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Chapter2

# Chapter2 - Chapter 2 THE STRUCTURE OF COMPUTERS A computer...

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Chapter 2 THE STRUCTURE OF COMPUTERS A computer is a complex object composed of wires, silicon chips with electronic circuits on them, and so on, but we will not be trying to follow circuit diagrams and worrying about how to build a computer. What we will be interested in is the various main parts of a computer and what the function of each is. In this way your programming will be more intelligent; you will have a better idea of what is going on inside the computer. FUNCTIONAL UNITS We have already mentioned a number of things about computers. They have a memory where programs, numbers, and alphabetic information can be recorded. They can add, subtract, multiply, and divide. This means they have a part called the arithmetic unit . They can read information from a keyboard input and output results on a screen. They may also have a printer. The printer may output a whole line at a time or just one character at a time, like a typewriter. We say they have an input (for example, keyboard) and an output (a screen). Computers execute instructions in sequence. The part of the machine that does this is called the control unit . The arithmetic unit and the control unit are usually grouped together in a computer and called the central processing unit or CPU . So then the computer is thought of as having three parts, memory, input-output, and CPU.

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2 | THE STRUCTURE OF COMPUTERS Main Parts of a Computer We will look at these different parts in turn but first we must see how numbers and alphabetic information can be represented in a computer. CODED INFORMATION You are probably familiar with the way that information used to travel over telegraph wires in the form of Morse Code. Perhaps you know that each letter or number is coded as a pattern of dots and dashes. For example, the letter A is a dot followed by a dash, E is one dot, V is three dots and a dash. The letters are separated from each other by a pause with no dots or dashes. The famous signal SOS is ∙∙∙ - - - ∙∙∙ This is an easy one to remember in emergencies. The Morse Code was designed so that the signal could activate some noise-making device and the listener could then translate the coded message back into letters. Modern teletype machines can send messages much faster because the machines themselves can be used to decode the messages. For these, a character is represented by a pattern of pulses, each pattern being of the same length. Instead of dots and dashes, which are two different lengths of electric pulses, they use one basic time interval and in that time interval have either a pulse or a pause. Each character requires 5 basic time intervals and is represented by a sequence of pulses and pauses. We often write down a pulse as a 1 and a pause as 0, and then the pattern for b is 10011, i is 01100, l is 01001. The word bill would be transmitted as 10011011000100101001 Strings of ones and zeros like this can be associated with numbers in the binary system . In the decimal system the number 342 means 3 ×10 2 + 4 ×10 1 + 2 ×10 0 where 10 2 stands for 10 squared, i.e. 100, 10 1
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Chapter2 - Chapter 2 THE STRUCTURE OF COMPUTERS A computer...

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