Anatomy Of A Computer
A computer can accept input, process or store data, and produce output according to a set of instructions which are fed into it. A computer system can be divided into two components which are responsible for providing the mechanisms to input and output data, to manipulate and process data, and to electronically control the various input, output, and their storage. They are known as hardware and software. The hardware is the tangible parts of the computer. Whereas, the software is the intangible set of instructions that control the hardware and make it perform specific tasks. Without software, a computer is effectively useless.
Hardware is the physical components of a computer that includes all mechanical, electrical, electronic and magnetic parts attached to it. A computer consists of the following major hardware components:
- Input and output devices
- Central processing unit (CPU)
- Memory unit and storage devices
- Interface unit
A brief description of the most common hardware found in a personal computer is given in the next few sections.
The data and instructions are typed, submitted, or transmitted to a computer through input devices. Input devices are electronic or electro-mechanical equipment that provide a means of communicating with the computer system for feeding input data and instructions. Most common input devices are briefly described below.
Keyboard Keyboard is like a type-writer. A keyboard, normally, consists of 104 keys. These keys are classified into different categories which are briefly described below.
Character keys These keys include letters, numbers, and punctuation marks. On pressing any character key, the corresponding character is displayed on the screen.
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Function keys There are 12 functional keys above the key board which are used to perform certain functions depending on the operating system or the software currently being executed. These keys are placed at the top of the key board and can easily be identified with the letter F followed by a number ranging from 1 to 12.
Control keys Alt, Ctrl, Shift, Insert, Delete, Home, End, PgUp, PgDn, Esc and Arrow keys are control keys.
Navigation keys These include four arrows, Page Up and Page Down, Home and End. These keys are normally used to navigate around a document or screen.
Toggle keys Scroll Lock, Num lock, Caps Lock are three toggle keys. The toggle state is indicated by three LEDs at the right-top side of the keyboard. For example, on pressing caps lock, letters typed by the user will appear in upper case. On pressing again, letters are typed on the screen in lower case.
Miscellaneous keys These keys include Insert, delete, escape, print Screen etc. The keys on the keyboard are placed in a series of rows and columns called the key matrix. Each key holds a position with respect to a row and column. When a key is pressed, the key switch in that position closes a circuit, sending a signal to the circuit board inside the keyboard. The keyboard controller uses the x and y coordinates of the matrix position to determine which key was pressed, thereby determining what code is transmitted to the computer by the keyboard.
Mouse A mouse is the pointing device attached to a computer. It is used to move the cursor around the screen and to point to an object (such as icon, menu, command button etc.) on the computer video screen for the purpose of selecting or activating objects on graphical interface provided by the operating system or the software currently being executed and executing various tasks. It has two or three buttons for clicking. The mouse tracks the motion of the mouse pointer and senses the clicks and sends them to the computer so it can respond appropriately. The mouse can be connected to the system either through a USB connector or wirelessly through infrared radiation. A wireless mouse needs to be powered through batteries.
Scanner A scanner is a device that captures pictures or documents so that they can be stored in storage devices, seen on the video screen, modified suitably, transported to other computers, or printed on a printer. A personal computer with a scanner and printer can function as a photocopier.
Output devices mirror the input data, or show the output results of the operations on the input data or print the data. The most common output device is monitor or visual display unit. The printer is used to print the result. A hard copy refers to a printout showing the information. On the other hand soft copy means information stored on a storage device.
Monitor Computer display devices are commonly known as Visual Display Unit (VDU) or monitor. It operates on a principle similar to that of a normal television set. Various technologies have been used for computer monitors. They are also of different sizes. CRT (Cathode-ray tube) and LCD (liquid crystal display) monitors are the two common types which are widely used. The CRT is composed of a vacuum glass tube which is narrower at one end. One electron gun is placed at this end which fires electrons. The electron gun is made up of cathode (negatively charged) and one anode (positively charged). On the other side it has a wide screen, coded with phosphor. The beam of electron strikes on the surface of screen and produces an image by photo luminance process. There is some vertical and horizontal coil to deflect the electron beam in any position of the screen. An image is formed by constantly scanning the screen. To send an image to the screen, the computer first assembles it in a memory area called a video buffer. The graphics are stored as an array of memory locations that represent the colors of the individual screen dots, or pixels. The video card then sends this data through a Digital To Analog Converter (DAC), which converts the data to a series of voltage levels that are fed to the monitor. CRT monitors are too bulky and consume high power. Apart from these, users are very much concerned about potentially damaging non-ionizing radiation from CRT monitor.
