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Chapter 6

Storage and Multimedia: The Facts and More

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Benefits of Secondary Storage

Space Reliability Convenience Economy

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Magnetic Disk Storage

Data represented in magnetic spots

Each spot is one bit Magnetized spot = 1 Non-magnetized spot = 0

Hard disks, zip disks, and floppy disks are magnetic storage

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Optical Disk Storage

Data written and read with a laser Write-once, read-many (WORM)

commonly used for backup Some forms of optical storage

allow the user to write multiple times

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Compact Disks (Optical) Compact Disk Read-Only Memory (CD-ROM)

700 MB – about 450 floppies worth of storage Compact Disk-Recordable (CD-R)

Write (record) once Compact Disk-Rewritable (CD-RW)

Record multiple times Many PC’s now come with CD-R or CD-RW

drives

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Digital Versatile Disk (Optical)

Digital Versatile Disk Read-Only Memory (DVD-ROM) 4.7 GB – about the same as 7 CDs common storage format for movies replaces VHS tape format

Writable version of Digital Versatile Disk (DVD-RAM)

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Magnetic Tape Storage

Data represented in magnetic spots Primarily for backup Density

cpi = characters per inch bpi = bits per inch

Inexpensive Sequential access to data Slow

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Diskette

Flexible plastic with metallic coating

3 ½ inch 1.44 MB Low capacity – small files Hard plastic jacket Portable

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High-Capacity Portable Disks

Can hold larger files Zip disk - 100 MB or 250 MB Super Disk - 120 MB Jazz Disk - 1 or 2 GB Data compression can be used to

store more data in the same area

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Hard Disk

Rigid platter with metallic coating Typical PC has 20 GB to 100 GB

hard drive Non-portable, permanently

mounted inside the system unit

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Redundant Array of Independent Disks (RAID)

Disk mirroring Simplest form of RAID reduces possibility of data loss maintains multiple copies of the same data on

different drives Data striping

Higher level of RAID Spreads data across several disks Uses check disk to rebuild lost data

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How Disk Drives Work

Disks rotate, then access arms move read/write heads in or out

All access arms move together

Only one read/write head can operate at a time

Read = Data transfer from the drive to memory

Write = Data transfer from memory to the drive

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Logical Layout of a Disk

Tracks Concentric circles Standard floppy

has 80 tracks on each side

Hard disks may have 1000 or more tracks per surface

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Logical Layout of a Disk

Sector Fixed size: usually

512 bytes

Zone recording assigns more sectors to outer tracks ↓↓↓↓↓↓

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Logical Layout of a Disk

Cluster 2 to 8 adjacent sectors Data is read into memory one cluster

at a time Data is stored one cluster at a time # of sectors in a cluster depends on

the OS

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Logical Layout of a Disk

Cylinder Same track on each platter Large files stored in

cylinder Access arms all move

together Faster access if data is on

the same cylinder

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Disk Access Speed

Access time (3 factors) Seek time Head switching Rotational delay

Data transfer rate Disk cache

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Data Organization Character – letter, digit, or symbol

($, ?, *, etc) Field – set of related characters Record – collection of related fields

Key field – a field that is different for each record

File – collection of records Database – collection of files

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Data Access

Sequential Records stored in order by a key field Slow retrieval Used on tape storage

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Data Access

Direct Also called random access Hashing algorithm determines the

address of a record Collision – if an address is already

used, another record must find a different address

Cannot be used on tape

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Data Access

Indexed Compromise between sequential and

direct Records stored sequentially An index tells where to find each key

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Data Processing

Batch processing Collects transactions and processes

them later Master file Transaction file Used when changes don’t have to be

processed immediately Bank statements, Payroll, etc

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Data Processing

Transaction processing Real-time, happens now Online – computers / terminals

connected directly

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Multimedia Hardware Requirements

CD-ROM, DVD-ROM Drive speed

Sound card Speakers MPEG – Motion Picture Experts

Group Widely accepted format for storing

video