communications and software

1 Computer systems, communications and software

1.1 Components of a computer system and modes of use A computer system is made up of hardware and flow of data and information. The storage device is both software. In this chapter we explain the terms relating an input device and an output device. to a computer system that will be used throughout the 1.1 a, b rest of the coursebook. Definitions and purposes of hardware devices The hardware is the physical electronic and electrical input device main memory (hard disk drive) processor (CPU) output device components that make up the computer system. For example, these include the motherboard, a keyboard storage device and a monitor. In this section, we explain terms relating Figure 1.1.11 1 The typical hardware of a computer system. to hardware devices that we use throughout the rest of the text. In Chapter 1.4, we look at various types of hardware Peripheral devices are hardware devices that in more detail. are outside the central processing unit (CPU). They are normally connected to the computer by internal wiring (buses), cables or wireless technology. A 1.1 a, c Definitions and purposes of software types Software is the set of instructions that make the printer is obviously a peripheral as it is separate from computer hardware usable. These sets of instructions the computer. A hard disk drive is also a peripheral are known as programs. So, a computer program is a because it is not part of the CPU. This is less obvious piece of software. It is important to distinguish between because it is in the same box as the computer. operating system software and generic applications Most peripheral devices are one of three types: software. An input device transmits data from the user to Operating system software is the set of instructions the computer processor. A computer is only of that make the hardware of the computer available for value if it is possible to give it data and processing use. It includes programs: instructions. that allow the user to communicate with the An output device conveys the results of processing computer from the computer processor to the user. that control the movement of data around the A storage device can store data outside the computer processor. When a computer system is switched off, that turn instructions that people can understand data in the processor are lost. Data that are needed into instructions that a computer can recognise. for future use must be saved to a storage device Applications software is a set of instructions designed Figure 1.1.1 shows a typical set of hardware in a to make the computer do something for the user. For computer system. The arrows show the direction of example, the user might want to write a letter, create a report or create a slide show presentation. 2 Module 1

Software that can be used for many tasks is called generic applications software or general purpose applications software. Microsoft Office is a suite of generic applications, including Microsoft Word (a word processing application), Microsoft Excel (a spreadsheet application) and Microsoft PowerPoint (a presentation application). Other applications software carries out a specific sequence of tasks. For example, a payroll application can process payroll data, print payslips and generate BACS payments. It cannot process electricity billing data. In Chapter 1.2, we discuss types of system software in more detail. You will notice that applications software is covered throughout much of the rest of the syllabus, which should give you an idea of its importance. Chapter 1.7 looks specifically at applications software. Summary Hardware is the physical components that make up the computer system. Software is the set of instructions (programs) that make the computer hardware usable. An input device is a physical component that transmits data to the computer. A storage device is a physical component that stores data for use by the computer. An output device is a physical component displays data transmitted from the computer. Operating system software comprises the programs that make the computer hardware available for use, often unseen by the user. Generic/general purpose applications software comprises programs that can be used for many tasks, such as a word processor, spreadsheet software and database software. Test yourself 1. Explain the difference between hardware and software. [2] Hint The question mentions two items ( hardware and software ) and two marks are available. Therefore, you should make two points. Make sure that the second point is not just the opposite of the first. The word explain indicates that the two points need to say how the items differ. 2. Give two reasons why a computer system would need to have some type of external storage device. [2] Hint Notice that the question asks for reasons why the system needs external storage. Do not say what is stored or what sort of device might be used. There are two indicators in the question that you should make two points: the word in bold and the marks awarded. 3. Describe the difference between system software and application software. [2] Hint This question is similar in style to Question 1. Remember that you need to state two distinct points that are related to each other. 1.1 Components of a computer system and modes of use 3

