Critical errors like an open disk drive or adisconnected printer result in the DOS message "Abort, Retry,Ignore, Fail?" without saving the current display, thusruining the application screen.
Turbo Pascal 4.0 allows you to write procedures whichare labelled INTERRUPT which automatically save all theregisters and restore them when returning. In addition, youcan modify the registers within the procedure, and thosechanges are passed back to DOS. No more need for routineswritten in assembler !
I have used this new ability of TP4 to write a criticalerror handler unit. When it is incorporated in theapplication it will save the screen, ask for a response, passthe response back to DOS, and restore the application'sscreen intact.
Beginning with DOS 3.0 the DOS technical reference saysyou should not code to correct specific errors, even thoughyou can present a message which is quite specific. Instead,you are told to follow the suggested action indicated by DOSthrough INT 59h, GetExtError. This is now the preferredmethod of error trapping according to the Blue Bible. Theregisters will also let you know the location of the error,generally, and the type of error, generally, as shown below.
Other critical error handlers in the public domain don'tuse this format probably because it is only useful for thosewho use DOS 3.x. This program checks the DOS version and ifit is < 3.x finds the DOS error number by looking at lo(DI)which contains the old DOS critical error number and thenconverts it to the new extended error numbers by adding 19 toit. It then extracts what other information it can from theregisters. You can then use the error message pointed to bythe resulting number, and the other information, but youdon't get a suggested correction, you may get unexpectedanswers in the future, and you transgress.
See the program listing for the specific codes and theirmeanings.
Other methods are available for dealing with errors aswell. They are included in the unit.
You can disable IO checking and call IOResult, thenhandle the error in a more specific way than just allowingabort retry ignore fail. If you use the INT24 unit from theEditor Toolbox you can trap critical errors this way as well,and take specific action to correct the situation.
Finally, you should have an exitprocedure to clean upand to catch errors you otherwise wouldn't expect, likeheap space errors. That's here too.▄j ▄î
Printer Errors are another story. Although the printerif off will trigger INT24h, if it is only off line or out ofpaper, no trap takes place. I've included a check for theprinter status in the code using INT 17h which may be useful. Be careful though, because NONE of these traps an error withmy HP ThinkJet. Most other printers I've tried respond asexpected.
Another value of the program is the use of the heap tostore the error messages. Although TP4 allows multiple codesegments so that code can exceed 64K, it still only allowsone data segment, and so only 64K of data. In addition,typed constants have been moved to the data segment as well,so that older trick of hiding them in the code segment nolonger works. To overcome this limitation it is necessary touse the heap to store data. That is why I chose to use thepointer structure to hold the long list of error messages.
