Memory Units:-Ø A bit is the smallest
storage unit of memory.
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8bits
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=
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1Byte
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1024 bytes
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=
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1 Kilo Byte (KB)
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1024 KB
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=
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1 Mega Byte (MB)
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1024 MB
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=
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1 Giga Byte (GB)
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1024 GB
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=
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1 Terra Byte (TB)
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1024 TB
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=
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1 Peta Byte (PB)
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1024 PB
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=
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1 Exa Byte (EB)
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1024 EB
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=
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1 Zetta Byte (ZB)
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1024 ZB
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=
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1 Yotta Byte (YB)
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The computer
memories can be divided into following categories:
Primary
Memory
Secondary
memory
Cache
Memory
Primary Memory
Ø Primary
memory or main memory is a Metal Oxide Semiconductor (MOS) memory used for
storing program and data during the execution of the program.
Ø It
is directly accessible to CPU.
Ø Broadly primary
memory can be of two types – RAM (Random Access Memory) and ROM (Read only
memory).
Random
Access Memory (RAM)
Ø In case of RAM,
the memory can be accessed from any desired location randomly.
Ø The
instructions and data that we input into the computer are stored in
the RAM of the Computer.
Ø It is a
read/write memory, so data can be both read from and written to the RAM.
Ø It is a
volatile memory and loses its contents when the power is switched off or
interrupted.
Ø Nowadays RAMs
are available in gigabytes.
Ø The normal memory
access time of a RAM is 20-80 ns.
Ø RAM can be
broadly classified into two categories:
I.
Dynamic RAM (DRAM)
II.
Static RAM (SRAM).
SRAM
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DRAM
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It is faster in operation
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It is slower in operation
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It don’t require refresh cycles
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It require refresh cycles
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Its memory cell contains 1 transistor and 1 capacitor
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Its cost per memory is very high
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Its cost per memory is less.
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It is not commonly being used as it is expensive
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It is commonly used everywhere as it is less expensive
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Used in cache memory of processors
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It is used in computers, smart phones, tablets as normal RAM
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Ø The special device where computer start-up instructions
are stored is called ROM
Ø It is non-volatile in nature, i.e., its contents are
not lost after power is switched off.
Ø Data stored in it are can be read and cannot be altered
by any one.
Ø ROM chips are not only used in computers but also in electronic
devices like Washing machines, Microwave oven etc.
Ø Once a ROM chip is being programmed it can’t be changed.
If it is to be corrected then we have to replace it with a new one.
Programmable Read Only
Memory (PROM):
This type
of ROM can be
programmed even after
its manufacture using a PROM
programmer circuit. But once a PROM is
programmed, it becomes just like ROM i.e. it cannot be changed.
Erasable Programmable Read Only Memory (EPROM): In this type of ROM, the contents can be erased and
the memory can be reprogrammed. To erase the data, an EPROM is exposed to ultraviolet
light and then it can be reprogrammed using a PROM programmer circuit.
When the
EPROM is in
use, then it
behaves like a ROM, that means the information can only be
read.
Electrically Erasable Programmable Read Only Memory (EEPROM): The contents of this type of ROM can be erased
and then reprogrammed using electric signals. This makes it an excellent back
up for RAM whose contents are lost when the power is switched off. When the power
is returned, the contents of EEPROM are copied back into the RAM and
the computer continues working without any data loss. Nowadays RAMs and
EEPROMs are integrated in a single chip.
Cache Memory
Ø Cache memory is a special high speed memory made up of
high speed static RAMs.
Ø It is used to hold frequently accessed data and
instructions.
Ø It is placed between the CPU and the main memory.
Whenever some data is required, the CPU first looks into cache.
Ø If data is found, we call it a cache
hit and the information is
transferred to the CPU.
Ø In case of a miss, the main memory is accessed.
Ø So access of frequently
used data becomes very fast with cache memory.
Ø There are two types of cache memory:
i.
L1 cache
ii.
L2 cache
L1 Cache
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L2 Cache
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Its memory capacity is low.
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Its memory capacity is high.
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It is present inside CPU.
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It is soldered on mother board.
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It is implemented using SRAM.
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It is implemented using DRAM.
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It is assessed first for any data.
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It is accessed if data is not
found in L1 Cache.
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It can be accessed faster.
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Its access time is slower.
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Secondary Storage Media
There
are the following main types of storage media.
(a)
Magnetic
(b)
Optical
(c)
Solid State
Magnetic storage media: Examples of magnetic
storage media are hard
disks, floppy disks and magnetic tapes.
Magnetic media is coated with a magnetic sensitive layer and this layer
is magnetized in
clockwise or anticlockwise
directions, which then
are interpreted as binary 1s and 0s at reading.
Floppy Disk (Diskette): A
floppy disk is
a flexible disk made
up of Mylar
with a magnetic coating on it.
Ø It is packaged
inside a protective plastic envelope.
Ø These were one of the oldest type of portable storage
devices that could store up to 1.44 MB of data but now they are no longer in
use.
Hard disk: A hard disk
consists of one or more circular disks called platters which are mounted on a common
spindle.
Ø Each surface of a platter is coated with a magnetic material.
Ø Both
surfaces of each
disk are capable
of storing data
except the top
and bottom disk where only
the inner surface
is used.
Ø The information is
recorded on the surface of the
rotating disk by magnetic read/write heads.
Ø These heads are joined to a common arm known as access
arm.
