COM.POTER BITS
By JENNIFER BONNITCHA
A guide to hard disc drives
The first law of computing says that software
expands to fill the available storage space. Sooner
or later, if you've been using a computer with
floppy discs, you'll he wondering about the
advantages of a hard disc drive. If you use your
computer a lot, adding a hard disc to it will he a
big step forward.
A hard disc drive represents
salvation for harassed floppy disc
users. When you depend only on
floppy discs, you soon find you need
an ever increasing number of floppies to store your files. As their
number increases, you will have
trouble keeping track of them all
and storing them for easy access.
Wouldn't it be so much nicer to
have it "all there" permanently, in
the computer?
Another important consideration
these days is the amount of disc
space programs take up. In the
good old days you could happily run
W ordStar on a single floppy disc
machine and store your documents
on the same disc. Today, the increased memory requirements of
graphics-based programs, so-called
"power-users" and lazy programmers has lead to the need for greatly increased storage. Many popular
PC programs such as Excel and
Ventura just won't work on a floppy
disc machine; you must have a hard
disc drive.
And while many programs can be
run on single or dual floppy disc
drive machines, the speed advantage of a hard disc drive is quite
dramatic. Typically you can expect
the hard disc drive to read and
Fig.1: this diagram shows
the concept of
interleaving although a
hard disc has many more
sectors than suggested
here.
HARD DISK INTERLEAVING
write data at around five to 10
times the speed of a floppy disc
drive.
As well, you can't beat the convenience of having your word processing, database, spreadsheet,
graphics and accounting software
all readily at hand without having
to boot them up from separate
floppies.
Just a couple of years ago, you
could purchase a staggering 20
megabytes of hard disc storage for
"just a few" thousand dollars. Today, the same hard disc with controller is likely to set you back $500
to $600 although most buyers would
now tend to go for 40 megabytes,
for around $700 or so. And if you
shop carefully, 80 megabytes or so
can be purchased for around
$1200. As you can see, the extra
capacity more than justifies the
price difference.
Hard disc standards
The hard disc itself is just a
magnetic recording device. What
the disc receives, how fast and
where it will be placed is determined by DOS (the Disc Operating
System) and the controller board.
So if you buy a hard disc, you must
also have a hard disc controller.
The hard disc controller handles
the task of formatting the disc for
later use, encoding the data it
receives and instructing the hard
disc where to place the read/write
heads.
A hard disc drive actually contains several aluminium discs
which are coated with particles of
ferric oxide which form the
magnetic storage. The read/write
heads record (write) and replay
(read) data to and from the disc in
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75
COMPUTER BITS - CTD
much the same way as on an audio
cassette deck or a video cassette
recorder except that the signal format is different.
The read/write head differs considerably for floppy disc drives,
high capacity drives and fixed
discs. In all cases though, it serves
two basic functions:
(1). It reads the data stored on the
disc dependent on the polarity of
the magnetic fields at a given point
on the disc.
(2). It stores or writes data onto the
disc by magnetising the disc surface in one direction or the other
(ie, north poles or south poles) for
binary ls and 0s.
Flying heads
Read/write heads ride on a very
thin layer of air and so don't normally come into contact with the
disc surface.
The 10Mb XT fixed disc consists
of two 5.25-inch aluminium platters
with each of the four sides capable
of holding 2.5Mb of data. Both the
platters and recording heads are
sealed inside a container which includes an air filter to prevent dust
particles coming into contact with
the highly sensitive surface and
recording head mechanisms.
The platters are fixed on a spindle which rotates at approximately
3600 RPM. The rapidly spinning
sealed disc creates sufficient air
flow for the recording heads to
"fly" or "float" at around 1.5µm
above the platters' surfaces.
The PC AT also has two platters
and four surfaces but unlike the
XT, the AT 20Mb hard disc has six
read/write heads. So while the AT's
rotational speed of 3573 RPM is not
significantly different from the
XT's, the AT can access its data at
two times the speed of the XT's
hard disc.
Since the hard disc coating contains finer oxide particles than a
floppy disc, information is packed
more densely on its surfaces. They
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also spin about 10 times faster than
a floppy disc and so a hard disc is
able to pack in large amounts of information then access it very
quickly.
For example, data is stored on
the IMB PC's 360Kb floppy disc at
48 tracks per inch while the XT's
10Mb hard disc contains 345 tracks
per inch. A mechanism called an
actuator finely positions the
read/write heads across the disc to
locate the required information.
