An introduction to computers and computer systems
An introduction to computers and computer systems

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1 The personal computer

You will find that all three of the examples match with the functional block diagram of the laptop computer given in Figure 1 in Session 2, although the tasks they have to perform mean that the individual components which perform the functions of the blocks within the diagram are quite different.

You looked at how the components of a computer could be related to the functional block diagram. Figure 1, which is a functional block diagram for the laptop, shows the data flow between the specific components of the laptop. Notice that in this diagram there are the specific input and output devices and items of secondary memory instead of the generic items seen previously in the functional block diagram in Session 2. That diagram is a generic for any computer; whereas Figure 1 below is its specific for the laptop.

This is a functional block diagram with the following labels: Processor; Main memory; Hard disk; Secondary memory subsystem (hard drive); SD card reader; Secondary memory subsystem; Keyboard; Keyboard input subsystem; Mouse; Mouse input subsystem; Monitor; Monitor output subsystem; Speakers; Speakers output subsystem (sound card).
Figure 1 Functional block diagram for a laptop.

Other PCs may have some additional input devices such as a stylus, plus some additional output devices such as a printer.

The PC is a general-purpose computer. It can run different software programs at the user’s request, and hence can be used for a variety of different applications. Typical examples are word processing, sending and receiving email, playing games, browsing the web, and sound and image recording and playback.

The following quote from the book A Shortcut through Time, The Path to Quantum Computing by George Johnson shows that even those long familiar with the concepts of how PCs work can still find them fascinating. (A register is a part of a processor and the term ‘disk drive’ is often used to describe either a floppy disk or hard disk.)

With a modern PC we blithely double-click an icon on the desktop summoning a flow of data from the disk drive – the pattern of bits that configures thousands of little switches to act as a word processor or a web browser or an MP3 player – temporary little structures, virtual machines. They are allowed to exist only as long as they are needed. Then they are wiped away and replaced with other structures, all built from 1s and 0s.

It is hard to believe sometimes how well this works. You can call up a movie trailer in a window and drag the image around the desktop, causing millions of bits to pour through the computer’s hidden registers. It is overwhelming to try and imagine the precise coordination going on behind the screen. Ultimately though it all comes down to shuffling 1 s and 0s, flipping little switches on and off.

(Johnson, 2004)

In Session 4 you looked at how data can be represented by bits – two bits can represent four items, three bits eight items, four bits sixteen items, etc. This is fine if, for example, you want to represent a clearly defined set of data such as the letters of the alphabet and numbers. You also saw how the images of the ‘movie trailer’ in the above quote can be represented in your computer, even though images are a more complex issue of data representation. As you will see from the next two examples, electronic kitchen scales and a digital camera, this issue of data representation exists in all computers.

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