Lecture 4:

Secondary Storage

I. Secondary Storage (Hard Drives)

Secondary StorageSecondary Storage: holds data and programs for future use by providing permanent nonvolatile (non – changeable) storage

Processing data can be held in secondary storage Secondary storage is also known as external

storage or auxiliary storage. Secondary StorageSecondary Storage has several advantages that

allow users to store an unlimited amount of data and programs convenience – locate and retrieve stored data and

programs quickly and easily inexpensive reliable large capacity

II. Data: Sizes from smallest to largest

a BITBIT is the smallest unit of data a computer can store

a unique combination of bits represents a character

a BYTEBYTE = 8 bits a KILOBYTEKILOBYTE (KB) = 1024 bytes

(typical high speed download rate)

a MEGABYTEMEGABYTE (MB) = 1,000 kilobytes a GIGABYTEGIGABYTE (GB) = 1,000 megabytes a TERABYTETERABYTE (TB) = 1,000 gigabytes

III. Hard Drives / Magnetic Disks

common use of secondary storage - backup hard drive / hard disk – made of rigid metal and flexible

plastics with an easily magnetized substance Hard Drive – made out of aluminum, huge storage capacity

and very quick data access Data – is magnetically recorded onto “tracks”

An address is assigned to every track on a hard drive This “system of addresses” is known as random accessrandom access – (also known as direct access)

Hard disks spin constantly (i.e. 7200 rotations per minute) Data is written and read by a “read/write head” that floats on a

pocket of air Disk access timeDisk access time – the time it takes between entering a

request for data (opening a file) and actually receiving that data – measured in milliseconds

IV. CD / DVD Technology

CD-Rom and DVD-Rom drives read and write data using a laser. Both CD and DVD drives read lasers OPTICALLY OPTICALLY rather than MAGNETICALLY.

Currently, there are two types of lasers:1) Red lasers – older technology2) Blue lasers – newer technology that allows

drives to read and write larger amounts of data at faster speeds (is a larger laser and is used in high definition)

CD’s and DVD’s (continued)

RED LASER MEDIUMSRED LASER MEDIUMSRegular CD’s (CD-R or CD-RW):

hold 700 MB of data or 80 minutes of audio

Regular DVD’s (DVD+ / DVD- / DVD-RW): single layer DVD’s hold 4.7 GB dual layer DVD’s hold 8.5 GB

BLUE LASER MEDIUMSBLUE LASER MEDIUMSHD-DVD (High Definition DVD):

HD-DVD’s utilize 1080p single layer DVD’s hold 15 GB dual layer DVD’s hold 30 GB

Blu-Ray (High Definition DVD): Blu-Ray also supports 1080p single layer DVD’s hold 25 GB dual layer DVD’s hold 50 GB

HD – DVD vs Blu-Ray

HD – DVD Blu-Ray