1.2 System software 1.2 a The purpose of an operating system An operating system (OS) is the main item of system software used by a computer system. The OS is a set of programs designed to run in the background on a computer system and provide an environment in which application software can be executed. Most operating systems comprise a large set of programs, only some of which are stored in the computer s memory all the time. Many of the routines available in the OS are stored on the hard drive so that they can be accessed when required. This saves space in the computer s main memory. When you are using applications software, you are not communicating directly with the computer hardware. Your applications software communicates with OS program modules that communicate with the computer hardware on its behalf. Without an operating system, a computer is useless no matter how many programs you have. There are many different types of operating system. Each type makes the computer behave differently and is appropriate to a given computer system and its applications. We look at some of the different types of OS in this chapter and again in other parts of the syllabus. All operating systems have some common purposes: The OS must manage the physical resources of the computer. Some resources are limited and must be managed to maximise the use of the computer system: A simple system has only one processor. Secondary storage is of a fixed size. Some input/output devices (e.g. printers) are shared. The OS provides a means of communication (the user interface) between the human user, or the outside world, and the computer. 1.2 b The OS provides a platform on which the applications software can run. The OS hides the complexity of the hardware from the user. The OS controls access to the computer system so that you can put passwords on your files to stop other people seeing them. It controls the access rights of different users. Whether or not a certain feature is classified as part of the operating system or as a utility program varies considerably across operating systems. We consider utility software in more detail at the end of this chapter. Types of operating system Batch When computing was still a new science, there were not enough machines to satisfy the demand for processor time. There was a speed mismatch between the user sitting at the keyboard, who was very slow, and the processing by the computer, which was very fast. One solution to this problem is to buy more machines. Another solution is to make the machines work more effectively by taking away the slowest part of the system the human being. Nowadays we might well opt to buy more machines! When computers were very expensive, an aim for efficiency improvements gave rise to batch processing. A batch processing operating system does not allow for interaction between the user and the processor during the execution of the program. Lots of programs (or lots of data to be run through the same program) are collected together (to form a batch). The batch operating system then controls their passage through the computer. 4 Module 1

Nowadays, batch processing is used for applications where: there are large amounts of data to be processed the data are very similar in nature and require similar processing the computer system has identifiable times when it is not being used and so has available processor time the application does not require interaction by a user. Typical examples of applications which use batch processing include: the processing of payroll information the production of bank statements from customer files at periodic intervals; e.g. all accounts with surnames starting A E could be processed on the fifth of each month, surnames F J on the tenth of each month, and so on the production of utility (gas, electricity and telephone) bills from customer records. Real-time A real-time operating system can react quickly enough to affect the next input or process that needs to be carried out. There is a continuous cycle of input processing output. Most real-time systems are based on the control of some process in the real world (where the processing takes place on a timescale of milliseconds). Some information processing applications require the processing to be done within seconds; it still fits the description of the continuous cycle and is known as a pseudo real-time operating system. The following examples of real-time applications show why immediate response can be vital: A chemical plant has a reaction vessel in which the temperature is critical to the result of the process. The temperature is monitored by a computer that accepts input from a sensor. The computer uses the sensor data to make decisions about adjusting the heating elements in the vessel. A delay in the decision-making process might corrupt the reaction. A robot trolley is controlled by a processor that takes input from a sensor following a black line on the floor. The processor makes decisions about steering to keep the trolley on the black line. The trolley would very soon lose its direction if it was not steering quickly enough. A catalogue shop processes orders. The code for a product is input and the system compares it with information in its files. When it finds the correct code, it can report to the user the quantity of that product in stock. It is necessary to record a purchase before the next shopper s request is processed otherwise the second person might be sold the same item. Single-user A single-user operating system controls a system which has only one user, and their programs, at any one time. A perfect example of a single-user system is the PC or laptop that you may have at home. Only one person uses the computer system at any one time. Note that it does not mean a system that only ever has one user! This means that security measures are important so that the system only allows access to files that belong to the appropriate user. Multi-user Again, as the name implies, this type of operating system allows the computer to service more than one user simultaneously. A multi-user operating system has a single (normally powerful) computer which is connected to a number of terminals. These terminals are not computers, although they may have a very limited amount of processing power. They rely on the main computer to carry out any processing. The computer sends a message to each of the terminals in turn, asking if it wants any processing to be carried out. This process is called a time-share system or a round robin system. Each of the small amounts of processor time is called a time slice. The length of a time slice varies according to the system but is typically about a hundredth of a second. A configuration in which a central computer (a server) processes data from several terminals (see Figure 1.2.1 on page 6) is called a thin-client network. 1.2 System software 5

