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COMPUTER BITS
By JENNIFER BONNITCHA
What to do when your
computer goes bung; Pt.4
Many computer problems can be quickly
diagnosed using the CHKDSK command. This
month, we take a look at CHKDSK error
messages and also cover power supply &
keyboard problems.
The DOS program CHKDSK is one
way to analyse the directory and
file allocation table on a hard or
floppy disc. The end result is a table
showing the number of files currently stored on the disc, together
with an availability report of disc
space and memory. It looks like this:
xxxxxxxx bytes total disk space
xxxxxx bytes in x hidden files
xxxxxx bytes in x directories
xxxxxxxx bytes in x user files
xxxxx bytes in bad sectors
xxxxxxxx bytes available on disk
xxxxxx bytes total memory
xxxxxx bytes ·free
Problems with a faulty File
Allocation Table (FAT) can often be
detected through CHKDSK. The
FAT records the position of each file
on a disc and the location of each
piece of information the file contains, and allocates free space so
you can create new files. Rather
than actually recording the contents of each sector, the FAT actually records information in groups of
sectors called clusters.
CHKDSK error
When CHKDSK detects an error,
you will often see messages like:
xx lost Clusters found in xx Chains
or
File x Cross-Linked on Cluster x
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SILICON CHIP
If you specify the IF (Fix) switch
with CHKDSK, you will be asked to
convert lost chains to files. If you
answer Yes, then CHKDSK gives
each fragment a name, starting
with FILE0000.CHK then FILE000l.CHK
and so on. You can then examine
the contents of the files using, for
example, the TYPE co~mand. If you
answer No, the lost cluster pointers
are erased.
Somehow the DOS file structure
of the disc has been damaged. This
typically occurs when a program
manipulates the FAT or directories.
Sometimes a normally well-behaved
program can go berserk. Sometimes
a program terminates before any
open files are closed.
A lost cluster is a unit of storage
with a pointer saying the cluster is
in use by a file but no directory entry knows anything about that
cluster. Since files may be fragmented on a disc, DOS is reporting
a discrepancy in the FAT - the link
to another part of the file has been
lost.
File allocation table
When a disc is formatted, the
FORMAT command creates an empty Root directory to hold the FAT.
Every disc in fact has two FATs, so
DOS can compare the contents of
each. If one copy is damaged, DOS
will use the other. However DOS
can't decide which of the FATS is
more correct than the other. If a
discrepancy is detected, DOS tells
you:
File Allocation Table bad for drive
X
This means the copy in memory
of one of the allocation tables points
to nonexistent blocks. Possibly the
disc has been incorrectly formatted. However, if the problem occurs
on a disc you have been using for
some time, reach for your backup
discs. Programs like Norton Utilities, PC Tools and Mace Utilities
can help rejoin the files created by
CHKDSK.
CHKDSK is OK as a quick way to
sort out simple problems. Software
programs like Norton Utilities and
Mace Utilities enable you to make a
duplicate of the FAT on an unused
area of the disc where it can be
read if the original or backup FAT is
damaged.
Another method of checking for
problems is to use a disc optimisation program. These programs
move all data firstly to the centre of
the disc then progressively back to
the outer edges. The advantages
are two-fold. First, the files are
positioned at the outer edge of the
disc so the read/write heads don't
have to travel so far to locate the
data and second, the files are no
longer fragmented, thus reducing
the possibility of lost chains.
Certainly operating systems have
become smarter when it comes to
allocating disc space, but large files
and discs nearing full capacity still
require allocation, dependent on
Magic Box Shares One PC Between Two Terminals
Does your computer sit idle on your desk for most of
the day? Why not let someone else use it while you are
doing other things? By using this device, it is possible to
let someone else share your machine while you stay at
your desk. Called the "PC-Share Magic Box", it enables
two video monitors and two keyboards to be connected
to any PC XT, AT, PS/2/25 or 30, or PCAT-386.
the available space. It's a good idea
to run disc optimiser programs at
regular intervals to really springclean your disc.
Other FAT errors include the
message:
Probable Non-DOS Disk
This often occurs on a hard disc
when the FAT has been altered. The
implication is that somehow the FAT
has changed and should be given
priority attention. Of concern is the
fact that something (or someone)
has overwritten a part of the disc
normally out of reach. FORMAT and
FDISK traditionally twiddle with this
area of the disc, however you could
have conflict with device drivers or
memory resident software.
The obvious treatment is to reformat the disc. Once again Norton
Utilities, Mace and PC Tools etc can
help you, which is great if you don't
really have lots of time to spare.
You do, however, need to have a
working DOS disc available to boot
In normal use, one user can watch the other's work as it
is produced but if you are sharing an AT-286 or AT-386,
it is possible to blank the unused station 's monitor by
pressing the NUM LOCK key several times.
Not a bad idea, eh? PC-Share is available from Rod Irving Electronics (7 4 Parramatta Road, Stanmore) for
$299 plus the cables .
the computer initially.
The problem is with the first
value of the FAT, the Media
Descriptor Byte. It is normally
defined as F8 for the hard disc and
if CHKDSK detects anything else, the
Non-DOS message eventuates.