Nowadays, LCD monitors are replacing CRTs and becoming the de-facto choice to the users because of its size, display clarity, low radiation emission and power consumption. An LCD display produces a image by filtering light from a series of cold cathode fluorescent lamps (CCFLs).through a layer of liquid crystal cells. Gradually, CCFL backlighting technology is being replaced by low-power light-emitting diodes (LEDs). A computer screen that uses this technology is sometimes referred to as an LED display.
Printer The printer is a device that prints any data, report, document, picture, diagrams, etc. Printers are categorized based on the physical contact of the print head with the paper to produce a text or an image. An impact printer is one where the print head will be in physical contact with the paper. In a non–impact printer, on the other hand the print head will have no physical contact with the paper.
The Dot matrix printer is considered as an Impact printer and Laser printer is considered as Non-impact printer. In a dot matrix printer, the printer head physically ‘hits’ the paper through the ribbon which makes the speed of the printer relatively slow. The printer head consist of some two dimensional array of dot called ‘dot matrix’. Every time when it strikes the paper through ribbon its dots are arranged according to the character which is going to be printed. The ink in the ribbon falls on the surface of the paper and thus the character get printed. In inkjet printer, instead of a ribbon one ink cartridge holds the ink in it. They are placed above the inkjet head.
The printing head takes some ink from the cartridge and spreads it on the surface of the paper by the jet head. This ink is electrically charged. An electric field is created near the paper surface. Thus the small drops of ink are arranged in the surface according to the character it prints. These printers are fast and capable of printing good quality graphics. The laser printer uses a laser beam to create the image.
Central processing unit (CPU) Central Processing Unit or CPU can be thought of as the brain of the computer. Most of the processing takes place in CPU. During processing, it locates and executes the program instructions. It also fetches data from memory and input/output devices and sends data back.
Physically, it is an integrated circuit (IC) silicon chip, mounted on a small square plastic slab, surrounded by metal pins. In the world of personal computers, the term microprocessor and CPU are used interchangeably. It is more accurate to describe it as a CPU on a chip because it contains the circuitry that performs processing. The CPU itself can be divided into different functional units which are described below-
Registers These are high-speed storage devices. In most CPUs, some registers are reserved for special purposes. For example, the Instruction Register (IR) holds the current instruction being executed. The Program Counter (PC) is a register that holds the address of the next instruction to be executed. In addition to such and other special-purpose registers, it also contains a set of general-purpose registers that are used for temporary storage of data values as needed during processing.
Arithmetic logic unit (ALU) It is the part of the CPU that performs arithmetic operations, such as addition and subtraction as well as logical operations, such as comparing two numbers to see if they are the equal or greater or less.
Control unit (CU) The control unit coordinates the processing by controlling the transfer of data and instructions between main memory and the registers in the CPU. It also coordinates the execution of the arithmetic logic unit (ALU) to perform operations on data stored in particular registers. It consists of
- an instruction decoding circuit that interprets what action should be performed.
- a control and timing circuit directs all the other parts of the computer by producing the respective control signals.
Nowadays, a high-speed memory, called cache memory, is embedded with the CPU chip. This improves the computer performance by minimizing the processor need to read data from the slow main memory. The CPU’s processing power is measured in terms of the number of instructions that it can execute per unit time. Every computer comprises of an internal clock, which emits electronic pulses at a constant rate. These pulses are used to control and synchronize the pace of operations. Each pulse is called a clock cycle which resembles a rectangular wave with a rising half of the signal and a falling half. In other words, a full clock cycle is the amount of time that elapses between pulses of the oscillating signal. Each instruction takes one or more clock cycles to execute. The higher the clock speed, the more instructions are executed in a given period of time. Hertz (Hz) is the basic unit of computer clock frequency which is equal to one cycle per second. CPU speed has been improved continuously. It is typically measured in megahertz (MHz) or gigahertz (GHz). One megahertz is equal to one million cycles per second, while one gigahertz equals one billion cycles per second. Nowadays, multiple processors are embedded together on a single integrated-circuit chip, known as multi-core processor e.g. a dual-core processor has two CPUs and a quad core processor has four CPUs.
