introduction to computers

MODULE 5 MCA-102 DIGITAL SYSTEMS & LOGIC DESIGN ADMN 2014-‘17
Dept. of Computer Science And Applications, SJCET, Palai Page 92
5.1 BASIC COMPONENTS OF A COMPUTER
The following five units (also called "The functional units") correspond to the five basic operations
performed by all computer systems.
Fig 5.1 Basic components of a computer
Input Unit
An input unit performs the following functions.
1. It accepts (or reads) the list of instructions and data from the outside world.
2. It converts these instructions and data in computer acceptable format.
3. It supplies the converted instructions and data to the computer system for further processing.
Output Unit
The following functions are performed by an output unit.
1. It accepts the results produced by the computer which are in coded form and hence cannot be easily
understood by us.
2. It converts these coded results to human acceptable (readable) form.
3. It supplied the converted results to the outside world.
Storage Unit
The Storage Unit or the primary / main storage of a computer system provides space for storing data and
instructions, space for intermediate results and also space for the final results.
The specific functions of the storage unit are to store:
1. All the data to be processed and the instruction required for processing (received from input
devices).
2. Intermediate results of processing.
3. Final results of processing before these results are released to an output device.
Computer’s memory can be classified into two types; primary memory and
secondary memory.
a. Primary Memory can be further classified as RAM and ROM.

• RAM or Random Access Memory is the unit in a computer system. It is the place in a computer
where the operating system, application programs and the data in current use are kept temporarily
so that they can be accessed by the computer’s processor. It is said to be ‘volatile’ since its contents
are accessible only as long as the computer is on. The contents of RAM are no more available once
the computer is turned off.
• ROM or Read Only Memory is a special type of memory which can only be read and contents of
which are not lost even when the computer is switched off. It typically contains manufacturer’s
instructions. Among other things, ROM also stores an initial program called the ‘bootstrap loader’
whose function is to start the operation of computer system once the power is turned on.
Fig 5.2 types of computer storage
b. Secondary Memory
These include devices that are peripheral and are connected and controlled by the computer to enable
permanent storage of programs and data. Examples are CD ROM, Hard Disk and Digital Video Disk
Central Processing Unit (CPU)
The main unit inside the computer is the CPU. This unit is responsible for all events inside the computer. It
controls all internal and external devices, performs "Arithmetic and Logical operations". The control Unit
and the Arithmetic and Logic unit of a computer system are jointly known as the Central Processing Unit
(CPU). The CPU is the brain of any computer system.
Arithmetic and Logic Unit (ALU)
The arithmetic and logic unit (ALU) of a computer system is the place where the actual execution of the
instructions take place during the processing operations. All calculations are performed and all
comparisons (decisions) are made in the ALU.

Control Unit
The control unit directs and controls the activities of the internal and external devices. It interprets the
instructions fetched into the computer, determines what data, if any, are needed, where it is stored, where
to store the results of the operation, and sends the control signals to the devices involved in the execution of
the instructions.
5.2 I/O DEVICES
Input/output devices are required for users to communicate with the computer. In simple terms, input
devices bring information INTO the computer and output devices bring information OUT of a computer
system. Examples of Input Devices are Keyboard, Mouse, Joystick, Scanner, Light Pen, Touch Screen etc
5.3 PRINTERS
Printers are used to produce paper (commonly known as hardcopy) output. Based on the technology used,
they can be classified as Impact or Non-impact printers.
• Impact printers use the typewriting printing mechanism wherein a hammer strikes the paper through
a ribbon in order to produce output. Dot-matrix and Character printers fall under this category.
• Non-impact printers do not touch the paper while printing. They use chemical, heat or electrical
signals to etch the symbols on paper. Inkjet, Deskjet, Laser, Thermal printers fall under this
category of printers.
When we talk about printers we refer to two basic qualities associated with printers: resolution, and speed.
Print resolution is measured in terms of number of dots per inch (dpi). Print speed is measured in terms of
number of characters printed in a unit of time and is represented as characters-per-second (cps), lines-per-
minute (lpm), or pages-per-minute (ppm).
5.4 DISPLAY DEVICES
A display device is an output device for presentation of information for visual or tactile reception,
acquired, stored, or transmitted in various forms. When the input information is supplied as an electrical
signal, the display is called electronic display.
CRT MONITOR [cathode-ray tube]
A CRT monitor contains millions of tiny red, green, and blue phosphor dots that glow when struck by an
electron beam that travels across the screen to create a visible image. In a CRT monitor tube, the cathode is
a heated filament. The heated filament is in a vacuum created inside a glass tube. The electrons are
negative and the screen gives a positive charge so the screen glows.
LCD Monitor

