Chapter 1 : Computer System

Introduction

The word "computer" comes from the Latin word "computare" which means to calculate. So a computer is normally considered to be a calculating device that can perform arithmetic operations at fast speed.

Definition

In simple terms, any electronic machine that can perform IPOS (Input Process Output Storage) cycle is known as computer.

An electronic device which accept raw data, process the data, stores the data for future use and gives the desired information (result) as output, such a device which have memory is called computer.

Characteristics of Computer

Following are some of the features of a digital computer:

1. Speed — Computer can calculate very complex data at a very fast rate that human being may not be able to do. The speed of the computer is measured in terms of fraction of second.

2. Diligence — It Means, is absolutely free from physical tiredness and mental fatigue. Because a computer has maximum capacity to perform any kind of tasks repeatedly at the same level of timing periods.

3. Versatile — Computers have multi-purpose capacities. A computer can do any types of jobs at the same time. That's why computers are versatile devices. At the same level of period of timings, it provides loading an environment to run many of tasks as we needed.

4. Reliability — It is the most recent demand and means in real life of all human beings. Men are imperfect but it has capable to make copy from them. So the modern age is depending onto it because of its capabilities a lot.

5. Automatic — Computer is an automatic machine. Everything that is given to computer are processed and done by computer automatically according to the instructions provided.

6. Accuracy — The result of the computer is 100% accurate if the input data are correct.

7. Storage Capacity — Computers have data storage devices. It stores the data permanently or temporarily. There are mainly two types of storage devices (Primary and Secondary). The capacity of the storage is measured in terms of bytes. The small unit is bit (0 or 1). Computers stores the data form of bit.

8. Word Length — A computer can process 16 bits, 32 bits and 64 bits data at a time. The number of bits the computer can process at a time known as word length.

Application (Use) of Computer

Capability and Limitation

Advantages and Disadvantages of Computer

Advantages

1. Computer is faster than human being and any other devices ever invented.
2. It is always accurate and reliable than human.
3. It is versatile i.e. master in all types of work and fields like education, hospital, banks etc.
4. It can be used for communication among many users.
5. It never feels exhausted and tired as human being.

Disadvantages

1. Computer can be used for computer crimes like hacking, creating and sending viruses, etc.
2. It has increased dependency on machine.
3. It is still expensive so each and every people cannot afford the computer.
4. Repair and maintenance is required frequently.
5. Skilled manpower is required to work in the computer.

History of Computer

Generation of Computer

The advancement in Hardware technology with the span of time is termed as Computer generation. Or the development of the computer took place a stage and each stages is called the generation of computer. There are Five Generations:

First Generation (1945–1955)

Second Generation (1956–1965)

Third Generation (1964–1971)

Fourth Generation (1971–At Present)

Fifth Generation (Future)

Types of Computer

Classification of Computers

On The Basis of Operation

Analog Computer — The computer which can process analog quantities (continuous data) is called an analog computer. It is specially designed to compute physical forces such as temperature, pressure etc. Presley is an example of analog computer.

Digital Computer — The computer which works on discrete data (discontinuous data, binary system or 0 or 1) is known as digital computer. For Example, IBM PC, IBM compatible, Apple/Mac. computer etc.

Hybrid Computer — A Computer, which has combined features of both analog and digital, is known as hybrid computer. It can convert data from analog to digital and digital to analog. These computers are used for scientific research and hospitals. Some of the characteristics of Hybrid Computer are:

• It is a combination of qualities of analog as well as digital computers.
• It can process both continuous and discrete data.
• It has capability to convert one type of data into another.
• It is the special purpose machine.
• It usually has high cost compared to analog and digital computer.

Difference Between Analog and Digital Computer

On the Basis of Size

Super Computer

• Most powerful and fastest digital computer
• Special purpose computer
• Performs billions of instructions per second
• Computing capacity equal to 40,000 microcomputers
• Uses parallel processors
• Requires large space
• Requires temperature-controlled room
• Uses: weather forecasting, scientific research, satellite communication, nuclear research, molecular modeling, study of DNA structure
• Examples: CRAY X-Mp/24, PARAM, ANURAG, etc.

Mainframe Computer

• High processing power but lacks flexibility
• Designed for large-scale data processing
• Large in size, occupies about 1000 sq. ft. area
• Supports more than 100 terminals
• Used in banks, insurance companies, airlines, railway reservation systems
• Also used in networked systems
• Examples: IBM 1401, ICL 2950/10, ICL 39, CYBER 170, etc.

Minicomputer

• Smaller in size and cost compared to mainframe computer, hence called mini computer
• First released in the 1960s
• Capabilities lie between mainframe and microcomputer, so also called midrange computer
• Supports about 50 terminals
• Used in small business organizations, industries, universities, banks, etc.
• Examples: Prime 9755, VAX 7500, HCL, MAGUM, etc.

