On Monday, 30 November 1998,
     MIKE RUSKAI wrote to IVAN TODOROSKI about Guess who's back... ;)

 MR> [snip]
 IT>> And here is a great way to TOTALLY and IRREVOCABLY freeze your
 IT>> OS/2 system. Mouse will stop, and not even Ctrl-Alt-Del or
 IT>> Ctrl-Alt-NumLck-NumLck will work!

 IT>> Take any DOS assembler (even DEBUG will do) and create a simple
 IT>> program (.COM or .EXE, no matter) with this piece of code in it:

 IT>> cli
 IT>> x:
 IT>> jmp x

 IT>> Execute this in a DOS window.

 IT>> Hope you closed all the applications first... ;)

 MR> That will only work on early-step 486's from Intel, and non-Intel
 MR> processors.

  I have an AMD 486, so there you have it! :) I didn't know this part, I
  thought it worked on all x86 processors.

 MR> All late-step 486 and higher processors for Intel have instructions to
 MR> allow fast interrupt virtualization, which OS/2 takes advantage of.  The
 MR> DOS window can be closed easily.

      Yeah? Cool! And I thought this was a flaw in OS/2... wonder if
      Win95 or WinNT uses this virtualization... will try this on some
      of my friends! :)

 MR> Of course, this has absolutely nothing at all to do with priority.
 MR>  The "cli" instruction clears the interrupt flag (i.e. disables
 MR> interrupts), and the rest is just a loop.

    Quite right. I didn't mention this in the context of my discussion
    about priorities, it was just an amusing side-note.

   I wonder if dosemu on Linux is vulnerable to this?

 MR> Any system with a watchdog timer (with any CPU) will also not be locked
 MR> up.

   What exactly is a watchdog timer?

                                                            - Ivan -

.!. If we KNEW what we were doing it wouldn't be RESEARCH!
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