Flat panel displays encompass a growing number of technologies enabling video displays that are lighter
and much thinner than traditional television and video displays that use cathode ray tubes, and are usually
less than 4 inches (100 mm) thick. They can be divided into two general categories: Volatile or Static
Volatile
Volatile displays require pixels be periodically refreshed to retain their state, even for a static image. This
refresh typically occurs many times a second. If this is not done, the pixels will gradually lose their
coherent state, and the image will "fade" from the screen.
Examples of volatile flat panel displays
 Plasma displays
 Liquid crystal displays (LCDs)
 Organic light-emitting diode displays (OLEDs)
 Light-emitting diode display (LED)
 Electroluminescent displays (ELDs)
 Surface-conduction electron-emitter displays (SEDs)
 Field emission displays (FEDs)
 Nano-emissive display (NEDs)
Static
Static flat panel displays rely on materials whose color states are bitable. This means that the image they
hold requires no energy to maintain, but instead requires energy to change. This results in a much more
energy-efficient display, but with a tendency towards slow refresh rates which are undesirable in an
interactive display.
Examples of static flat panel displays
 electrophoretic displays (e.g. E Ink's electrophoretic imaging film)
 bichromal ball displays (e.g. Xerox's Gyricon)
 Interferometric modulator displays (e.g. Qualcomm's iMod, a MEMS display.)
 Cholesteric displays (e.g. MagInk, Kent Displays)
 Bistable nematic liquid crystal displays (e.g. ZBD)
5.5 SCANNERS
Scanner is an input device used for direct data entry from the source document into the computer system. It
converts the document image into digital form so that it can be fed into the computer. Capturing
information like this reduces the possibility of errors typically experienced during large data entry. The
most common types are flatbed scanners, sheetfed scanners, photo scanners, and portable scanners.
5.6 MOTHER BOARD

This is the main circuit board of the PC. It contains all the basic, core components of the computer. It
usually contains:
 The CPU, which plugs into a socket designed for a particular CPU's pin arrangement. Because a
motherboard has sockets that can only accept certain types of CPU, it is important to make sure
when upgrading your CPU that your motherboard can accept it.
 Memory chips - these hold data and programs that the CPU is currently using.
 Input/output ports ("I/O") such as connectors that hard disk drives, floppy disk drives and CD-ROM
drives plug in to, serial port sockets, parallel port sockets and USB port sockets.
 BIOS chips (Basic Input Output System) - the BIOS chips are PROM (Programmable Read Only
Memory) chips that contain the most basic information that a computer needs to start up and
operate. The BIOS contains bootup information, details of what sort of CPU is installed, what hard
disks are available, how the motherboard should behave etc.
Fig 5.3 Motherboard components
 Real time Clock (RTC) so the computer knows the time and date. The RTC needs a battery to keep
the clock running when the computer's power is turned off.

 Slots for the system's main memory, typically in the form of DRAM chips, Slots for one or more
hard drives to store files. The most common types of connections are Integrated Drive Electronics
(IDE) and Serial Advanced Technology Attachment (SATA). A slot for a video or graphics card
 Chipsets is a group of small circuits that coordinate the flow of data to and from key components of
a PC. This includes the CPU itself, the main memory, the secondary cache and any devices situated
on the buses. The chipset also controls data flow to and from hard disks, and other devices
connected to the IDE channels. A computer has got two main chipsets:-
NorthBridge (also called the memory controller) is in charge of controlling transfers between the
processor and the RAM, which is why it is located physically near the processor.
SouthBridge (also called the input/output controller or expansion controller) handles communications
between slower peripheral devices.
• BUS:
A bus is simply a circuit that connects one part of the motherboard to another. The more data a bus
can handle at one time, the faster it allows information to travel. The speed of the bus, measured in
megahertz (MHz), refers to how much data can move across the bus simultaneously
Fig 5.3 buses in motherboard

