Some senseless babbling from Ivan Todoroski to Mike Ruskai
on 02 Dec 98  03:58:00 about Guess who's back... ;)...

 IT> On Monday, 30 November 1998,
 IT> 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.

 IT> I have an AMD 486, so there you have it! :) I didn't know this part,
 IT> 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.

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

WinNT virtualizes interrupts in software.  That means such a "program" 
won't lock it up.  That also means that any program which is 
interrupt-intensive will crawl, to put it mildly.  People that have 
attempted to run Doom or the DOS version of Quake in NT will know what I'm 
talking about.

Win95 doesn't do either.  It will lock up on any processor, unless there's 
a watchdog timer.
 
 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.

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

 IT> I wonder if dosemu on Linux is vulnerable to this?

I couldn't say, since I haven't gotten around to installing it yet.  It's 
on my when-I'm-really-bored list.  

Something to consider is that to get the documentation on these 
instructions from Intel requires signing an NDA.
 
 MR> Any system with a watchdog timer (with any CPU) will also not be locked
 MR> up.

 IT> What exactly is a watchdog timer?

It's a little timer that's independant of the CPU, that issues a 
non-maskable interrupt if normal interrupts have been disabled for a set 
period of time.  Some IBM PS/2 machines have it, as part of the MCA bus.  
Not sure what other machines have it.

Mike Ruskai
thanny{at}home.com


... Had to give away the kids - The cat got allergic.
--- Renegade v05-11 Exp