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7.2 Adding 2's complement integers

The leftmost bit at the start of a 2's complement integer (which represents the presence or absence of the weighting −128) is treated in just the same way as all the other bits in the integers. So the rules given at the start of Section 7.1 for adding unsigned integers can be used.

Example 7


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2.7 Input and output considerations

So far in Section 2 I have focused on how the data is represented, or encoded, inside the weighing-scales computer. But how does it get into the computer? And how does it get out again in a form that users can recognise? These are big questions, and ones that later parts of the course will be going into in some detail. But I can sketch some answers here.

Weight is the most important input in the kitchen scales. To detect a weight, sensors are placed under the scalepan. They produce an e
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2.3 Representing numbers: fractions

In the denary system, a decimal point can be used to represent fractions, as in 6.5 or 24.29. One way of encoding fractions uses an exactly analogous method in binary numbers: a ‘binary point’ is inserted.

Some examples of 8-bit binary fractions are:

  • 0.0010110

  • 110.01101

  • 0101110.1

The weightings that are applied to the bits after the binary point are, reading from left to right, 1/2, 1/4, 1
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14.3 Personal Digital Assistants

Personal Digital Assistants (PDAs) or handheld computers are small, portable computers. They each contain a small processor and have specially written operating systems. Two popular types of PDA at the time of writing (early 2005) are those running the Palm OS operating system and those using the Windows Mobile operating system, (also called Pocket PC). There is a range of applications purposely written for PDAs, but many also use special versions of popular applications like Microsof
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4.2.3 Second computer (the FirstClass server)

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 Ope
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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
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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
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13.3 Optical storage

A CD-ROM (Compact Disk Read Only Memory) uses a laser-based optical form of storage. This type of disk has been used for many years to distribute music and computer software. A CD-ROM drive is needed to read the disks. Data is locked into the disk during manufacture, and cannot afterwards be changed.

There are two other types of CD device for computers: CD-R (CD-recordable) and CD-RW (CD-rewritable). With the right sort of CD drive in your computer, you can ‘burn’ data (that
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13.1 Introduction

I'll now introduce you to some different storage media and devices. As the uses for ICTs have expanded and developed, so has the need to store ever larger amounts of data. I've quoted some figures for storage capacity in this section but, given the rapid rate of development in ICT systems, some of these figures may be out of date when you read this unit.


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11.7 Applications

Most people buy computers in order to run applications. There are many different examples of software application, including word processors and spreadsheet, database and graphics packages. Some are combined together in ‘office’ suites, such as the StarOffice applications you can find on the Open University's Online Applications disk.

Word-processing software, such as Microsoft Word, allows you to create, edit and store documents. You can produce very professional-looking do
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11.6 Operating systems

A computer requires software just to look after itself and to manage all its components; this is called the operating system. The operating system handles communication with the other software on the computer and with the hardware resources of the machine, such as the processor and memory. The operating system provides a means of running the computer's application programs. It also provides a standard user interface with windows, buttons and menus so that users can interact with the co
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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 ide
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11.4 Computer software

The electronic components and other equipment that make up your computer system are known as hardware. In order to make the computer do things, such as help you to produce your TMAs, edit photographs or draw diagrams, you also need computer programs, which are called software.


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4.3.1 Confidentiality, integrity and availability

To preserve the value of an information asset, an organisation needs to sustain simultaneously its scarcity and its shareability within their respective regions. This is the critical high-level information security goal for any information asset; it is the entire rationale of an information security management system.

To maintain the security of an information asset, an organisation must:

  • either make the information asset unavailable in i
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7.2 Should I unsubscribe from mailing lists?

Many spam messages have a line at the bottom offering to unsubscribe you from a mailing list, but you should be very wary of doing this. Quite often the senders of the spam will use the ‘unsubscribe’ option to verify that your email address is live. They may then sell your address to other people for use in spamming. So using the unsubscribe option can increase your spam rather than reduce it. Our advice is never to use the unsubscribe option unless the mail you receive is from a well-kno
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7.2 An overview of RFID

The technology behind RFID is relatively straightforward and has been in use in some form for many years. You may have even used it yourself or seen it in use – for example in a ‘proximity card’ entry system in buildings, and in pet identification where a microchip is inserted just below an animal's skin. But you are likely to hear a lot more about it in the future and increasingly to see it deployed. This is because, at the time of writing (early 2005), the technology is receiving a st
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7.3 Quality of customer service

This is probably the key determinant of customer satisfaction. Most online transactions go through flawlessly. But some don't. Goods fail to arrive, or are delayed, or delivered to the wrong address. Or what is delivered doesn't match what you ordered. With a real-world bricks-and-mortar retailer, the solution is obvious: you phone or turn up at their premises and speak to a human being. But what do you do if an online order goes wrong? Whom do you phone? And where is the number for customer
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1 Introducing eBay

One of the earliest popular applications of the Web was an online auction system called eBay. Think of it as an enormous car-boot sale. In an ordinary (offline) auction system, you brought things you wished to sell to an auctioneer who catalogued and then sold them on your behalf at a physical auction attended by people who placed bids for the goods. There would be a series of bids with the auctioneer encouraging people to bid against each other, thereby raising the selling price. The highest
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Acknowledgements

The content acknowledged below is Proprietary (see terms and conditions) and is used under licence.

Grateful acknowledgement is made to the following sources for permission to reproduce material within this unit.

Figures

Figure 6 NanoElectronics Japan

Figure 30 The Cottingley Fairies © Science and Society Picture Library

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7.2 Frameworks

A framework is a set of classes with well-defined interactions which are designed to solve specific problems. Frameworks are usually developed for a specific domain of application, say financial management or document preparation. They go some way towards a complete solution, but require some degree of customisation, usually through the creation of subclasses within the framework or by overriding operations. Coplien suggests:

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