BIOS
Basic input/output system, the built-in software that determines what a computer can do without accessing
programs from a disk. On personal computers (PCs), the BIOS contains all the code required to control
the keyboard, display screen, disk drives, serial communications, and a number of miscellaneous functions.
The four main functions of a PC BIOS
 POST - Test the computer hardware and make sure no errors exist before loading the operating
system. Additional information on the POST can be found on our POST and Beep Codes page.
 Bootstrap Loader - Locate the operating system. If a capable operating system is located, the
BIOS will pass control to it.
 BIOS drivers - Low level drivers that give the computer basic operational control over your
computer's hardware.
 BIOS or CMOS Setup - Configuration program that allows you to configure hardware settings
including system settings such as computer passwords, time, and date.
5.7 SECONDARY STORAGE DEVICES
Secondary storage device is any volatile medium that holds data until it is deleted or overwritten.
Secondary storage is about two orders of magnitude cheaper than primary storage. Consequently, hard
drives (a prime example of secondary storage) are the go-to solution for nearly all data kept on today's
computers. There are several types of secondary storage media used today in the world, each of these can
be compared to each other in terms of portability, speed and capacity.
Currently the most common forms of secondary storage device are:
 Floppy disks
 Hard disks
 Optical Disks
 Magnetic Tapes.
 Solid State Devices
5.7.1 Hard Disk
HD Is a data storage device used for storing and retrieving digital information. The primary
characteristics of an HDD are its capacity and performance. Capacity is specified in unit
prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes.

Five Main Components
Platters - The aluminum alloy disks upon which data is stored. It's a magnetic media, somewhat akin to the
surface preparation on a cassette tape. However, the surface of the platter is magnetically formatted (on
both sides) into sectors and tracks where digital information is written.
Spindle - the platters or disks spin on the spindle, which is run by a motor on the drive. I guess you could
say it's kind of like the axle on a wheel.
Read/Write Heads - The heads move across the platters to write data to, and read data from the platters.
There's a read/write head for each side of each platter. Access is random, meaning that the heads can jump
straight to the information they want without having to fast-forward or rewind past unneeded information.
Head Actuator - Controls the read/write heads. The heads are at the end of an actuator arm which is
attached to the actuator.
Circuit board - Receives commands from the hard drive controller and translates them in order to move
the head actuator, which moves the read/write head across the platters to the required position.
Fig 5.4 Hard Disk Internal Structure
Data is recorded onto the magnetic surface of a platter in exactly the same way as it is on floppies
or digital tapes. Essentially, the surface is treated as an array of dot positions, with each domain’ of
magnetic polarization being set to a binary 1 or 0. The position of each array element is not identifiable in
an absolute sense, and so a scheme of guidance marks helps the read/write head find positions on the disk.
The need for these guidance markings explains why disks must be formatted before they can be used.
When it comes to accessing data already stored, the disk spins round very fast so that any part of its
circumference can be quickly identified. The drive translates a read request from the computer into reality.