Refer to your favourite utility program for instructions on setting the
descriptor right.
FE
160K single sided disc
FC
180K single sided disc
FF
320K double sided disc
FD
360K double sided disc
F8
hard disc
F9
720K, 1.2M or 1.44M disc
F0
other
The power supply
Heading the list of likely computer troublemakers is the power
supply. It uses a switching
regulator to convert the mains
voltage into something the computer can use, usually 5V and 12V
DC rails. The electronic components and circuits of the motherboard, adapter cards and disc
drive logic boards use the 5V
power, while the disc drive motors
and fans use the 12V power.
Naturally, the system needs a
steady supply of both to function
effectively.
At system startup time, the
power supply completes a series of
internal checks to ensure that adequate power levels are achieved
prior to allowing the PC to continue.
When it is satisfied that all is well,
a signal is sent to the motherboard.
If the signal is not sent, the computer will go no further.
Common symptoms of a faulty
power supply are:
(1) power switch on;
(2) fans running;
(3) hard disc running;
(4) not much else happening.
Failure can occur due to overloading, power surges and normal
AUGUST 1990
71
defects brought about by conditions
such as thermal expansion or contraction (read overheating, etc).
The fan in a power supply can also
fail. This problem is quite easy to
recognise - prior to failure, the
fan runs at an increasingly slow
rate and in its death throes, tends
to make strange noises.
Power supply problems can occur when you purchase additional/more powerful disc drives,
8087 chips, hard discs and memory
devices. Remember that power is
needed to run all these devices and
the more devices you add, the more
you overtax a possibly inadequate
power supply.
Poorly engineered power supplies can also run hot, causing the
system to do likewise. Repeated
heating and cooling of electronic
components can lead to system
failure since the hotter the component becomes, the shorter its life
expectancy.
Repeated heating and cooling
causes metal to harden and become
brittle. Since many components are
soldered, the same principle applies. Devices connected via sockets
can also work loose, thus causing
possible corrosion of the sockets
and connector joints.
When a power supply "blows", it
is frequently the result of cracked
or broken solder joints. Keeping the
unit at a constant temperature will
certainly prolong the life of your
system and this is one reason why
you will hear some people say they
leave their computer running constantly. Only you can judge.
When replacing a power supply,
make sure you specify the kind of
PC you are using, as well as the
desired wattage. The shape of the
power supply varies from model to
model (ie PC/XT to AT etc ), together
with the size, screw hole positions,
number and type of connectors and
switch position.
Keep on keying
The keyboard is the most frequently used and most obvious
method of communicating with your
PC and so it is in your best interests
to treat it gently. It is really an intelligent device - in fact, it is a
computer in its own right.
Keyboards also contain built-in
memory to enable them to perform
their own self-test when powered
on. There are now three main types
of keyboards in the compatible
world:
(1) the 83-key PC & XT keyboard;
(2) the 84-key AT keyboard; and
(3) the 101-key enhanced keyboard
(with and without LED panel indicators for CAPS, SCROLL & NUM
LOCK).
Any of the PC or XT type systems
cannot operate the lights because
they do not have the necessary interface for light operation. Thus
you need different keyboards
depending on your computer; the
PC/XT keyboard is the same,
Keep liquids and dust away from keyboards and give the keyboard a vacuum
or brush down at regular intervals. It's also a good idea to use a dustcover to
protect the keyboard when it is not in use.
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SILICON CHIP
although usually switchable as appropriate, while the AT uses the
enhanced keyboard.
Often the ROM BIOS of the PC/XT
may not be able to operate the
enhanced keyboard correctly. This
is typical when the NUM LOCK light
does not come on automatically
when the system unit is turned on.
Problems with the keyboard can
also arise if it is not securely attached to the system unit or if it
is removed while the computer is
switched on. Most computers cope
very well with this but some will
lock out the newly plugged in
keyboard. Make sure you save any
work on the screen if you need to
change the keyboard for any
reason, just in case you have to
reboot.
As with discs, liquids can mean
death for a keyboard. Spilled drinks
running between the keys can
dramatically increase the risk of
short circuiting since water is a
conductor of electricity and you
may need to replace the keyboard.
Turn off the computer ASAP.
However all may not be lost disassemble the keyboard and use
distilled water to wash, yes wash,
the keyboard. All circuit boards are
washed during the manufacturing
process. You need to make sure the
keyboard is perfectly dry before
you attempt to use it again.
Keyboards also suffer from the
mere fact of constant use. Under
seige from a heavy-handed, greasyfingered operator, it is little wonder
that the keys eventually give up the
ghost and start sticking. Also, as
dirt and dust settles between the
keys, they will require greater
pressure from the pinkies to type
the correct letters.
To prevent this problem, vacuum
the keyboard regularly and keep it
covered, particularly if you work in
a dusty environment. Turn the
keyboard upside down and give it a
good shake, especially if there are
foreign objects like pins and paper
clips trapped inside.
If you are feeling adventurous,
carefully remove the keytops and
gently spray with compressed air or
brush around the key connectors.
Take care not to remove too many
keytops at once unless you are completely familiar with the keyboard
layout.
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