern techniques are:

  1. Pipelining
  2. SuperScalar
  3. Super Pipelining

These are techniques for executing more instructions during a given time. They will be discussed elsewhere.