- An integrated circuit, or IC, is a matrix of transistors and other electrical components embedded in a small slice of silicon.
- A microprocessor is a digital electronic component with miniaturized transistors on a single semiconductor integrated circuit (IC). One or more microprocessors typically serve as a central processing unit (CPU) in a computer system or handheld device allocating space to hold the data object.
Components such as the input device, output device, and CPU are not sufficient for the working of a computer. A storage area is needed in a computer to store instructions and data, either temporarily or permanently, so that subsequent retrieval of the instructions and data can be possible on demand. Data are stored in memory as binary digits, called bits. Data of various types, such as numbers, characters, are encoded as series of bits and stored in consecutive memory locations. Each memory location comprises of a single byte which is equal to eight bits and has a unique address so that the contents of the desired memory locations can be accessed independently by referring to its’ address. A single data item is stored in one or more consecutive bytes of memory. The address of the first byte is used as the address of the entire memory location. CPU uses registers exclusively to store and manipulate data and instructions during the processing. Apart from registers, there are mainly two types of memory that are used in a computer system. One is called primary memory and the other secondary memory.
Primary memory Primary memory is the area where data and programs are stored while the program is being executed along with the data. This memory space, also known as main memory, forms the working area of the program. This memory is accessed directly by the processor. A memory module consists of a large bank of flip-flops arranged together with data traffic control circuitry such that data can be stored or read out on or from a set of flip-flops. A flip-flop can store a binary digit. These flip-flops are grouped to form a unit memory of fixed length and each of which is identified by a sequence number known as a memory address. These type are called Random Access Memory, or RAM, where any location can be accessed directly, and its stored contents get destroyed the moment power to this module is switched off. Hence, these are volatile in nature. Primary memory devices are expensive. They are limited in size, consume very low power, and are faster as compared to secondary memory devices. There is another kind of primary memory increasingly being used in modern computers. It is called cache memory (pronounced as “cash”). It is a type of high speed memory that allows the processor to access data more rapidly than from memory located elsewhere on the system. It stores or caches some of the contents of the main memory that is currently in use by the processor. It takes a fraction of the time, compared to main memory, to access cache memory. The management of data stored in the cache memory ensures that for 20 per cent of the total time, during which the cache is searched, the data needed is found to be stored in cache. As a result the performance of the computer improves in terms of speed of processing.
Secondary memory Secondary memory provides large, non-volatile, and inexpensive storage for programs and data. However, the access time in secondary memory is much larger than in primary memory. Secondary storage permits the storage of computer instructions and data for long periods of time. Moreover, secondary memory, which is also known as auxiliary memory, stores a huge number of data bytes at a lesser cost than primary memory devices.
- The memory unit is composed of an ordered sequence of storage cells, each capable of storing one byte of data. Each memory cell has a distinct address which is used to refer while storing data into it or retrieving data from it.
- Both RAM and cache memory are referred to as primary memory. Primary memory is comparatively expensive, and loses all its data when the power is turned off. Secondary memory provides less expensive storage that is used to store data and instructions on a permanent basis.
There are some operations common to both primary and secondary memory devices. These are as follows.
Read During this operation, data is retrieved from memory.
Write In this operation, data is stored in the memory. Using read and write operations, many other memoryrelated functions such as copy and delete are carried out.
Unit of memory The memory’s interface circuit is designed to logically access a byte or a multiple of a byte of data from the memory during each access. The smallest block of memory is considered to be a byte, which comprises eight bits. The total memory space is measured in terms of bytes. Thus, the unit of memory is a byte. The capacity of memory is the maximum amount of information it is capable of storing. Since the unit of memory is a byte, the memory’s capacity is expressed in number of bytes. Some units used to express the memory capacity are as follows:
- Kilobyte (KB) = 1024 bytes
- Megabyte (MB) = 1024 Kilobytes
- Gigabyte (GB) = 1024 Megabytes
- Terabyte (TB) = 1024 Gigabytes
- Petabyte (PB) = 1024 Terabytes
- Exabyte (EB) = 1024 Petabytes
- Zettabyte (ZB) = 1024 Exabytes
- Yottabyte (YB) = 1024 Zettabytes
The size of the register is one of the important considerations in determining the processing capabilities of the CPU. Word size refers to the number of bits that a CPU can manipulate at one time. Word size is based on the size of registers in the ALU and the capacity of circuits that lead to those registers. A processor with a 32-bit word size, for example, has 32- bit registers, processes 32 bits at a time, and is referred to as a 32-bit processor. Processor’s word size is a factor that leads to increased computer performance. Today’s personal computers typically contain 32-bit or 64-bit processors.
