5 -14 Beyond Level E - Computer Systems

In this section, we will look at how the inside of a computer is organised and also at some of the important programs that must always be present when the computer is switched on.
This section will be concerned with:

The Central Processing Unit (or CPU for short);

Computer Memory (RAM and ROM);

The Operating System and

Computer Networks.

Introduction

In order to work, a computer needs some sort of "brain" or "calculator" to perform sums or calculations and somewhere to store the instructions that we load.

The brain or calculator is called the Central Processing Unit and the storage space is called memory.

The Central Processing Unit

The Central Processing Unit or CPU is the device that does the calculations and the searching and sorting. The CPU is also the part of the computer which carries out the instructions contained in computer programs or software.

The CPU is a special type of microchip like this one:-

The Central Processing unit is made up of different parts:-

a part to perform calculations and comparisons- the ARITHMETIC and LOGIC UNIT;

a part to control the rest of the computer hardware - the CONTROL UNIT;

and a part to act as temporary storage within the processor - REGISTERS.

Here is a diagram showing the parts of a central processing unit:-

ALU = Arithmetic and Logic Unit, CU = Control Unit, REG = Registers

Computer Memory

When we load software from a floppy disk, hard disk or CD-ROM, it must be stored somewhere inside the computer. The main storage space for programs inside the computer is called MEMORY (or MAIN STORE).

There are two types of computer memory inside the computer, RAM and ROM.

RAM

RAM stands for RANDOM ACCESS MEMORY.

This is really the main store and is the place where the programs and software we load gets stored. When the Central Processing Unit runs a program, it fetches the program instructions from RAM and carries them out.

If the Central Processing Unit needs to store the results of calculations it can store them in RAM.

Random Access Memory can have instructions READ from it by the CPU and also it can have numbers or other computer data WRITTEN to it by the CPU.

When we switch a computer off, whatever is stored in RAM gets erased.

ROM

ROM stands for READ ONLY MEMORY.

The CPU can only fetch or read instructions from Read Only Memory (or ROM). ROM comes with instructions permanently stored inside and these instructions cannot be over-written by the computer's CPU.

ROM memory is used for storing special sets of instructions which the computer needs when it starts up.

When we switch the computer off, the contents of ROM do not become erased but remain stored permanently.

Here is a diagram showing the relationship between the Central Processing Unit and Computer Memory (RAM and ROM):-

Measuring Computer Memory

The more memory (RAM) a computer has, the better it will work, but how do we measure how much a computer's memory will store?

We can think of memory as a series of little boxes (called memory locations), each of which can store a piece of computer data (the data might be a number, a letter or an instruction).

It is number of boxes or locations that a computer's memory has, that gives us a measure of the size of its memory.

We measure computer memory in units of thousands of locations and in units of millions of locations.

ONE THOUSAND memory locations (actually 1024) is known as a KILOBYTE (Kb) of computer memory.

ONE MILLION memory locations (actually 1,024,000) is known as a MEGABYTE (Mb) of computer memory.

Here are some examples:-

Note:- the same units of measurements are used to describe the storage capacity of floppy disks, hard disks and CD-ROMs. Hard disk storage is becoming so large that we need a new unit of storage to describe how much they can store - 1 Gigabyte which is 1024 Megabytes.