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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 70 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. 72 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. ~