- - The n u m b e r of sectors currently free for storing data on (excluding
directories)
- - T h e location of the first directory sector and the first data sector
free.
The Disc Drive Unit
P r o b a b l y the most obvious difference between disc and tape storage is
the speed of operation with a disc system, it is possible to write data to a
disc and read data from a disc m a n y times faster than f r o m a tape
system. Disc systems are also generally more reliable and there is less
likelihood of the data being spoiled as it is being transferred to and from
the disc.
T h e disc drive unit which holds the disc has no external controls (unlike
a tape recorder which has a n u m b e r of controls - - Record, Playback
etc). This means that control of storage and retrieval of data from disc is
entirely with the c o m p u t e r and therefore m o r e sophisticated data
handling features can be built into the disc than is possible for a tape
cassette storage system which usually requires manual operation of the
'press b u t t o n ' controls.
Because of the way in which a disc drive unit is engineered, it takes only
seconds for an item of data to be found and loaded into the computer.
With a tape sytem, the speed at which the tape moves means that it can
take minutes rather than seconds.
The devices which actually record and read data on the disc surface are
called the read/write heads and they m o v e radially under control of the
c o m p u t e r . The disc itself is spinning at high speed, so the read/write
head can very quickly get to any sector of any track. Disc drive units
c o m e in single or double-sided versions, the double one having two sets
of read/write heads (one for each side of the disc). With a double sided
drive you d o n ' t have to turn the disc over in the drive in order to write to
both sides (as you have to with a single sided drive).
