Fullscreen HTML5Med Res (??MB)HTML4

COMPUTER BITS BY JENNIFER BONNITCHA What to do when things go wrong Most times when you use your computer, things go according to plan and the desired result is achieved. From time to time, however, things go horribly wrong and knowing how to fix the problem is quite an art. You can divide most computer problems neatly into two categories: (1) those causing physical damage, and (2) those which are logical. Physical damage in terms of computer files means that something affects the physical location of the file on a disc, thus preventing a part of the file from being written to or read. Logical damage, on the other hand, is contentrelated and this can mean that a DOS area (for example the File Allocation Table - FAT) or the contents are corrupt. Most computer users have at some time had the experience of a program terminating unexpectedly- for example the Unrecoverable Application a little data loss through to corrupted files. Graceful program termination generally closes any open files, deletes any temporary files created during the session, frees-up allocated memory and restores the original hardware/ software interrupt table values, if altered. In order of importance, the first process - closing open files - is vital. Deleting temporary files is a neat and tidy way to end a session, while releasing memory and restoring the interrupt table are very important unless you decide to turn the computer off or perform a restart. If DOS and your program don't cooperate in the removal of temporary "If DOS and your program don't co-operate in the removal of temporary files, then you most definitely should remove them yourself so that they don't tie up disc space." Error from Windows 3. Programs which terminate in a graceful, orderly manner seldom, if ever, cause problems. Inelegant, awkward terminations can occur when you choose the Abort option from DOS during a critical stage of the program, the system locks up or you lose power or reboot your computer during program execution. You may end up with anything from 40 SILICON CHIP files, then you most definitely should remove them yourself so that they don't tie up disc space. Open files When DOS closes a file, any information or instructions pending for the file are written, the directory time/ date stamp is updated, as is the FAT, and finally the file handles are freed. It is imperative that DOS and the pro- gram cooperate when closing files since when a program closes a file it directs DOS to write all information the program holds in its buffers. DOS, on the other hand, writes all information and updates the directory entry and FAT. If only DOS closes a file, there is the chance information may be "trapped" in the program's buffers. If a program stops before it closes a file, you lose information, the subdirectory information may not be correct and the FAT may not be updated correctly. IfDOS stops during file closing, information may or may not be recorded, the subdirectory entry may not be correct and the FAT may or may not contain the correct chain entries for the file. In combination, disagreement between the file, subdirectory entry and FAT causes logical inconsistency, thus the disc is logically damaged. While this can cause considerable problems, there is hope! Unexpected program termination has one saving grace - you can usually guess how much data may have been lost. Word processing and spreadsheet programs generally lose the most recent revisions while database and accounting programs may have pieces lost. You may find that the file has been corrupted or is littered with garbage. In the case of a database, you may also find that any related files could be affected. The most important question in this instance is: could DOS and the program clean-up prior to termination? While the current information is lost, the system's integrity should be intact. Memory disagreements As mentioned above, when a program terminates in an orderly fashion, memory used by the program is freed for further use. Memory (or RAM) is used both by DOS (and freed automatically) and a program (which it should free on termination). Note that DOS 5 handles memory allocation in a different way. This article describes earlier versions to avoid complication. A future article will describe the differences in DOS 5. When a program starts, DOS , depending on the program type, allocates conventional RAM to the program: COM: first 64K EXE: whatever is established by the linker that creates the EXE file. This information is stored in the file's preamble. Any other memory used by the pro- mer. If the problem was a disc error, the program halts immediately and DOS bypasses the clean-up stage! The Ctrl C/Ctrl Break combination may cause a program to terminate prematurely. DOS will restore any changes to the, interrupt table, however if the program made any modifications, you should restart DOS to reset the interrupt table. This is not a bad idea in any case, just to be sure. How far is too far? Knowing when to turn off your computer or restart DOS is part of the art. Try to make sure all foreground and background programs have terminated, the system is at the DOS prompt and that the disc drives have completed any writing "Unexpected program termination activity. Remember, however, that the has one