Lecture No 11
Storage Involves Two Processes
The materials on which data is stored.
The physical material on which data is stored is called
a storage medium.
E.g. Diskette is a storage medium.
The hardware that writes data to or reads data from
a storage medium is called a storage device.
E.g. A floppy disk drive is a storage device.
Two Categories of
High-capacity floppy disks
Compact Disk Read-Only Memory (CD-ROM)
Digital Video Disk Read-Only Memory (DVD-
DVD Recordable (DVD-R/DVD), DVD Rewritable
Magnetism Allows Data Storage
Hard disks, diskettes, high-capacity floppy disks and
tapes have a magnetic coating on their surface that
enables each medium to store data.
Magnetic Storage Devices
How Magnetic Storage Works
A magnetic disk's medium contains iron particles,
which can be polarized when given a magnetic
charge in one of two directions.
Each particle's direction represents a 1 (on) or 0 (off),
representing each bit of data that the CPU can
A disk drive uses read/write heads containing
electromagnets to create magnetic charges on the
Random particles Current flow
(no data stored) (write operation)
As the medium
rotates, the head
writes the data
Preparing Disks for Storage
Before a magnetic disk can be used, it must be
formatted—a process that maps the disk's surface
and determines how data will be stored.
During formatting, the drive creates circular tracks
around the disk's surface, then divides each track into
Formatted Disks Have Three
Main sections, called Tracks
Track subsections, called Sectors
Groups of sectors, called Clusters
A logical format is the labeling of tracks and
The Logical Format has Four Disk
Master boot record
File allocation table
Master Boot Record (MBR)
This program determines whether the disk contains
the basic components of an operating system
necessary to run successfully.
It stores the master boot record (MBR), a small
program that runs when you first start (boot) the
File Allocation Table (FAT)
A log created during the logical formatting process
that records the location of each file and status of
each sector on the disk.
The top folder or directory in the disk’s folder
Enables the user to store data on the disk in a logical
Place where data or program files are actually
3.5-inch Diskettes (Floppy Disks)
Diskettes are used to transfer files between computers
as a backup medium.
Spin rate: 300 revolutions per minute (rpm)
Disk Density=Total no of Sectors * Total no of bytes
that each sector can hold.
Storage Capacity of HD disks is 1.44 MB
Spin rate: from 3,600 to
Storage capacity ranges
from several hundred
MB to more than 200
High rotational speed
allows more data to be
Commonly used for
hard disk backup.
Used to connect new components like
memory and expanding storage capacity to a
Up to 512MB memory.
Two Ways to Measure Storage
Average Access Time
Data Transfer Rate
Average Access Time
Amount of time the device takes to move its
read/write heads to any spot on the medium.
Measurement should be an Average Time.
Important measure of performance for storage
devices and memory.
Access Time Measured in Two Ways
Storage devices: milliseconds (ms)
Memory devices: nanoseconds (ns)
Examples of Average Access Time
Floppy Disk: 100 ms
Hard disk: 6 to 12 ms
High-performance hard disk: 4 or 5 ms
CD-ROM: 80 to 800 ms
Tape drives: slowest at a few seconds to a few
Data Transfer Rate
Amount of time it takes for one device to transfer
data to another device.
Measured in units of data per second
Note: MBps= Megabytes/sec, Mbps =megabits/sec
8bits = 1 byte
Hard drives are the fastest
Floppies, the slowest
When the PC’s performance slows down,
disk maintenance, or disk optimization,
can speed things up again
File or Data Compression
Technology for making files smaller to free up
File compression enables to store more data on a disk
effectively increasing the disk’s capacity.
File Compression is performed by a software.
Utilities such as Windows' DriveSpace enable you
to compress the entire contents of your hard disk.
File Compression Software
PKZIP and WinZip: Windows-based PC’s
Ways to Optimize Disk Performance
Delete temporary files
Uninstall little-used software programs
Run a disk scanner to look for and fix errors
Defragment the disk.
Another factor to determine how quickly a drive can
read and write data is the Type Of Controller that
Storage Devices need a controller to act as an
interface between the Drive and CPU.
Personal Computers use one of two drive-interface
standards for built-in disk drives: EIDE or SCSI.
Other type of interface (USB & FireWire) to attach
additional disk drives and other devices to a
EIDE -- Enhanced Integrated Drive Electronics
SCSI – Small Computer System Interface
USB – Universal Serial Bus
FireWire – IEEE 1394
Most new computer systems use EIDE drive-
Latest version supports data transfer rates of
Variants: Fast IDE, ATA, Fast ATA, Ultra-ATA,
ATA 66 etc.
Earlier developed to connect third-party peripheral
devices to mainframe computers.
Allows high data transfer rates than EIDE.
Normally found in Servers, Workstations etc.
USB & FireWire
All sort of peripheral devices can be attached to a
computer through USB or FireWire port.
Support High Data transfer rates.