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. 1 Representing data in computers: introduction A computer is designed to do the following things:
receive data from the outside world;
store that data;
manipulate that data, probably creating and storing more data while doing so;
present data back to the outside world. In the next few sections I am going to examine in more detail the data that a computer receives, stores, manipulates and presents. I 18.2 Using e-commerce Many people now have internet connections and this offers many benefits to both businesses and their customers. From a customer's point of view, e-commerce has a number of advantages. Shopping can be done from home; you can probably find what you need without trudging from one shop to another and waiting in queues. You can also purchase goods 24 hours a day, every day. From the point of view of a business, e-commerce also offers a number of advantages. There is a potentially wide 16.2.4 Sending data As the items are scanned into the checkout computer, information about the price of each item may be shown on a small display so that the customer can see the price. Sometimes there is a beep as each item passes the bar code reader, to tell the checkout operator the item has been identified. Once all the items have been scanned, the total amount is displayed. Once the customer has paid, they are given a printed till receipt which shows all the items purchased, the price of each and total expe 15 Computers and communication systems working together The combination of communication systems and computers has produced powerful new systems not possible when these technologies are used separately. In section 15–19, I'll be using an ICT system in a supermarket as an example, as it is something that you have probably experienced. The material in this study session is not intended to be a comprehensive examination of how ICT systems are used in supermarkets; I'll just be focusing on some of the supermarket's activities in order to highlight t 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 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 4.2.1 First computer (your computer) 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 14.1 Introduction Now that I have introduced you to the processes carried out by a stand-alone computer, I will move on to discuss what happens when computers are linked. 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 13.2 Magnetic storage As I mentioned earlier, your computer has a hard disk which provides a permanent storage area for your computer's programs and the files you create. When you save files to your computer's hard disk, you are using a magnetic storage medium. Data stored in magnetic form can be changed once it has been stored, so if you run out of space you can delete some files to make room or, if you want to edit a file, you can make the necessary changes and then save it again. At the time of writing, a mediu 12.2 Bytes of data You will recall from Section 6.2 that a binary digit, or bit, can have one of two values: either a 0 or a 1. In a computer, bits are assembled into groups of eight, and a group of eight bits is known as a byte. The abbreviation used for a byte is B, so 512 bytes would be written as 512 B. Although this course will use ‘b’ for bit and ‘B’ for byte, you should be aware that not everyone makes this clear distinction. A byte of data can represent many different things in a co 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 11.3 Memory An essential component of a computer is the memory which it uses to hold data currently being used by the processor. This is the random access memory (RAM), the computer's working memory in which programs and data are stored so that they can be accessed very quickly by the processor. The processor stores data in RAM and retrieves data from it as it carries out its manipulations. The more RAM a computer has, the faster the computer programs will run. RAM memory is used and reused and an 11.2 The processor The processor can be thought of as the ‘brain’ of the computer in that it manages everything the computer does. A processor is contained on a single microchip or ‘chip’. A chip is a small, thin slice of silicon, which might measure only a centimetre across but can contain hundreds of millions of transistors. The transistors are joined together into circuits by tiny wires which can be more than a hundred times thinner than a human hair. These tiny circuits enable t 6.1 Networks Next I'll be looking more closely at the ‘network’ block in Figure 8, and in particular at the links that must be present before communication can take place. I'll introduce you to just a few of the forms that these links can take; links may be physical ones, such as cables, or they 5 The processes My description of the three subsystems of ‘means of conveying a message’ has indicated some important processes that each carries out. These are shown in Figure 8. The key processes are those that will always be carried out and they are shown in bold; the other processes may or may 2.3 Models of an ICT system To help me to introduce you to important ideas about ICT systems, I'm going to take a three-stage approach. ICTs involve conveying, manipulating and storing data. This is going to be the basis of my approach. Firstly, in the next few sections, we'll look at ICT systems where the primary function is to convey data. We can think of these systems as communication systems and I'll use a mobile phone system as an example. In sections 8–14, I'll focus on ICT systems wher 2.2.2 Drawing the boundary Deciding where to place the system boundary is an important consideration in that we have to think about what to include and exclude. This isn't always an easy decision to make and it often depends on the perspective of the person viewing the system. The system maps in Figures 1 2.2 A system map One way of explaining and analysing a system is to represent it in a graphical form, known as a system map. I'll use the example of a system for making an appointment with a doctor in a health centre to illustrate this point. In this example, the health centre uses a computerised booking system and the patient may phone or visit the health centre to make an appointment. Therefore, the system includes a patient, a receptionist, a doctor, and a computerised booking system. The example sh
Example 7
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