Micro Computer

• Based on microprocessor
• Small, low cost digital computer
• Requires small space — can be placed on table or kept inside a briefcase
• Has CPU on a single chip
• Used in office, house, school, shop, store
• Smallest types: laptop, notebook, palmtop, PDA
• Examples: IBM PC, Apple/Macintosh

Categories of Microcomputer

Microcomputers are divided into two categories:

• Non-portable category
• Portable category

A. Non-Portable Category

1. Personal Computer (PC)

• Small computer, fits on normal-sized office table
• Designed for use by one person at a time
• Most common computer
• Found in office, college, home, hospital, shops

2. Workstation Computer (WC)

• 5 to 10 times faster than PC
• Larger main and backup memory
• Larger, high-resolution monitor
• Accelerated graphics handling capability
• Suitable for architectural/engineering design, modeling, animation, video editing

B. Portable Category

1. Laptop

• Portable, easy to carry
• Suitable for people on the move
• Lightweight, has all basic features of a computer
• LCD (Liquid Crystal Display) monitor
• Can operate on AC or DC (battery) power, or both

2. Notebook

• Designed for people on the move
• Smaller in size compared to laptop
• Reduced keyboard, screen, functionality, and software support

3. Tablet PC

• Looks similar to laptop/notebook but more convenient
• Differs in accessories, screen, software used
• Allows writing directly on screen using a pen instead of keyboard
• Has handwriting recognition software that digitizes handwritten text

4. Palm Top

• Portable PC, size of a palm
• Very small memory and features compared to laptop/notebook
• Used for storing phone numbers, addresses, pictures, small data
• Has electronic writing pad and light-sensitive electronic pen for input

5. PDA (Personal Digital Assistant)

• Limited in tasks it can perform
• Modern PDAs include: colored screen, scientific calculator, digital diary (phone numbers/addresses), web browser, cell phone, audio-video, games

Concept of Computer Architecture and Organization

• The design, arrangement, construction or organization of the different parts of a computer system is known as Computer Architecture.
• It is the conceptual design and fundamental operational structure of a computer system.
• It is a framework and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the CPU performs internally and accesses addresses in memory.
• Computer Organization refers to the operational units and their interconnections that realize the architecture specification.
• Computer organization deals with physical aspects of computer design, memory and their types as well as the microprocessor design.

Components of Computer System

A computer system consists of the following components:

1. Input Hardware

• Most computers cannot accept data in human forms like speech or handwriting
• Data must be presented in a way that is easily convertible into electronic form
• Commonly done by typing data into a keyboard, which converts it into machine-sensible form
• Keyboard is just one of many input devices
• In some cases, machine-readable documents or media are produced as part of the input process

2. Output Hardware

• Provides processed results to the user
• Result is taken from main storage and fed to an output device
• Two forms of output:
  • Hardcopy output: Printed form (via printer)
  • Softcopy output: Displayed on monitor screen (like a TV screen)

3. Processing Hardware

• Instructions are obeyed and necessary arithmetic operations carried out on data
• Arithmetic-Logical Unit (ALU):
  • Performs arithmetic and logical operations
  • Usually not a physically separate component (same for control unit)
  • Operations occur at very high speed (e.g., millions of numbers summed in under a second)
• Control Unit:
  • Controls the operation of the computer
  • Fetches instructions from main storage
  • Interprets instructions
  • Issues necessary signals to system components
  • Directs all hardware operations needed to obey instructions

4. Storage Hardware

• Data and instructions are stored and held until needed for processing
• Instructions dictate what actions are to be taken on data
• Results of actions are held until required for output

Fig. Elements of computer system showing logical structure

Microprocessor

Basic Concepts

• Multipurpose, programmable, clock-driven, register-based electronic component
• Fabricated on a single IC (Integrated Circuit) chip
• Capable of performing Arithmetic and Logical Unit (ALU) operations
• Communicates with other connected components
• General purpose device, applicable to a wide range of applications
• Reads binary instructions from an input device or primary memory
• Accepts binary data as input, processes data according to instructions, provides results as output
• First microprocessor: Intel 4004 developed by Intel Corporation in 1971
• Functions like an "artificial brain"

Functions of the Microprocessor

• Carries out processing instructions in and out (fetch) from main memory
• Gives commands to all parts of the computer system
• Controls the sequence of operations
• Controls the storage of data or instructions
• Performs arithmetic calculations (addition, subtraction, etc.)
• Supervises and controls I/O (Input/Output) devices, in addition to execution

Clock Speed

• Every microprocessor/CPU contains a clock along with the Control Unit (CU)
• Clock generates the speed at which instructions are executed
• Synchronizes all components of the computer
• Also known as Clock Rate
• Measured in Megahertz (MHz) or Gigahertz (GHz)
  • 1 MHz = 1 million cycles per second
  • 1 GHz = 1 billion cycles per second
• Greater the speed → more instructions performed
• Today's PCs: clock speeds up to 5 GHz

Problem with High Clock Speed: High clock speed generates heat due to friction, which can damage the microprocessor chip.

