Digital Electronics: Principles, Devices and Applications

Computer Fundamentals 607Secondary or auxiliary memory cannot be accessed by the CPU directly. It is accessed by theCPU through its input/output channels. Secondary memory has a much greater capacity than primarymemory, but it is much slower than the primary memory. It is used to store programs and data for futureuse. Most commonly used secondary memory devices include the hard disk, floppy disks, compactdisks (CDs), USB disks and so on. The hard disk is used for storing the high-level operating systems,application software and the user data files. Floppy disks have a limited capacity of 1.44 MB and havebeen replaced by CDs and USB drives. Floppy disks, CDs and USB drives are also referred to asoff-line storage devices as they can be easily removed from the computer. Different types of memoryare covered in Section 15.4.15.1.3 Input/Output PortsA port is a physical interface on the computer through which the input and output devices areconnected to and interact with the computer. Ports are also used as an interface to connect twocomputers to each other. The ports on the computer can be configured as input and output portsthrough software. These ports are of two types, namely serial ports and parallel ports. Serial portssend and receive one bit at a time through a single wire pair. Parallel ports send multiple bitsat the same time over a set of wires. Serial ports are used to connect devices such as modems,digital cameras, etc., to the computer. Parallel ports are used to connect printers, scanners, CDburners, external hard drives, etc., to the computer. Serial and parallel ports are discussed in detail inSection 15.8.15.2 A Computer SystemFigure 15.2 shows the block diagram of a typical computer system. The diagram basically showsthe interconnection of the computer with the commonly used input/output devices. Input devicesconvert the raw data to be processed into a computer-understandable format. Some of the commonlyused input devices include the keyboard, mouse, scanner and so on. Output devices convert theprocessed data into a format understandable by the user. Commonly used output devices includethe monitor, printer, cameras, and so on. Input and output devices are discussed at length inSection 15.7.15.3 Types of Computer SystemComputers can be classified into various types, depending upon the technology used or the size andcapacity or the applications for which they are designed.15.3.1 Classification of Computers on the Basis of ApplicationsBased on the application or the purpose, computers are often classified as general-purpose computersand special-purpose or dedicated computers. General-purpose computers are comparatively moreflexible and thus can be used to work on a large variety of problems including business and scientificproblems. For instance, banking applications such as financial accounting, pay-roll processing, etc., atthe head-office level would require the services of a general-purpose computer. The size and capacityof a general-purpose computer could of course vary, depending upon the quantum of data and the