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CPT 105 · Franklin College · Erich Prisner · 2002-2007

Magnetic Disk Storage

surface consists of iron oxide or some other magnetizable oxide.

1.44MB Diskettes

The so-called "Floppy disks". invented in 1967 as a "floppy" 5.25 inch (369 kilobyte) disk. The rigid 3.5 inch diskette appeared around 1985.
Advantage: Small, inexpensive, movable, more or less reliable.
Disadvantage: small capacity and not fast enough.

What about data compression?

Other 31/2 inch "Diskettes"

Zip disk: 250 MB. much higher quality magnetic coating.
other types are also available, some can work with ordinary floppy disks.

Hard Disks

Invented in the 50s.

A "pack" of disks in an airtight case (to prevent dust and other particles to go on the disks. Both sides of each disks may be used. Separate read/write head for each side of the disk. Only one head may be active at any time. Then it should come close to the disk surface but may NOT touch it (otherwise "head crash") Access arm contains all these read/write heads.

Organization of data on Hard Disks

Disk Access Speed

also important: data transfer rate (between 5 and 40 megabyte/sec) that the drive can deliver to CPU.

average access time below 10msec now.

Improve performance by using disk cache: "Read whatever is around, store it in cache, you may need it soon".

Magnetic Tape

May be a dying technology. Advantage: It is inexpensive. Disadvantage: It is sequential, data cannot be accessed directly, but you have to go through the whole tape when you want to get some data. Is still used for backup purposes.


redundant array of independent disks

in both methods, one should be able to recover data even if one disk crashes.

Optical Disk Storage

Writing: A laser "burns" tiny spots on the disk.
Reading: Laser scans the disk.
Main difference to magnetic storage devices: Rewriting/erasing is more complicated. Doable only with advanced material.

Compact Disks

CD-ROM ("compact disk---read only memory")
CD-R ("compact disk, recordable")(once only)
CD-RW ("compact disk, rewritable")


"digital versatile disk", former "digital video disk". two layers on one side, can be written on both sides, uses laser with shorter wave length than CD. DVD-ROM drive can also read CDs.

Magneto-Optical Disks

Combines the erasing/rewriting possibilities of magnetic disks with high storage capacity of optical disks. Laser makes part of surface "soft", magnet organizes tiny magnetic crystals before material hardens.

Solid state Technology

no moving parts. Example: Flash memory, used for digital cameras. chips with two-transistor cells.

How data is organized on disks

3 Methods to organize data

Processing stored data