terminal 1 terminal 2 terminal 3 server stores: - application programs - users data files network cable terminals do not have any processing capability server server processes each terminal s application program Figure 1.2.1 A multi-user computer system. Since terminals do no processing this is a thin client network. There is a limit to the number of terminals that can be serviced. The system needs to be fast enough that a user at a terminal does not have to wait for too long before being given processing time. The system can be speeded up if the computer ignores terminals that are not active. Terminals can send a signal to the computer when they need processor time. Such a system is only satisfactory if the response time for all users is acceptable; no user should have to wait for a long time for a program to respond. Multi-tasking A multi-tasking operating system allows several applications to be available simultaneously. On a single-user system, you are probably used to having a number of processes loaded in main memory at the same time. One window may show a spreadsheet while another shows a word processing application; you may also be connected to the Internet through an email client; and you may be playing music while you work. It appears that more than one task is running simultaneously but they aren t, they just appear to be. The OS can switch between tasks so quickly that it seems to the user as if they are all being done at once. The method used to do this is very similar to the multiuser OS. The tasks are allocated time slices and the computer goes from one to the other servicing them. Microsoft Windows is a multi-tasking operating system. You can view a list of the running processes in the Task Manager operating system utility (Figure 1.2.2). In Chapter 3.1, we further consider multi-tasking in Windows. 1.2 c Network A network comprises a number of computers linked together for the purposes of communication and the sharing of resources. Networks are important because they allow hardware and software to be shared. They require the computers to run a network operating system. Often, one of the machines is used to control the rest of the system; this machine is called the server. A network OS must carry out tasks such as: control of access to the network management of the filing system management of all applications programs available from the server management of all shared peripherals. A common misunderstanding is to confuse network systems with multi-user systems. A multi-user system has many users using one computer at the same time; a network system has many computers each using shared hardware and software. In Chapter 3.8, we consider networking in detail. Application requirements for operating systems In an examination, you will be asked to identify applications that require batch processing and applications that require real-time processing. Applications that require batch processing include: payroll, the production of bills and the production of bank statements. Be very careful in choosing any other application it must be clear that there are good reasons for using batch processing. For example, simply to say that an ATM machine uses batch processing is not enough. Requests for bank statements 6 Module 1

Figure 1.2.2 The Windows Task Manager utility. are batched for later processing but the PIN for a customer must be checked in real time. You must be specific and justify your choice. For example, you could write Because statements must be sent out by post, they can be processed while the computer is not doing other things. It makes sense to process a large number of statements at the same time as a batch. Control applications, in which the results of a process are used to inform the next input, use realtime processing. A good example would be the control of a robotic machine on a production line. Information systems also require real-time processing. It is necessary to update the file of information before the next enquiry is dealt with. The classic example is the airline (or theatre) booking system. If a customer decides to buy a ticket for a flight, the number of tickets available must be updated before the next person makes an enquiry, otherwise another person may be sold the same seat. In general terms, an examination question will ask you to decide which of these two types of processing is most appropriate for a given application. It is not 1.2 d sensible to try to memorise a list of applications for each type. You must learn to identify the characteristics of each of the types of processing for a given application. Types of user interface The user interacts with a computer system through its user interface. The user gives input (e.g. the click of a mouse) and receives responses from the computer. A user interface (also called a human computer interface (HCI)) consists of both hardware and software. When a user types instructions into a computer and the computer responds by displaying details on a screen, then that is an interface. The keyboard and the screen are the hardware components; the software components of the interface allow the computer to understand the typed instructions. In the early days of computing, people could use a teleprinter instead of a monitor. The teleprinter was similar to a typewriter. As the user typed commands on it, the computer would print the commands and its response on paper. The hardware 1.2 System software 7