Word Length

• Number of bits a processor can process in a single instruction
• Also called word size
• Determines:
  • Amount of RAM that can be accessed
  • Total number of pins on the microprocessor
• Higher word size → faster CPU instruction execution
• Word length ranges from 4 bits to 64 bits (depending on type of microcomputer)
• Most current microprocessors use 32-bit or 64-bit architecture

Components of Microprocessor

The microprocessor consists of mainly three components:

1. Arithmetic Logic Unit (ALU)
2. Control Unit (CU)
3. Register Array (Set of Registers)

1. Arithmetic and Logic Unit (ALU)

• Performs mathematical computations and logic operations
• Controls the speed of these operations as directed by the CU
• Also executes comparisons and logical testing

Calculations performed by ALU:

• Addition, subtraction, multiplication, division
• Logical operations: AND, OR, XOR, NOT
• Increment and decrement
• Left or right shift (content of accumulator shifted left/right by one bit)
• Clear (content of accumulator or carry flag made zero)
• Exponential, logarithmic, trigonometric, and floating-point operations

2. Control Unit (CU)

• Controls the entire operation of the computer
• Receives signals from CPU to move data between microprocessor, memory, input, and output devices
• Acts as the nerve system of the computer system
• Consists of multiple components: Decoder, Flag, Quartz crystal, Control logic circuits

Functions of the Control Unit:

1. Controls and coordinates sequence of data movements around ALU, registers, memories, input and output devices
2. Fetches instructions from memory, decodes and interprets them, and executes the task
3. Issues control signals that control hardware components within the CPU
4. Gives orders to ALU and directs operation of the whole computer system
5. Generates timing/clock speed to count the instruction cycle of the computer
6. Provides status of every device connected to the computer to check its functionality
7. Receives external instructions/commands and generates a sequence of control signals

3. Register Array

• Registers = temporary data holding places in the microprocessor
• Small amounts of high-speed memory contained within the CPU
• CPU contains multiple registers for program execution
• Used to handle movement of information between various units of the computer and to speed up processing
• Most CPUs today have 32-bit or 64-bit registers

Registers are needed during processing for:

• Address of the next instruction to be executed
• Current instruction being decoded
• Results of calculations of frequently used small amounts of data

Functions of Common Registers

Bus System

A bus is a collection of wires, chips, and slots inside the computer through which data is transmitted from one part of the computer to another. Often compared to a highway — pathways on which data travels. It is a set of parallel distinct wires, serving different purposes, allowing devices to communicate with the CPU.

Functions of the Bus:

• Carries information from one component to another
• Carries data, address, or control signals
• Allows one component to interact with others

Types of Bus

Fig. Bus System

Memory

Primary Memory

Also called system memory / main memory / internal memory. Directly associated with CPU — short-term storage.

Three tasks: holds data for processing · holds instructions for processing · holds processed data waiting for output/storage.

Features: directly accessible to CPU · faster and more expensive than secondary memory · semiconductor memory · used for smaller, temporary storage.

RAM (Random Access Memory)

• Read/write memory — CPU can access any location randomly.
• Volatile — data lost when power is off.
• Stores: instructions waiting/being executed, data waiting/being processed, data waiting for output.

ROM (Read Only Memory)

• Read-only; non-volatile — data permanent even after power off.
• Used for permanent storage of boot programs.

Types of ROM:

• PROM — Programmable once by user using a ROM burner.
• EPROM — Erased using UV light (entire data erased); reusable.
• EEPROM — Erased/reprogrammed electrically, byte by byte; fastest erase; no circuit removal needed.

Cache Memory

• High-speed memory placed between RAM and CPU.
• Holds most frequently used instructions to reduce CPU waiting time.
• Cache hit — data found in cache (fast). Cache miss — fetched from main memory (slow).
• L1 cache: smaller, faster, checked first. L2 cache: larger, slower.
• Types: Internal cache (inside CPU) and External cache (outside CPU).

Buffer

• Temporary storage space to hold data before processing (used when data arrives faster than it can be processed).
• Uses FIFO (First In First Out) arrangement.
• Print buffering = spooling (frees computer while printer works in background).
• Located mainly in RAM; used for I/O processes.

Secondary Memory

Also called auxiliary / backup memory. Non-volatile, slower, and cheaper than primary memory. Stores large volumes of data permanently; transferred to primary memory when needed.

