The logic operations introduced here are summarised in Table 1, which is an example of what is known as a â€˜truth tableâ€™. It shows what the result (â€˜outputâ€™) of each logic operation is for all possible combinations of â€˜inputâ€™ values. You may find this format a useful one for remembering t
Author(s): The Open University

The exclusive-OR operation (usually abbreviated to XOR, pronounced â€˜ex-orâ€™) combines two binary words, bit by bit, according to the rules:

• 0 XOR 0 = 0

• 0 XOR 1 = 1

• 1 XOR 0 = 1

• 1 XOR 1 = 0

In other words, the result is 1 when either bit is 1 but not when both bits are 1 or both bits are 0, or the result is 1 when the two bits are different and 0 when they are the sam
Author(s): The Open University

The OR operation (occasionally called the inclusive-OR operation to distinguish it more clearly from the exclusive-OR operation which I shall be introducing shortly) combines binary words bit by bit according to the rules:

• 0 OR 0 = 0

• 0 OR 1 = 1

• 1 OR 0 = 1

• 1 OR 1 = 1

In other words, the result is 1 when either bit is 1 or when both bits are 1; alternativel
Author(s): The Open University

Just as multiplication can be turned into repeated additions, so division can be turned into repeated subtractions. And just as shifting a binary integer one place to the left equates to multiplying by two, so shifting a binary integer one place to the right equates to dividing by two.

As I indicated at the start of this section, subtraction is converted to addition by replacing the number to be subtracted by its additive inverse, which in the case of binary arithmetic is its 2's complement. An example should make this clear.

## Example 9

Subtract the signe
Author(s): The Open University

Sound, such as speech or music, is an analogue physical quantity that varies with time, and so the ideas you have already met in Section 2.5 about converting analogue weights to digital form are relevant here too. In particular, samples of the sound will have to be taken, and each sample will have to be quantised to the nearest binary code in the digital representation.

It's important to appreciate that sound such as speech or music varies rapidly with time, and so samples of it will ha
Author(s): The Open University

There are two basic methods of representing still images in a computer: bit maps (also sometimes called raster graphics or raster images) and vector graphics (also sometimes called geometrical-shape graphics or graphics metafiles). Bit maps are usually used when there is a great deal of detail, as in photographs, or when there are irregular shapes, such as in drawings of natural objects. Vector graphics are usually reserved for line and blocked-colour drawings consisting of regu
Author(s): The Open University

Sometimes a quantity that is to be represented in a computer has only two possible values, either true or false. An example of such a true/false quantity in the kitchen scales is the one that represents whether the scales are to weigh in metric or in imperial measure. The value of this true/false quantity is given by the true/false response to the statement â€˜the most recent push of the input button made the measuring system metricâ€™.

Author(s): The Open University

If computers encode the denary numbers of the everyday world as binary numbers, then clearly there needs to be conversion from denary to binary and vice versa. You have just seen how to convert binary numbers to denary, because I did a couple of examples to show you how binary numbers â€˜workâ€™. But how can denary numbers be converted to binary? I'll show you by means of an example.

Author(s): The Open University

Study note: You may like to click on the link below to the Numeracy Resource as you study Section 2. It offers additional explanations and extra practice on some of the topics, and you may find this useful.

Click on the 'View document' link below to open the Numeracy Resource.
Author(s): The Open University

The network conveys the data on items purchased through to the database server. It also conveys data such as revised prices and special offers from the database server back to the checkout terminal. In both cases this may involve selecting an appropriate route through the network and manipulating, storing or retrieving data.

Author(s): The Open University

The term 'network' is used to describe some very different interconnected systems. In a home setting, you might have just two computers linked together to share documents and devices (such as a printer and a scanner) and to use the same internet connection. This setup is a network, albeit a small one. At the other end of the scale is a multinational company with a network of computers distributed all over the world.

A network belonging to a single organisation, where the computers are c
Author(s): The Open University

The computer on the right of Figure 11 receives the data, manipulates it and then stores it. The computer then typically sends some kind of response back via the network, which may require the computer to retrieve some stored data.

The computer in this example is one of the Open University's FirstClass servers. A server
Author(s): The Open University

4.2.2 Network

In the same way as in the network shown in Figure 8, this network conveys the data to the receiver, selecting the most appropriate route for it to travel. In order to do this, the network may need to manipulate and store or retrieve data.

Your computer sends the FirstClass message into the internet, via your ISP connection, a
Author(s): The Open University

In the block diagram, the computer receives data from the user and sends it into the network. It will manipulate and also store and retrieve data.

If you send a message to a FirstClass conference, your computer receives the message from you as data via the keyboard. The computer manipulates the data into a form that can be sent into the network, in this case the internet via your internet service provider (ISP). Your computer will also store or retrieve relevant data, such as details of
Author(s): The Open University

13.4 Flash memory

Flash memory is an electronic form of memory which can be used, erased and reused. A flash memory card is a small storage device used to store data such as text, pictures, sound and video. These cards are used in portable devices such as digital cameras and in small portable computers, such as Personal Digital Assistants (PDAs).

A USB flash memory, sometimes called a 'memory stick', is a small storage device which is completely external and connects to the computer via a USB port
Author(s): The Open University

11.5 Programming languages

A computer program is written in a programming language and contains the instructions that tell the computer what to do. Developers write new software using specialised programming languages. The resulting programs (or 'source code') can be converted into the low-level instructions understood by the processor. There is a wide range of programming languages to suit different types of task; if you look at advertisements for programming jobs in newspapers or online you will get an idea of
Author(s): The Open University

6.2 Working with bits

You may have met the term bit, perhaps in connection with computers. The term 'bit' is also important in communication systems. It is an abbreviation for 'binary digit'. A binary digit can have just one of two values: it can be either 1 or 0. Pulses can be represented by 1s and 0s, that is, as bits, and so it is convenient to think of streams of 1s and 0s being conveyed along the communications link.

The rate at which the 1s and 0s are conveyed is known as the data rate or
Author(s): The Open University

4.1 Introduction

I'll now look at what these components do in the communication system, using the mobile phone system as an example.

Author(s): The Open University

2.2.1 Subsystems

An important aspect of systems is that each component can be considered as a subsystem. In the health centre appointments system, the 'computerised booking system' may be a complex system in its own right involving a number of computers networked together. Figure 2 shows this view of the system with 'computerised booking
Author(s): The Open University