Data Storage

Lecture 3CSCI 1405, CSCI 1301

Introduction to Computer ScienceFall 2009

Main memory cells

• Cell: A unit of main memory (typically 8 bits which is one byte)

Main memory & address

Memory techniques

• Random Access Memory (RAM): Memory in which individual cells can be easily accessed in any order

• Capacity:– Kilobyte: 210 bytes = 1024 bytes ~ 103

– Megabyte: 220 bytes = 1,048,576 bytes ~ 106

– Gigabyte: 230 bytes = 1,073,741,824 bytes ~109

Mass Storage (Secondary Storage)

1. Magnetic Systems (Disk, Tape)2. Optical Systems (CD, DVD)3. Flash Drives • Larger capacity• Less volatility• Slower: Requires mechanical motion- more time• On-line: connected and readily available to the

machine• Off-line: requires human intervention before

being accessed

Magnetic systemsTwo kinds:1. sequential access secondary storage: searching for a

certain data means starting from the beginning of the file sequentially till the data is reached ( like magnetic reel tape and cassette )

2. Direct access secondary storage: Data can be reached directly without going through the stored data (like magnetic disk )

Magnetic Disk• The most widely used storage medium in computers today

• Data is stored by magnetizing particles on the storage medium.

• Capacity of disk storage system depends on the number of disk used and the density in which the tracks and sectors are placed

– Lower-capacity system consists of a single plastic disk known as diskette or floppy disk (off-line storage). 3 1/2 inch have a capacity of only 1.44 MB

– High capacity system like hard-disk (consist of 5-10 rigid disks mounted on a common spindle )

Magnetic Disk

• Head, track, sector, cylinder• Access time = Seek time + Rotation delay/latency time• Transfer rate: the rate at which data can be transferred to or from the

disk

Floppy Disks and Drives

– Single plastic flexible disk

– Typically 3½ inches in diameter.

– Typically hold 1.44 megabytes.

– Inserted into floppy disk drive to be read from or written to.

Floppy Disks and Drives, Cont’d.

• Disk is divided into tracks, sectors, and clusters.

Hard Disk System:

• Usually (5-10) rigid disks mounted on a common spindle and combined into a disk pack

• Disk drive - a device that allows data to be read from or written to a disk

• Disk drive for personal computers contained within computer housing

• Can be internal or external.• Large computer systems may have several external disk

drives• Capacity in gigabytes• Accessing files much faster than accessing files on

diskettes

Disk Packs

• Each disk has its own access arm with read/write head

• Most disk packs combine disks, access arms, and read/write head

Reading/Writing Data

• Access arm moves read/write head over particular location

• Read/write head hovers a few millionths of an inch above disk and doesn’t touch the surface of the disk.– If head touches platter, a

head crash occurs and data is destroyed

– Data can be destroyed if head touches miniscule foreign matter on surface of disk

How Data Is Organized

Organized into tracks, sectors, clusters, and cylinders

•Track•Sector•Cluster•Cylinder

Disk Access Speed

• Access time - the time needed to access data on disk

• Three factors– Seek time– Head switching– Rotational delay

• Once data found, next step is data transfer

Optical Systems:

Like Compact Disk (CD):

• These disks are 12 cm (approximately 5 inches) in diameter and consist of reflective material covered with a clear protective coating

• The capacity of the CD about 600 to 700 MB.• Information is recorded on them by crating

variations in their reflective surfaces.• This information can be retrieved by means of a

laser beam. That monitors irregularities on the reflective surface of the CD. (Laser scans disk and picks up light reflections from disk surface)

CD storage

Optical Systems:

• Information on these CDs is stored on a single track that spiral around the CD, this track divided into units called sectors. Each with its own identifying marking s and a capacity of 2KB of data

• The distance around the spiraled track is greater from the outer edge of the disk than the inner portion

• To maximize the capacity of a CD: information is stored at a uniform linear density over the entire spiraled track which means that more information is stored in a loop around the outer portion of spiral than in a loop around the inner portion

Optical Systems: Compact Disks

Categorized by read/write capability:

• CD-ROM - drive can only read data from CDs– stores up to 700 MB per disk– Primary medium for software distribution

• CD-R - drive can write to disk once– Disk can be read by CD-ROM or CD-R drive

• CD-RW - drive can erase and record over data multiple times– Some compatibility problems trying to read CD-RW

disks on CD-ROM drives

Optical Systems: Digital Versatile Disk (DVD)

• Constructed from multiple, semi-transparent layers– DVD drive can read CD-ROMs– Capacity up to 17GB– Allows for full-length movies– Sound is better than on audio CDs

• Several versions of writable and rewritable DVDs exist

Flash memory media

• Bits are stored by sending electronic signals directly to the storage medium where they causes electrons to be trapped in tiny chambers of silicon dioxide.

Flash memory media

• Flash memory media, cont’d.

– Common forms of flash memory:

• Flash memory sticks and cards. Commonly used with digital cameras, digital music players, handheld PCs, notebook computers, smart phones, etc.

• Flash memory drives, such as USB flash drives.– with capacities of up to a few GB, are available for

general mass storage applications– packaged in small plastic cases approximately

three inches long with the removable cap on one end to protected the unit's electrical connector when the drive is off-line

Thank You