Storage Devices and
Stored Data
Just the Facts
Why Secondary Storage is Needed
- Memory is limited in size and is temporary (volatile).
Magnetic Disks
- Metal or plastic disk coated with magnetic oxide.
- Types
- Disk Packs
- Hard Disks
- Diskettes
Magnetic Disk Storage
- Data is recorded as magnetic spots on concentric tracks
.
- A direct-access storage device (DASD) is a secondary
storage device that allows you to go directly to the record you
want.
Disk Packs
- Common on mainframes and minicomputers - no longer manufactured.
- Up to 11 disks arranged one above the other.
- Information written on both sides of platter.
- Read/write heads move together on access arms.
- Data is recorded on tracks on one cylinder.
Hard Disks
- Similar to disk pack, but metallic, not plastic.
- Usually 5 1/4" or 3 1/2"
- Enclosed in PC so doesn't need special environment (also called
Winchester disks).
- Data can be accessed 2 to 10 times faster than on a diskette.
Diskettes
- Also called floppy disks
- Two primary types
- 3 1/2" Diskette
- 5 1/4" Diskette
3 1/2" Diskette
- Double Sided / Double Density (DD): stores 720 Kbytes.
- Double Sided / High Density (HD): stores 1.44 Mbytes.
5 1/4" Diskettes
- Double Sided / Double Density (DD): stores 360 Kbytes.
- Double Sided / High Density (HD): stores 1.2 Mbytes.
Diskette Storage
- Instead of using the cylinder storage system, the tracks are
divided into sectors during formatting. This is because only one
disk is used.
- A list of the diskette's contents, the file allocation table
(FAT) is recorded on a specific sector & track.
Optical Storage
- Data is recorded with a laser hitting a layer of metallic
material, producing tiny spots on the disk's surface.
- Data is read by scanning the disk with a laser, picking up
reflections from the spots on a lens.
CD-ROM (Compact Disk Read-Only Memory)
- Similar to audio CD media.
- Can hold up to 600 Mbytes of information.
- Slower access time than magnetic disks.
Write-Once, Read Many (WORM) Disks
- Written once by user, then becomes a read-only media.
Magnetic Tape
- Information recorded in binary form by magnetization of spots
on mylar tape coated with ferrous oxide.
- Inexpensive means of storing large amounts of information.
- Useful for operations that require sequential processing and
for backing up.
Magnetic Tape Storage
- Two types of magnetic tape:
- Classified by density - bytes per inch (BPI), and number of
tracks. Most common number of tracks is 9 (9th track is a parity
bit). Tape drive starts & stops at interblock gaps.
Cartridge Tapes
- 1/4" wide tape stored in a cartridge similar to audio
or video cassette tape.
- Can store up to 250 Mbytes of information.
- DAT, 8mm tape similar to 8mm video tape.
- Can store Gbytes of information.
- Commonly used for backups of Pcs.
Methods of Processing Data
- Batch processing
- Transactions are collected in groups, to be processed later.
- Transaction processing
- Transactions are processed in any order they occur.
- Real-time processing can obtain in time to affect the
current activity.
- Many computer systems combine batch and transaction processing,
gathering and retrieving data in transaction mode, but using batch
mode for updating and reporting.