Sec (1.3): Mass storage (secondary storage). File storage and retrieval: Field: set of characters Record: set of fields File: set of records Data base:

Sec (1.3):

Mass storage (secondary storage)

File storage and retrieval:

Field: set of charactersRecord: set of fieldsFile: set of recordsData base: set of files

Mass storage (secondary storage)Like magnetic disk, CD, DVD, magnetic tapes and flash drives

Advantages of mass storage over main memory:1. include less volatile2. large storage capacity3. low cost4. many cases, the ability to remove the storage medium from the machine for

archival purposes

Disadvantages: A major disadvantage of the mass storage systems is that they typically

require mechanical motion and therefore require significantly more time to store and retrieve data then a machine's main memory, where all activities are performed electronically

On-line and Off-line:

Means that the devices that can be either attached or de-attached from a machine

On-line: means that the device or information is connected readily available to machine without human intervention

Off-line: means that human intervention is required before the devices or information can be accessed by the machine.

Magnetic system:

It is significant to two kinds:1. sequential access secondary storage2. Direct access secondary storage

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 )

Direct access secondary storage Data can be reached directly without going through the stored

data (like magnetic disk )

Magnetic system: sequential access secondary storage1 ) Reel Magnetic Tape: Used in mainframe computers It has a plastic strip coated with one side with magnetized material

(Chromium Dioxide) It is width 1/2 inch and it is length commonly 2400 feet (some reel

with 300, 600 or 1200 feet ) The strip is divided into 7 or 9 tracks (9 common ) Strip with 9 tracks stores about 6250 characters per inch. In strip with 18 tracks, 38000 characters can be stored in 1 inch

The devices that reads the tape is called tape drive it consists of: Two reels one for source and the other for destination R/W head which reads or writes the data on the strip Control unit which control in reading and writing operations

Magnetic system: Direct access secondary storage

2 ) magnetic disk Note: the 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 a 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 )

Mass Storage: Magnetic Disk Systems

Magnetic disks: the most widely used storage medium in computers today

Data is stored by magnetizing particles on the storage medium.

Capacity depends on # of disks & density of tracks and sector placed on

Mass Storage: Magnetic Disk Systems

Two common types:

Floppy disks: Diskette (low-capacity)Hard disks: (high-capacity)

Floppy Disks and Drives

Floppy disk characteristics

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.

Floppy Disks and Drives, Cont’d.

Using floppy disks Must be inserted into the proper drive in the proper direction. Should not be removed when the disk is being accessed.

High-capacity removable magnetic disks and drives Zip disks (750 MB) SuperDisks (240 MB)

Hard Disk System:Hard disk characteristics 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

TrackSectorClusterCylinder

Disk Access Speed

Access time - the time needed to access data on disk

Three factorsSeek timeHead switchingRotational 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.

Optical Systems:CD-DA: (Compact Disk Digital Audio): Which technology applied to audio recordings

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

Note: the distance around the spiraled track is greater from the outer edge of the disk than the inner portion

Note: 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

Mass Storage: Optical Systems

Provides inexpensive and compact storage with greater capacity ( approx. 5 inches diameter)

Information is recorded by creating variation in reflective services

Information is retrieved by a laser beam that monitors irregularities on the reflective surface

Laser scans disk and picks up light reflections from disk surface

Optical Systems: Compact DisksCategorized by read/write capability:

CD-ROM - drive can only read data from CDs CD-ROM 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 Which are constructed from multiple layers that serve as

distinct surfaces when viewed by precisely focused laser provide storage capacities of several GB. Such disks are storing lengthy multimedia presentations, including entire motion pictures

Mass Storage: 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.

Chip-based storage. This technology is suitable for off-line

storage of data

Flash memory media

Note: data stored in flash can be accessed in small byte-size units as in RAM applications.

Repeated erasing slowly damages the silicon dioxide chambers, meaning that current flash memory technology is not suitable for general main memory application

Since flash memory is not sensitive to physical shock, it is potential in portable applications is enticing

Flash memory media

Flash drives, with capacities of up to a few GB, are available for general mass storage applications

These units are 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

Commonly used with digital cameras, digital music players, handheld PCs, notebook computers, smart phones, etc.

Flash memory media

Flash memory media, cont’d.

Common forms of flash memory:

Flash memory sticks.

Flash memory cards—CompactFlash, Secure Digital (SD), MiniSD, MultiMedia (MMC), SmartMedia, and xD Picture cards.

Flash memory drives, such as USB flash drives.