4DOS Critical Error Handler Version 2.00 Revision B WHAT IS IN THIS FILE ? This file includes a quick overview over 4DOS24H2 and a quick instal- lation manual. It also includes an overview of the files included in the archive, some technical information together with bugfixes and changes in this version/revision. You are encouraged to read all of this file before installing 4DOS24H2.COM - it will take a few minutes. If you face problems you can turn to the section dealing with trouble shooting in the manual. WHAT IS IT ? 4D24H20B.ZIP contains a new critical error handler designed to replace the one present in the otherwise excellent 4DOS command interpreter from JP Software. (The normal critical error handler is the piece of code printing the notorious message 'Abort, Retry, Ignore ...' when you have forgotten to close the drive door to the diskette drive - it also called an interrupt 24h handler). It is inspired by 4DOS24H from Patrick Philippot but completely rewritten and optimized in almost any possible way. WHY USE IT ? 4DOS24H2 writes much more comprehensive information in a pop-up window and accepts more flexible user input than does the original handler. It saves and restores the user screen and is able to utilize enhanced text modes, e.g. screen mode 22h (44 rows by 132 columns) on Tseng based SuperVGA cards. It tries to detect graphics modes and acts by calling the original 4DOS handler which will activate the speaker. Contrary to other external 4DOS critical error handlers (that I am aware of) this one is also operational during shells. QUICK INSTALLATION Extract the file 4DOS24H2.COM from 4D24H20B.ZIP to your 4DOS directory and put the following line in your existing 4START.BTM file (or create one if you have not already got one): [d:][path]4DOS24H2.COM were 'd:' is your boot drive and 'path' is the path to the directory were you keep 4DOS24H2.COM. If you are running an appropriate memory manager or DOS version 5.x or later you can also load the handler high by using the proper command (for DOS 5.x it would be LOADHIGH). Reboot your computer to make the handler take effect. WHAT'S IN THE 4D24H20B.ZIP ARCHIVE ? The archive includes (besides this README file) a documentation file, two assembled versions of the handler and the source code. The two assembled versions are identical except for the number of columns in their pop-windows -- 4DOS24H1.COM uses 40 columns while 4DOS24H2.COM uses 41. The last one is generally the nicest but cannot handle the old 40 column screen modes, so if you use a 40 column screen mode regularly you should consider using 4DOS24H1.COM. TECHNICAL DETAILS The handler is written in assembler for maximum efficiency (source code is included) and uses only 2448 bytes of memory (4DOS24H2.COM). It can be loaded high using 4DOS 4.0 or later and an appropriate memory manager (e.g. DOS 5.0 or QEMM). CHANGES IN REVISION B FROM INITIAL RELEASE A BUG-FIX If you switched on the printer echo facility by pressing Ctrl-P and your printer was off-line you would inevitable end up in a deadlock when pressing the first time. If you had a printer you would have to switch it on-line to return from the handler. If you had no printer at all the only salvation was to press Ctrl-Alt-Del. This was not a special feature of 4DOS24H2 as the built-in handler in 4DOS behaves the same way (at least in version 4.01). In this revision you can press Ctrl-P to switch off echoing to the printer before pressing 'a' to abort the operation. As a minor curiosity you will have to press 'a' twice to abort. This is not a bug in 4DOS24H2 but due to the way the printer echoing facility is implemented. A COMMAND LINE SWITCH In this revision a command line switch has been added for debugging purposes. If you append '-M', '/M' or just 'M' the handler will print the load address and the exact memory usage when going resident. The gross memory usage is a bit higher than stated by the handler because DOS allocates memory in 16 byte chunks. A bit slack must therefore be expected.