The various types of memory used in a computer system differ in speed, cost, size, and volatility (permanence of storage). They can be organized in a hierarchy. The memory hierarchy in the computer system is depicted as:
It shows that on moving down the hierarchy, the cost per bit of storage decreases but access times increases (i.e., devices are slow). In other words, from top to bottom, the speed decreases while the capacity increases and the prices become much lower. Of the various memories specified in the hierarchy, those above the secondary memory are volatile and the rest are non-volatile. While designing a computer system, there must always be a balance on all of the above factors, namely speed, cost, volatility, etc. at each level in the hierarchy. The devices in a computer system other than the CPU and main memory are called peripherals. Popular peripheral devices include printers, digital cameras, scanners, joysticks, and speakers.
The interface unit interconnects the CPU with memory and also with the various input/output (I/O) devices. The instructions and data move between the CPU and other hardware components through interface unit. It is a set of parallel wires or lines which connects all the internal computer components to the CPU and main memory. Depending on the type of data transmitted, a bus can be classified into the following three types:
Data bus The bus used to carry actual data.
Address bus memory or Input/output device Addresses travel via the address bus.
Control bus This bus carries control information between the CPU and other devices within the computer. The control information entails signals that report the status of various devices, or ask devices to take specific actions. A model of the bus-based computer organization is shown in Fig. 1.2.
Most of the computer devices are not directly connected to the computer’s internal bus. Since every device has its own particular way of formatting and communicating data, a device, termed controller, coordinates the activities of specific peripherals. The processor reads from the input devices or writes on the output devices with the help of the device controllers. Each input device or output device has a specific address. Using these addresses, the processor selects a particular I/O device through the associated device controller for either transferring data or any control commands.
All the components in the computer system are mounted and connected together by an electronic circuit board called motherboard or main board. To make all these things work together the motherboard provides some kind of physical connection among them. (See Fig. 1.3).
In general, a motherboard consists of the following.
CPU socket This holds the central processor which is an integrated chip along with the system clock, cache, cooling fan, etc.
Memory sockets These sockets hold the RAM card that contains RAMs.
Interface module This is for the hard disk, floppy disk, and CD-ROM drives.
ROM integrated chip This is embedded with the basic input/output system software.
Ports and expansion slots A port is used to connect a device with the bus. Physical ports include serial and parallel ports, to which peripheral devices such as printers, scanners, and external storage devices can be attached. The slots are used to attach accessories such as graphics (video) cards, disk controllers, and network cards. There are two different standards for expansion slots: ISA (Industry Standard Architecture) and PCI (Peripheral Component Interconnect). Most common types of ports and slots are briefly described below.
ISA slots These are for connecting ISA compatible cards.
PCI slots These are for connecting I/O devices.
Advanced graphics port (AGP) V ideo card is inserted into this slot.
Parallel port The parallel port is also known as the printer port, or LPT1. It is capable of sending eight bits of information at a time.
Serial ports these are sometimes called communication ports or COM ports. There are two COM ports, COM1 and COM2. size of COM1 is larger than that of COM2. COM1 has 25 pins and used for connecting Modems. COM2 is 9 pin port used for interfacing serial mouse. D-type connectors are used to with there ports.
USB (universal serial bus) This is also a serial port but data rate is more than the serial port. USB is used as a general purpose communication channel in Personal Computers. Many different devices, such as mouse, keyboards, hard disk drives, portable CD-ROM/DVD drives, pen-drives, scanners, cameras, modems and even printers are usually connected to these ports.
CMOS The CMOS in a Personal Computer stands for Complementary Metal Oxide Semiconductor memory. It is a type of RAM that stores the necessary attributes of system components, such as the size of the hard disk, the amount of RAM, and the resources used by the serial and parallel ports etc. Since RAM loses its content when the power is switched off, a small battery, on the motherboard, powers the CMOS RAM even when the computer power is switched off thereby retaining its stored data.