Interleaving
The disc surface is divided into
sectors by a magnetic numbering
system. Since the surface of a hard
disc moves so rapidly past the
read/write heads, most computers
can't transfer data to and from the
disc fast enough to keep up with
consecutively numbered sectors.
Sectors are therefore not usually
numbered sequentially to allow
time for the controller and computer to transfer and process the
data being read from the disc. Instead the sectors are interleaved,
with the sequence of the sectors
called the "interleave factor".
For an AT computer, the sectors
on the hard disc are often organised with two sectors separating the
first and second sectors (1 **2**3
- an interleave of 1 to 3). The
slower XT uses an interleave of 1 to
6 (1 *****2*****3). An interleave
of 1:1 means data is placed in sequential sectors. If the interleave
factor is incorrect, the files aren't
transferred to and from the hard
disc as fast as possible and the
overall performance of the computer is slowed.
If the interleave is set too fast,
the computer can't process the data
as rapidly as it appears under the
read/write heads and so must wait
for an entire revolution to read the
remaining data. On the other hand,
if the interleave is too slow, the
computer is ready to read the next
sector before it appears so time is
lost waiting for the sector to
appear.
Head crash
A "head crash" describes the
loss of the air layer which brings
the heads and the disc surface into
contact, usually with disastrous
Yet another option in the hard disc field is this completely removeable hard
disc drive. Called the Portadisk, it is a standard 40Mb drive which fits into a
normal 5¼-inch drive slot. It is available from Electronic Solutions.
results. The hard disc surface is
very delicate so a head crash can
not only wipe out data stored at the
particular location where contact
is made, it can also tear a hole in
the coating although this situation
is very rare.
Access times
Look for information on the data
access time when you consider purchasing a hard disc. Anything rated
at 50 milliseconds is fairly fast,
with 20ms or less very fast; either
way, even the slowest hard disc
operates much faster than a floppy
disc.
Installation
Installing a hard disc in an existing PC is a relatively simple matter. The hard disc requires the installation of an adapter in one of
the system expansion slots of the PC
XT system unit. PC users may have
to first upgrade their power supply
because of the power requirements
of the disc, although "low power"
hard discs can be installed and used with the original supply included
with the PC. You need to check this
aspect before you purchase.
Other alternatives are the
removable hard disc, self-contained
drives which fit into a flexible,
removable cartridge, and removable hard cartridges.
Removable hard discs can be
taken out in seconds for transport.
Since the platters are rigid, two or
more can be packed closely
together in one drive to give decent
storage capacity. To counter the effects of moving, removable hard
discs are typically protected to
some extent by shock mounted
assemblies.
If the computer loses power
unexpectedly or you neglect to
"park" the read/write heads before
turning the power off, the spinning
disc no longer provides an air
cushion for the heads. The potential
for the heads crashing onto the
discs increases markedly. To
counter this, some makers provide
"auto-parking" which may be timeactivated or triggered by the ejection of the hard disc from its
receptacle.
Flexible cartridges
The flexible cartridge is typically
enclosed in a plastic case to protect
the disc surface. The beauty of this
system is its ability to squeeze
20Mb from floppy disc media while
remaining small and light - and it
provides a storage device which
can take a fair amount of rough
treatment. However, these benefits
don't come cheaply with prices
starting at about $1900 for a bottom of the range system.
While the system is certainly
durable, it is also slow and prone to
wear since, like the floppy disc, the
read/write heads actually touch the
media surface. In an effort to
reduce wear, the systems spin
anywhere between 600 RPM and
1800 RPM. Although this reduces
media wea:r, disc performance is
severely limited.
Combining the performance of a
hard disc with the convenience of
removable cartridges seems to be
the best of both worlds. The
removable hard cartridge separates the read/write mechanism
from the disc platter, which rests in
its own removable cartridge.
However, these systems generally
have no provision for auto-parking
and the cartridges are more prone
to damage if dropped.
Since the cartridges are typically
equipped with hinged or sliding
doors, it is inevitable that dust and
debris will ultimately find its way
onto the disc surface. Although the
presence of debris may not cause
severe data loss, expected media
life is undoubtedly shortened.
So what is the answer? If you can
take it out, eventually it will be
dropped. Therefore you should
choose a heavy duty product and
from this angle, the flexible disc
seems to be the best choice, particularly if you regularly travel or
send information by courier/post.
On the other hand, if you want to
store a lot of programs and data
and be able to access it very quickly
at any time, a coventional 40
megabyte hard disc drive is a very
hard proposition to beat.
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