There was a time when the cylinder/head/sector location that the computer worked out really was the data’s
location, but today’s drives are more complicated than the BIOS can handle, and they translate BIOS
requests by using their own mapping.
Reading Data
When the computer wants to read data, the operating system works out where the data is on the disk.
To do this it first reads the FAT (File Allocation Table) at the beginning of the partition. This tells the
operating system in which sector on which track to find the data. With this information, the head can then
read the requested data. The disk controller controls the drive’s servo-motors and translates the fluctuating
voltages from the head into digital data for the CPU.
More often than not, the next set of data to be read is sequentially located on the disk. For this
reason, hard drives contain between 256KB and 8MB of cache buffer in which to store all the information
in a sector or cylinder in case it’s needed. This is very effective in speeding up both throughput and access
times. A hard drive also requires servo information, which provides a continuous update on the location of
the heads. This can be stored on a separate platter, or it can be intermingled with the actual data on all the
platters. A separate servo platter is more expensive, but it speeds up access times, since the data heads
won’t need to waste any time sending servo information.
5.7.2 Compact Disc
A CD is 1.2 mm thick and rotates at a speed that varies between around 500 r.p.m. at the inside of
the disc to around 200 r.p.m. at the outside (the disc is read from the inside to the outside). The speed
reduces as the laser moves towards the outside of the disc to keep the linear read-out speed constant. This is
usually 1.2 m/s, corresponding to a theoretical maximum playing time of 74 minutes. The frequency
response of a CD is flat within ±0.5 dB from 5 Hz to 20 kHz, with a signal-to-noise ratio of better than 90
db.
A compact disc (CD) is a flat, round, portable metal storage medium that usually is 4.75 inches in
diameter and less than one-twentieth of an inch thick. Compact discs store items using microscopic pits
(indentations) and land (flat areas) that are in the middle layer of the disc. A high-powered laser light
creates the pits in a single track, divided into evenly spaced sectors, that spirals from the center to the edge
of the disc. A low-powered laser reads items by reflecting light off the disc surface. The reflected light is
converted into a series of bits that the computer can process.

Fig 5.5 CD structure
Some variations of the CD include:
CD-Audio (CD-A)
This kind of disc, commonly known as an audio CD, contains digitized recordings lasting up to 63, 74 or
80 minutes, depending on the length of the spiral on the disc, although the older 63 minute variety is now
very rare. Audio CDs can be played on almost any kind of computer CD-ROM drive, as well as on a
standard audio CD player.
CD single
he 8 cm ‘CD single’ was launched to take over the ‘singles’ function, with a maximum playing time of
around 20 minutes and the same sound quality as a normal 12 cm CD.
CD Graphics
CD Graphics makes it possible to store song lyrics and other information on a CD, which can be displayed
on a TV screen when a disc is played.
CD-ROM
Has a diameter of 12 cm and a storage capacity of 650 to 700 MB. A CD-ROM allows fast data access and
has a very high reliability. This is why it is now universally used to store computer software and data.
CD Video
Three discs sizes - 12, 20 and 30 cm - were developed. The 12 cm 'CD with video' was referred to as
'CDV'. It contained up to 5 minutes of (analogue) video (like LaserDisc) plus up to 20 minutes of CD

digital audio, which could be played on any CD player. The 12 cm discs were yellow; to distinguish them
from the silver audio CDs. Philips intended these discs to be an ideal carrier for video clips. 20 and 30 cm
CD Video discs were also introduced, with playing times of 40 to 120 minutes.
CD-i, the interactive CD
The CD-i combined CD sound with video, text, animation and interactivity. This means that users could
‘communicate’ with a CD-i player and the software on the discs. CD-i disc had a capacity of 650 MB,
allowing the storage of around 7,000 photos, 72 minutes of animation or 19 hours of speech. Or any
combination of these, as the CD-i titles showed. Picture quality was also excellent, thanks to digital video
with more than 16 million colors.
Video CD
A Video CD disc, with the same size as an audio CD, had a capacity of up to 74 minutes of digital audio
and video. This meant that 2 discs were needed for a full-length movie, although these could be packaged
in a box just as flat as a normal CD case, and much smaller than the same film on a video cassette or Laser
Disc.
CD-RW
CD-Rewritable discs were initially much more expensive, is an erasable disc you can write on multiple
times.
5.7.3 DVD
A DVD-ROM (digital video disc-ROM) is an extremely high-capacity compact disc capable of storing
from 4.7 GB to 17 GB. In order to read a DVD-ROM, you must have a DVD-ROM drive. You also can
obtain recordable and rewritable versions of DVD. A DVD-R (DVD-recordable) allows you to write on it
once and read (play) it many times. With the new rewritable DVD, called a DVD+RW, you can erase and
record on the disc multiple times.

introduction to computers

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