Understanding Computer Storage Devices and Media

Storage Devices

Storage Devices: devices that store information. They comprise:

Magnetic Disks

Magnetic disks: Hard drives, floppy disks have large capacity, non-volatile random access, but live slower. It is a closed box, partly comprising:

• Platters: Stored information resides here.
• Head: Reads and writes on the platters.
• Motors: Spin the platters and move the heads.

Components of Magnetic Disks

Platters: Glass, ceramic, or aluminum; joined by a shaft and a motor to rotate at 3600 and 7200 rpm, with a diameter of 5.5 inches.

Heads: Magnetic sensor that reads the charge and a magnet to change the charges for writing.

Spindle (Topic): Support for the disks.

Actuator: The motor that moves the heads.

Structure and Organization

Tracks (Structure-Runs): Concentric rings on the sides of each plate; the heads can move between tracks. Tracks are wider and more international.

Sector: The area into which a track is divided. The industry standard (the largest outer tracks, smaller inner tracks) uses ZBR (Zone Bit Recording), where the number of sectors per track is constant within the same zone but changes between different zones.

Logical Structure

Geometry, physics (BIOS), including:

• Boot Sector: The first sector/industry: head, cylinder, sector. Saves the partition table.
• Space Partitions: Assigned space for a partition, consisting of a number of cylinders.
• Free Space Partition: Space not yet assigned.

Capacity Calculation

Capacity: Number of cylinders x heads x sectors per track x sector size.

Performance Metrics

Seek Time: Time to move the head to the correct track, typically 8 to 12 ms.

Average Access Time: Time it takes for the head to position over the correct sector containing the data.

Rotational Speed: Measured in RPM (Revolutions Per Minute).

Average Latency: Time for the head to reach the correct sector once positioned over the track.

Transfer Speed: Speed at which data can be transferred to the hard disk drive.

Cache (Falls): Saves the latest data before using them to gain speed.

Buffer Track: Saves the contents of an entire track.

Optical Discs and Tape Drives

Optical Disc: CD, DVD; used the laser to read and write information.

Tape Drive Unit: Devices used for backup.

Longitudinal Recording: Divided into tracks.

Helical Scan: Used for audio and video tapes.

Optical Disc Characteristics

Characteristics: Outer diameter 120 mm, internal diameter 15mm, thickness 1.2 mm, wavelength 780nm, separation between tracks 1.6 microns, 0.85 microns deep pit, error correction included.

Comprising: A transparent surface, a polycarbonate layer, a reflective material layer, a protective material layer, and a label layer.

Optical Disc Types

Type:

• Master Copy Generated (ROM): A mold is made; copies can only be read.
• Recordable Only Once (R): Carry special ink; once recorded, data cannot be deleted.
• Rewritable (RW): Can be erased and rewritten using three power levels: fine (write), medium (erase), low (read).

Speed and Formats

Speed Reading: Capacities between 650 and 700 MB, recorded in a single track.

Lead-in Area: Table of contents.

User Data Area: Where the main data resides.

Lead-out Area: Marks the end of the data area.

Formats: CD-DA (Audio CD), CD-ROM (Data storage), CD-R (Recordable discs), CD-RW (Rewritable discs).