2.4 Computer systems (contd)
As I have already mentioned, the functional blocks shown in Figure 3 relate very closely to, even though they are not necessarily identical with, the computer's physical components. The computer's physical components are normally known collectively as the hardware. Software is a term often used to refer to a computer program or a collection of computer programs which enable a computer to carry out its tasks. As the course progresses you will find out a great deal about computer hardware and software, including the processor and the programs it runs. You will generate some programs yourself and look at the operation of the processor to see how it executes the instructions within them. Through this you will gain an understanding of what form the data used by the processor and memory must take, and hence understand the role of the input and output subsystems.
Box 1: Electrical signals and computers
The data that travels along the main computer bus does so in the form of electrical signals which can have one of two possible values: a near-zero voltage, known as ‘voltage low’, and a rather higher voltage, probably around 3 volts, known as ‘voltage high’. (3 volts is used to represent a ‘1’ that is most common at the time of writing [early 2004]. A couple of decades ago the value was usually nearer to 5 volts, and older systems still use this value. Some newer systems are already using voltages near 2 volts and this may well become the most common value in a few years.) Any electrical signal where the number of possible values that can be used is limited is known as a digital signal. If the number of possible values is limited to just two, as on the computer bus, then the signal is known as a binary digital signal, or simply a binary signal. (The ‘bi’ in ‘binary’ means ‘two’.)
So the electrical signals that travel along the computer bus, and hence to the output subsystems and from the input subsystems, are all binary signals. Unfortunately, the electrical signals produced by input devices or needed by output devices are not necessarily binary, or even digital. Hence an important task of the input and output subsystems is to transform between the binary signals on the computer bus and whatever signals are used by their particular input or output device.
Activity 2 (Self assessment)
For the PC shown in Figure 2, write down which of the functional blocks in Figure 3 the following items of hardware relate to (for simplicity, items that provide input functionality can be assumed to relate to input devices, rather than input subsystems, and similarly for items that provide output functionality):
Keyboard, display, 500 GB hard drive, 6 GB DDR3 RAM, speakers, touchpad, SD reader.
The keyboard and mouse relate to input devices. The display and speakers relate to output devices.
To decide whether the 500 GB hard drive relates to the secondary memory, to the secondary memory subsystem or to the combination of both you need to make an intelligent guess about what those who wrote the advert meant. In this case I suspect they probably meant the combination of the two.
The 6 GB DDR3 RAM relates to main memory.
The SD reader relates to secondary memory.
Activity 3 (Exploratory)
The laptop shown in Figure 2 contains a sound card and a network card. What problem arises if you try to relate these two items to the functional blocks of Figure 3?
The problem is that a sound card is used for both input and output (it takes inputs from a microphone and delivers outputs to one or more speakers). The network card is also used for both input and output (it both sends signals to and receives signals from the telephone network). Therefore these items do not at first seem to fit neatly with Figure 3. But remember that the diagram is showing functionality. So you can relate the input functionality of the sound card to the ‘input subsystem’ item in Figure 3 (the microphone would be the input device), and the output functionality of the sound card to the ‘output subsystem’ (the speaker or speakers would be the output device). In the case of the network card, its input functionality relates to the ‘input device’ in Figure 3 and its output functionality to the ‘output device’.
The terms input-output device and input-output subsystem are sometimes used where items have both input and output functionality. Hence a sound card is an input-output subsystem.
Finally, just as you are just becoming familiar with all of the terms I have been introducing, I need to add a word of caution. When you read books or other literature about computers you may find some of the terms I have defined used differently. This is not necessarily a problem, and is common when technical terms become part of everyday language. However, throughout your study of this unit you do need to make sure that you use the terms as defined here.
One term I have not used here that you might come across is computer system. Historically some people used the terms ‘computer’ and ‘computer system’ rather differently. But that is no longer the case, and nowadays the word ‘system’ tends to be omitted. A good example is the use of the term ‘personal computer’, which would several years ago have often been described as a ‘personal computer system’.