Magnetic Memory

Magnetic Disk

• Most common secondary storage; random access device.
• Circular disks (metal/Mylar plastic) coated with iron oxide.
• Data stored in concentric tracks divided into sectors.

Advantages: direct access · shared device · erasable & reusable · less vulnerable to corruption. Limitations: less efficient for sequential apps · less portable · needs dust-free environment.

Hard Disk

• Used for permanent mass storage (OS, programs, databases).
• Multiple platters; each side has a read/write head on a single access arm.
• Made of aluminium coated with iron oxide; standard size: 3.5 inch.
• Rotates at 3600–15000 rpm; average access time ~15ms.

Floppy Disk

• Also called diskette; removable round flexible Mylar plastic disk in a protective case.
• Data stored as magnetized spots on iron oxide film.
• Access time ~150–250ms. Two sizes: 3½ inch and 5¼ inch.

Magnetic Tape

• Thin plastic tape coated with magnetizable substance; sequential access only.
• Used mainly for backup storage and transporting data.
• Available in ½ inch, ¼ inch, 8mm, 3mm widths; cassette form = cartridge tape.
• Capacity: 500 MB to 4 GB+. Data transfer rate: 240 KB/sec.

Optical Disk

Removable disk; data written/read using laser beams. High storage capacity, relatively cheap.

Advantages: very low cost-per-bit · no mechanical head contact (reliable) · data permanent · compact and lightweight. Limitations: CD-ROM/WORM are read-only · slower than magnetic disk · sensitive to scratches/dust.

Other Storage Devices

Input Devices

An input device converts input data into a form acceptable to the computer. It is the medium of communication between user and computer.

Examples: Keyboard, Mouse, Trackball, Touchpad, Joystick, Light Pen, Touch Screen, Scanner, Bar Code Reader, OCR, OMR, MICR, Microphone, Digital Camera.

Output Devices

Output unit translates computer-processed information (0s and 1s) into human-understandable form.

Two types of output:

• Softcopy — shown on display screen or heard from speaker; untouchable, temporary.
• Hardcopy — printed on paper/material; touchable, permanent.

Monitor (Softcopy Output Device)

Display screen showing text and graphics; most common softcopy output device.

Classification by Colors:

Classification by Display Technique:

CRT Monitor

• Operates like a TV: electron gun sends beam toward phosphor-coated screen.
• Smallest phosphor dot = pixel; gun scans every pixel top-left to bottom-right.
• Color CRT uses three guns for Red, Green, Blue → called RGB monitor.

LCD Monitor

• Creates images using liquid crystal that becomes opaque when electrically charged.
• Used in laptops, cameras, clocks, microwave ovens, CD players.

LED Monitor

• Array of light emitting diodes turn on/off to display output.
• Popular for TVs, desktops, laptops, mobiles, tablets.

Gas-Plasma Display

• Uses gas (mainly neon) that emits light when electric current passes between electrodes.
• More expensive; used less often than LCD and LED.

Printer (Hardcopy Output Device)

Peripheral device that prints text, images, and figures on paper — permanent readable form.

Printer quality depends on: printing speed · resolution (dpi) · memory · color · cost.

Dot Matrix Printer (Impact)

• Print head of small pins (9, 18, or 24 pins) strikes inked ribbon; 24-pin gives best quality.
• Draft quality: 72 dpi; near letter-quality: 144 dpi.

Daisy Wheel Printer (Impact)

• Removable flower-like wheel with raised characters; hammer strikes the character.
• Speed: ~90 characters/sec; cannot print graphics.

Line Printer (Impact)

• Prints a whole line at once; up to 3,000 lines/minute.
• Types: Chain printer (rotating chain) and Drum printer (rotating drum).
• Used for mark-sheets, bank statements, bills, receipts.

Inkjet Printer (Non-Impact)

• Sprays electrically charged ink droplets from four nozzles (CMYK: Cyan, Yellow, Magenta, Black).

Laser Printer (Non-Impact)

• Entire page processed at once; uses toner (magnetically charged ink powder) transferred via drum.
• Resolution: 300 to 2400+ dpi.

Thermal Printer (Non-Impact)

• Uses colored waxes and heat to burn dots onto heat-sensitive paper.
• Highest-quality desktop color printing; requires expensive special paper.

Plotter

Specialized output device producing high-quality graphics in various colors.

• Especially useful for maps, architectural drawings, charts and graphs.
• Uses ink pen or inkjet mechanism; works on paper or plastic sheets.
• Much more expensive than printers.
• Types: Drum plotter, Micro-grip plotter, Flat-bed plotter, Inkjet plotter.

Hardware Interfaces (Ports)

A port is a socket on the outside of the system unit connected to an expansion board inside. It allows connecting peripheral devices to the computer.

Functions: connecting peripherals · transmitting data to/from peripherals · providing electrical power to low-power devices.