saving grace - you can system prompt is usually guess how much data may not always a guarantee of completion. have been lost." When you run backgram is requested when necessary, ground programs, including TSRs then freed. Since DOS can't automati- (Terminate and Stay Resident procally free any other memory when a grams), the DOS prompt won't indiprogram ends, memory must be freed cate an idle system. explicitly by the program. Thus, if a Since the computer is processing program stops suddenly, RAM can be in the background, any work taking trapped. Although programmers by place is out of view. The DOS prompt and large are careful to include a is an indication that any foreground "clean up" routine for unexpected work is complete, while the backprogram termination, programs can't ground program could still be chugging away. Don't always trust the disc handle all problems. activity light either, since this is meant Why is_it so? as an indicator rather than the absoYou can put the blame on three lute truth. You may notice the light remains on when audible disc access different occurrences: critical errors, CPU exceptions and the Ctrl C/Ctrl is complete. This is particularly true for floppy Break key combination. Badly behaved programs usui).lly disc drives. When a disc is selected cause the CPU exception, giving help- the drive light is turned on and then ful responses like divide by zero, di- turned off again when deselected. The vide overflow and unrecognised in- difference between light on and acstruction errors. The price you pay is tual activity is between half to one the loss of any un-saved work and the second; likewise when deselected. possibility of inconsistencies between The light may stay on until the motor stops completely. the FAT and directory entries. The hard disc drive, on the other Critical errors are generated by devices such as open disc drive doors hand, spins continuously, so the light and off-line printers. The DOS com- is a better indicator of use. Also, since mand processor, COMMAND.COM, in- the light only goes on during actual tercepts the problem and displays the read and write operations, you may device not ready message together find the inte:i;nal processing is taking with the option to Abort, Retry, Ig- place between read/write operations nore or Fail. Should you decide to and so the light is extinguished. Abort or Fail, DOS returns control to We'll look at more troubleshooting the program; the next reaction de- tips next month and discuss ways of SC pends on the ability of the program- repairing damaged files. GAS LASER INTRODUCTORY KIT 8 :~i~i ~~~~d a"~l~!:f:~ : t~~s~~~i~~i.~7n°:hrs~~~eC?t~ LASER INTRODUCTORY OFFER". The tubes are used but have plenty of life left in them. They are GUARANTEED! You I~~~~:~~:·ann!i7:u~ 0 i~~~;•1i:~\~!.';~:~~hst~~~~~ at about 3mW, and one of our reliable and efficient 12V UNIVERSAL LASER POWER SUPPLY KITS. A tube and the supply will easily slide into inexpensive 50mm SWV pipe, which makes for a compact self powered laser head. Casing is not supplied. Several of our customers reported that AT NIGHT, THE BEAM FROM THE SIEMENS WAS VISIBLE UP TO 4km AWAYI The kit comes with full instructions and it even includes a prewound transformer. The unlimited applications include laser pointers, surveying, optical experiments, medical field, holography, and fascinatinR laser dis~lays. Wr, are still offering this "TRIO $149 ~.ft~1~i~1~ 6~:R PICTURE) PASSIVE INFRARED SENSORS Experimenters delight! A brand new SmW 780nM• 820nM infra red laser diodes supplied with a collimating lens, heatsink, laser diode data, a suitable driver circuit and the components for this circuit. Visible in low light. Excellent for IR sights, data links, high quality audio links, ~~~t~~ro~~~~.eter s59 What are the expensive parts in a Passive element movement detector, as per ~u~iiiaJu:r:1eme:t htY~ sensor, plus a fresnet lens plus a white filter. We can supply these three components for: 5 ONLY or 10 for $80 9,90 For all the abovemen• tloned components! We can supply the same kit with a used, tested laser diode for $35. Limited stock. DECIMAL KEY PADS Positive snap action switches on a self adhesive backing. No connectors provided. Limited quantity. BIGGER LASER TUBES For the more serious applications we have some brand new, large 4mW and l0mW tubes. Both the tubes are of exceptional quality and would normally cost about three times this price. Call for a special deal for a tube and supply s155 10mW ONLY $386 co~~~ag~•tv 5 1,30 EA. or 10 for $10 LCD DISPLAY MODULES ~ ~~~ul~~~i\t s~~.':,. ~~~::l!~ !1~~!~u::ricb[~~di~~:y control circuitry already mounted underneath the PCB. Both the units have provision for backlighting, and only require a few milliamperes at SV to operate. We include comprehensive information. AT A SMALL FRACTION OF THEIR REAL VALUE. ~~t~hb'::~~lt:h~~: line module (16 X 2), s22 .50 s35 !1u;'~'::t~~~~~g2 (~~exd~flay module, We will atso have other LCD modules available, at similar low prices. A backlighting inverter kit is also available for $8. Melbourne Distributor: Electronics World (03) 723 3860 or (03) 723 3094 Closed from 24/12/91 till 14/ 1/ 92 OATLEY ELECTRONICS PO BOX 89, OATLEY , NSW 2223 Telephone: (02) 579 4985 Fax : (02) 570 7910 Certified p &p : S6 in Au st. NZ (Airmail) : S10 JANUARY 1992 41