and software components of the interface need to be appropriate to the use. For example, a keyboard and screen are needed for an interface that enables the user to make enquiries about theatre tickets; a printer would enable the user to print a booked ticket. There are many different types of interface. Their features vary depending on the application, the conditions in which it is to be used and the knowledge and skills of the user. From the many types of HCI, you are expected to be able to describe the five software interfaces discussed below and give a typical application for each of them. In Chapter 1.9, we discuss the hardware that is necessary to put the software interface into operation. Form-based interface If the majority of the input to a system is standardised, a typical interface is an on-screen form to be filled in. This type of interface is used by an operator inputting information while talking to a customer over the telephone. The interface: prompts the operator to ask each of the questions in turn makes the operator input the information in the correct order and ensures that nothing is missed out ensures that the information is input in the correct format makes the checking of the information easier. A form-based interface (see Figure 1.2.3) has a specific area for each piece of data. For example, there may be: a drop-down list for restricted input (such as the person s title or a date) boxes for textual information (such as the name or email address) check boxes for yes no information (such as a box to indicate acceptance of conditions). The form uses standard widget controls text boxes, radio buttons, check boxes and drop-down lists. When the user enters data, the cursor often moves automatically to the next box to be filled in. Sometimes a Figure 1.2.3 Form-based interface. 8 Module 1

box is highlighted to make it clear to the operator where the data are to be inserted. Some of the data are more important than others and the cursor does not move on until data have been supplied. The interface checks that the input is sensible for that box before moving on. When a person orders something from a supplier on the Internet, a form-based interface is used. The customer fills in a number of standard personal and payment details. Menu-based interface A menu-based interface is used in situations where the user does not know what options are available. Examples of this are information systems for tourists and the on-screen menus for digital television. A list of choices is made available. When the user selects an option, a further set of choices is displayed, and so on until the result is obtained. Imagine an information system at a train station in a popular holiday location. The first screen may ask for the general area of interest (accommodation, trips, shopping or entertainment). If the user selects accommodation, the next screen may offer a choice of types of accommodation (hotels, guest houses, etc.). The third screen may offer different price bands. Finally, the user may see a list of all the available properties that match the choices of type and price band. Such an information system often uses a touch screen because other peripherals are less appropriate. For example, a mouse connected to an information system in a train station might be vandalised or damaged by the weather. Graphical user interface A graphical user interface (GUI) uses windows, icons, menus, and a pointer (giving rise to the acronym WIMP). The user of a GUI expects to be able to view different applications or files on the screen at the same time. This is done by putting each into its own bordered area known as a window. The user can select a command by clicking on a small picture (an icon) that represents it. The user selects further options from menus. Icons and menu options are selected by using some sort of pointing device such as a mouse. In Figure 1.2.4, the main menu is displayed horizontally as text options (File, Edit, etc.). Selecting Shape from the menu opens a drop-down menu of Figure 1.2.4 Graphical user interface. 1.2 System software 9

selections. Selecting Rotate or Flip gives access to a final menu from which the user can select the final command Rotate Right. The user can save a file in one step by clicking the Save icon (it represents a floppy disk); in Figure 1.2.4 you can see this button directly under the Edit button. The whole principle of a GUI is to make using the system as simple as possible by hiding all the complicated bits! For example, when the user clicks an icon to bring a piece of software onto the screen, it involves only one action. In the background, the computer is executing a complicated process involving a lot of instructions. When the icon is chosen, the computer is simply told to run those instructions so the software will appear. The icon has hidden the complexity from the user. Natural language interface A natural language interface is sometimes referred to as a conversational interface. The computer displays or speaks questions that give the user the impression that the computer is talking to them. However, the system restricts itself to questions that will provoke very simple responses which the computer can understand. If the user does not give one of the expected responses, a message is produced which makes it clear that the user must try again. It is often assumed that a natural language interface is very close to a spoken language. This may be true but it is likely to be typed into the machine rather than actually being spoken. For example, some Internet search engines or large information systems can process natural language queries. This is the next step up from a simple keyword search, where the software picks out the keywords and then searches for matching documents or files. Computerised telephone systems may use voice input to select menu choices. A true spoken interface might be used by a disabled person to communicate with a computer. Command line interface In a command line interface, or command-based interface, the user types a series of commands at the keyboard and the computer displays appropriate output (see Figure 1.2.5). The computer does not Figure 1.2.5 Command line interface. 10 Module 1