The System Unit holds all the system components in it. It is sometimes called cabinet. The main components like motherboard, processor, memory unit, power supply unit, and all the ports to interface computer’s peripherals. Inside the unit all the components work together to give the service that the user needs.
Based on its use, cabinets are of two types.
(i) AT cabinets (or mini-tower)
(ii) ATX cabinets
AT cabinets are smaller and cheaper than ATX cabinets and are popularly called mini-tower cabinets. They are used for older processors and smaller motherboards. ATX cabinets, on the other hand, are marginally larger in size than AT cabinets and are more expensive as they come with more features such as powered sliding front panels and extra disk storage compartments.
- The motherboard is a printed circuit board which contains the circuitry and connections that allow the various components of the computer system to communicate with each other. In most computer systems, the CPU, memory, and other major components are mounted to wiring on the motherboard.
- The input, output, and storage equipment that might be added to a computer system to enhance its functionality are known as peripheral devices. Popular peripheral devices include printers, digital cameras, scanners, joysticks, and speakers.
Software provides the instructions that tell the hardware exactly what is to be performed and in what order. This set of instructions is sequenced and organized in a computer program. Therefore, a program is a series of instructions which is intended to direct a computer to perform certain functions and is executed by the processor. In a broader sense, software can be described as a set of related programs. But software is more than a collection of programs. It refers to a set of computer programs, which provide desired functions and performance, the data which the programs use, data structures that facilitate the programs to efficiently manipulate data and documents that describe the operation and use of the programs. A comparison between computer program and software is listed below (Table 1.2).
Table 1.2 Comparison between computer program and software
|Programs are developed by individuals. A single developer is involved.||A large number of developerd are involved.|
|Small in size and have limited functionality.||Extremely large in size and has enormous functionality.|
|The user interface may not be very important, because the programer is the sole user.||For a software product, user interface must be carefully designed and implemented because developers of that products and users of that product are totlly different.|
Nowadays, most of the software must be installed prior to their use. Installation involves copying several files to computer memory or requires a series of steps and configurations depending on the operating system and the software itself so that it can be run or executed when required. Software is generally categorized as system software or application software or utility software.
System software is designed to facilitate and coordinate the use of the computer by making hardware operational. It interacts with the computer system at low level. Examples of such software include language translator, operating system, loader, linker, etc. However, the most important system software is the operating system which is a set of programs designed to control the input and output operations of the computer, provide communication interface to the user, and manage the resources of the computer system, such as memory, processor, input/output devices etc. and schedule their operations with minimum manual intervention. Other programs (system and application) rely on facilities provided by the operating system to gain access to computer system resources. The loader is the system software which copies a executable program from secondary storage device into main memory and prepares this program for execution and initializes the execution. Hardware devices, other than the CPU and main memory, have to be registered with the operating system by providing a software, known as device driver, for communication between the device and other parts of the computer. This type of system software is used by printers, monitors, graphics cards, sound cards, network cards, modems, storage devices, mouse, scanners, etc. Once installed, a device driver automatically responds when it is needed or may run in the background. Modern operating system recognizes almost all connected hardware devices and immediately begins the installation process. Such a device, for which the operating system automatically starts the installation process, is called a plugand- play device. However, there are few hardware devices for which the user has to manually initiate the installation process.
Application software is designed to perform specific usages of the users. Microsoft Word, Microsoft Excel, Microsoft Power Point, Microsoft Access, Page Maker, Coral Draw, Photoshop, Tally, AutoCAD, Acrobat, WinAmp, Micro Media Flash, iLeap, Xing MP3 Player etc. are some of the examples of application software. There are two categories of application software, custom software and pre-written software packages. Software that is developed for a specific user or organization in accordance with the user’s needs is known as custom software. A pre-written software package is bought off the shelf and has predefined generic specifications that may or may not cater to any specific user’s requirements. The most important categories of software packages available are as follows:
- Database management software, e.g. Oracle, DB2, Microsoft SQL server, etc.
- Spreadsheet software, e.g. Microsoft Excel.
- Word processing, e.g. Microsoft Word, Corel Wordperfect and desktop publishing (DTP), e.g. Pagemaker.
- Graphics software, e.g. Corel Draw.
- Statistical, e.g. SPSS and operation research software, e.g. Tora.
- Without any software, the computer is called a bare machine, having the potential to perform many functions but no ability to do so on its own.
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