Storage holds data and information for future use. Common storage mediums include hard disks, CDs, DVDs, memory cards, and USB flash drives. Storage devices, like hard disks, record and retrieve data from these storage mediums. Hard disks use magnetic disks to store data in tracks and sectors. They have faster access times than optical discs but slower times than memory. Other storage media include floppy disks, optical discs like CDs and DVDs, and flash memory cards.

1. Storage Devices
Lecture 22
Zikria Mian

2. What is Storage
• Storage holds data, instructions and
information for future use.
– All the programs when started are loaded
from a storage device onto computer memory
– Users store a variety of data on mainframes,
servers, desktops, notebooks, handheld
devices, smart phones etc

3. Storage Medium
• A storage medium is the physical material
on which a computer keeps data,
instructions and information.
– Hard disks, CDs, DVDs, memory cards, USB
flash drives etc are all storage mediums or
secondary storage devices
– Storage capacity is the number of bytes a
storage medium can hold

4. Storage terms
• 1 KB = 2
10
Bytes = 1,024 bytes
• 1 MB = 2
20
Bytes = 1,048,576 bytes
• 1 GB = 2
30
Bytes = 1,073,741,824 bytes
• 1 TB = 2
40
Bytes = 1,099,511,627,776 bytes
– A storage medium is non-volatile, that is, items
on storage medium remain intact even when
power is removed

5. Storage device
• A storage device is the computer
hardware that records and/or retrieves
items to and from the storage media.
– Writing is the process of transferring data,
instructions and information from memory to
storage medium.
– Reading is the process of transferring these
items from a storage medium into memory.

6. – When storage devices write data onto storage
medium, they are generating computer output
– When storage devices read from storage
media, they are generating input for the
computer.
* however, storage devices are not categorised
into input or output devices

7. Speed
• The speed of storage devices and
memory is defined by access time.
– The time required to locate an item on a
storage medium
– The time required to deliver an item from
memory to the processor
• Access time of storage devices is slow as
compared to memory devices

8. Memory Most RAM
Storage
Hard Disk
Flash Memory cards
and USB drives
CDs, DVDs etc

9. Magnetic Disks
• Magnetic disks use magnetic particles to
store items on a disk’s surface
– Each particle on the magnetic disk represents
a bit
– Formatting is process of dividing the disk into
tracks, sectors and clusters (allocation unit)

11. Hard Disk
• A hard disk is a storage device that contains one or more
inflexible magnetic disks

12. • The magnetic disks in a hard disk rotate at
a speed from 5,000 to 15,000 rpm
• The data transfer rate of hard disks range
from 15 mbps to 320 mbps
• Hard disks improve their access time by
using disk caching
– Disk cache is a memory chip on hard disk that
stores frequently accessed items

13. • A group of two or more integrated hard
disks is called RAID (Redundant Array of
Independent Disks)
• Miniature hard disks are present in digital
cameras, portable media players, smart
phones etc
• A pocket hard drive or portable hard drive
also has a miniature hard disk
• External or removable hard disks

14. Hard Disk Controller
• A disk controller consists of a special-
purpose chip and electronic circuits that
control the transfer of data.
– An interface is required by the hard disk
controller to transfer data to and from the hard
disk
• SATA (Serial Advanced Technology Attachment)
• EIDE (Enhanced Integrated Drive Electronics)
• SCSI Interfaces
• SAS (Serial Attached SCSI)

15. Online storage
• Online storage is a service provided on
the web that gives hard disk storage to
users for free or for a nominal monthly fee
– To access data from other computers
– To store files like audio, video and graphics
on the internet instead of downloading onto
local hard disk
– To allow others to access your files
– To store backups

16. Other storage media
• Floppy disks : A flexible plastic film with a
magnetic coating
• Optical discs: A storage media that
consists of portable disc made of metal,
plastic and lacquer that is written and read
by a laser
– Read/write feature
– Double-sided discs

17. Disc type Features
CD-ROM Read only
CD-R Read and write
CD-RW Read, write and
erase
DVD-ROM, BD-ROM, HD DVD-
ROM
Read only
DVD-R, BD-R, HD DVD-R Read and write
DVD-RW, DVD-RAM, BD-RE, HD
DVD-RW
Read, write and
erase

18. • PC cards and Express cards (developed
by Personal Computer Memory Card
International Association)
• Flash memory cards (solid state media)
– Secure Digital (SD)
– Mini SD
– USB flash drives
– Smart card (sim, visa card, master card etc)
– Enterprise storage