Ø This arm moves over the surface of the rotating disk
Ø Information
is recorded on each of these disks
in the form of
concentric circles called tracks
which are further
divided into sectors.
Ø Hard drives however,
are not very portable
and are primarily
used internally in
a computer system.
Ø Today the hard disks have the storage capacity of
several gigabytes to terabytes.
Optical
storage media
On
an optical storage media information is stored and read using a laser beam.
Ø The data is stored as a spiral pattern of pits and
ridges denoting binary 0 and binary 1.
Ø Examples of optical media are CDs, DVDs etc.
Compact Disk: A compact disk or
CD can
store approximately 650 to 700
megabytes (MB) of data.
Ø To read the data, an infrared laser is beamed through
the CD's polycarbonate substrate.
There
are three types of CDs:
CD- ROM: It stands
for Compact Disk - Read Only Memory and
data is written on these disks at the time of manufacture.
Ø Data cannot be changed but can only be read by a laser
beam in the form of a continuous spiral.
Ø These are used for text, audio and video distribution
like games, encyclopedias and application softwares.
CD-R: It stands for
Compact Disk- Recordable. Data can be recorded on these disks but only once.
Ø So we can write data on these disks through a
read/write CD drive but after that the disk cannot be erased /modified.
CD-RW: It stands for Compact Disk-Rewritable.
Ø It can be read or written multiple times.
DVD: It stands
for Digital Versatile Disk or Digital Video Disk.
Ø A DVD holds 4.7 GB to 17 GB of data.
Ø Like CDs DVDs also
come in three varieties
a. DVD- ROM
b. DVD- R
c.
DVD-RW
Blue Ray Disk: This is the latest
optical storage media to store high definition audio and video.
Ø It looks like a CD or DVD but can store up to 27 GB of
data on a single layer disk and up to 54 GB of data on a dual layer disk.
Ø Where CDs or DVDs use red laser beam, the blue ray disk
uses a blue laser to read/write data on a disk.
Ø As the wavelength of the blue ray is shorter, more data
per unit area can be stored on the disk.
Ø This is because due to shorter wavelength, it is
possible to focus the laser spot with greater precision. Hence data can be
packed more tightly.
Ø Blue-ray Disc (BD) was developed to enable
recording, rewriting and playback
of high-definition video (HD), as well
as storing large amounts of data.
Solid State Memories
Ø The term ‘solid-state’
essentially means ‘no moving parts’.
Ø These devices
store data using
a special type of memory called flash memory.
Ø SSD, Solid-state drive (or flash memory) is used mainly
in digital cameras, pen drives or USB flash drives.
Pen
Drives: Pen
Drives or Thumb
drives or Flash
drives are the
recently emerged portable storage
media.
Ø It is an EEPROM based flash memory which can be
repeatedly erased and written using electric signals.
Ø This memory is coupled with a USB connector through
which it can be plugged into the computer.
Ø They have a capacity of 8gb/16gb/32gb etc.
PORTS
A
port is an interface between the motherboard and an external device. Different
types of port are available on motherboard as serial port, parallel port, PS/2
port, USB port, SCSI port etc.
Serial port (COM Port):
A serial port transmits data one bit at a time. Typically on older PCs, a modem,
mouse, or keyboard would be connected via serial ports. Serial cables are
cheaper to make than parallel cables and easier to shield from interference. It
is also called communication port.
Parallel Port: It supports
parallel communication i.e. it can
send several bits simultaneously.
Ø can send 8
bits (1 byte)
at a time
simultaneously (in parallel). Hence data transmission is faster
through these ports.
Ø Parallel ports are used to connect printers, scanners,
CD writers etc.
USB (Universal Serial Bus):
It is a newer type of serial connection that is much faster than the old serial
ports.
Ø USB is also much smarter and more versatile since it
allows the "daisy chaining" of up to 127 USB peripherals connected to
one port. It provides plug & play communication.
PS/2 Port: PS/2 ports are
special ports for connecting the keyboard and mouse to some PC systems. This
type of port was invented by IBM
FireWire Port: The FireWire port
has the ability to interact with a number of different devices since it
provides a single plug and socket connection for all devices.
Ø A FireWire port can provide an ideal way to connect a scanner
and digital camera/camcorder to a
computer system as the
data transfer is relatively faster than on USB and also
results in excellent quality.
Infrared Port: An IR port is a port which sends and receives infrared
signals from other devices. It is a wireless type port with a limited range of
5-10ft.
Bluetooth: Bluetooth uses
short-range radio frequencies
to transmit information
from fixed and mobile devices.
Ø These devices must be within the range of 32 feet, or
10 meters for Bluetooth to effectively work.
Ø A Bluetooth port enables connections for Bluetooth-enabled
devices for synchronizing.
Ø The devices that
are Bluetooth enabled
contain a small
transceiver chip that allows
them to communicate with other
Bluetooth enabled computer
or device.
Ø Data can be exchanged at the rate of about 2 megabit
per second.
EXCERCISE
a) Define word length of a microprocessor.
b) Name the two types of Primary Memory.
c) What is the purpose of System Clock?
d) Differentiate between CISC and RISC
processors.
e) Why do we use secondary storage? Name any two
secondary storage devices.
f) How is Computer’s internal memory important?
g) Why is it more appropriate to call RAM as
Read-Write memory?
h) What is the purpose of Cache memory?
i) Explain in brief the different ports and
their purposes.
j) Distinguish between the following pairs:
a. Primary memory and Secondary memory
c. RAM and ROM
d. Bluetooth and Infrared port
