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new HACKWRAP fix for MS-DOS7+, aka smashing the bug (Miscellaneous)

posted by geoffchappell(R), 14.12.2009, 10:41

> - I pick an unused bit in the SAME byte for a new flag which HACKWRAP, or
> dare I say FIXDOS ;=) will set during of Config.sys.

There's a luxury that comes with knowing that there will be no more DOS versions from Microsoft. It would not have occurred to me to define a new bit: I'm still thinking as I was back in 1996!

> An other instance of HACKWRAP, or a companion program will later unset
> the flag, restoring the usual DOS working (an oxymoron?)

To me, this is fundamentally unsound. What if your other instance doesn't load? You disturb the system without ensuring that you will un-disturb it.

> So, what FIXDOS/HACKWRAP does is we'll FLIP one bit in that instruction,
> in memory, that will change it to TESTing for both OUR new bit and THEIRS
> and the following JNZ will take the proper action in all cases, skipping
> over the bookkeeping whenever it is unwanted !

So, you will find the TEST instruction and patch it to recognise your new bit, and then also set your new bit before WRAPPER.SYS and clear it after. Why define a new bit to set and clear? Since you still must patch the TEST instruction, why not just patch the JNZ that follows it (and patch it back later)?

 

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