Backing Storage – Magnetic Disks
Higher Computing Systems - Peripherals Infosheet 3.12
Produced by S Lambert, R Simpson and HSDU for City of Edinburgh Council, 2004
Magnetic disks have been widely used as a fast backing storage media. The capacity of fixed disk systems has
been growing very fast over recent years and the cost per megabyte has dropped dramatically.
How does it work?
Consider the diagram of the disk drive. The disk is
coated with a magnetic film on both surfaces, and
two read/write heads are positioned on moving
arms so that they can be moved back and forth
very close to the disk surfaces. The disk is rotated
at a constant speed. With the moving arm in any
given position, data may be retrieved/recorded by
the read/write heads (using both sides of the disk).
The data is recorded onto concentric circular
tracks, each track divided into sectors.
The interface for the disk drive is called a disk
controller. One disk controller may be responsible
for several separate drives. Consider the case of
four drives. The interface has to be able to, select
one out of the four disk drives, control the head movement to select a given pair of tracks, select the appropriate
sector, and select which surface is to be read from or written to. Only then can it initiate the read or write action
involved in actual data transfer.
It must also provide extensive information for the processor. The processor has to be able to identify which drive
was last selected, which of the two disk surfaces is in use, which sector was last used, whether the head is
positioned over a specified track or still moving towards it, whether a read or write is in progress, etc. Control and
status functions of interfaces are important and can be more complex than the actual data transfer function.
Capacity
The capacity of hard disk systems has increased dramatically over recent years. Typically, a desktop computer
will have an internal 20 Gb hard disk. The capacity of portable disks, such as floppy or zip is not as great as a
hard disk. Zip disks, for example, can store up to 750 Mb.
Speed
The access time of a disk drive dictates its speed. The access time is defined as seek time + search time (or
latency).
Cost
Disk drives can vary considerably in price. Obviously the media has to be bought as well. The main factors are:
§ access time;
§ types of interface;
§ capacity of media;
§ type of drive (internal or external).
Examples Cost Type Interface Media Capacity
Iomega Zip Drive £76.99 External Firewire 750Mb
LaCie Hard Disk Drive £63.00 External USB2 40Gb
Compatability
Disk drives can be connected to computer systems depending upon the interface used.
Example
Iomega Zip Drive Firewire
Read/Write
Heads
fig3
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(17 pages)
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