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From: ralf@b.gp.cs.cmu.edu (Ralf Brown)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Switching from Protected to Real Mode
Message-ID: <86@b.gp.cs.cmu.edu>
Date: Mon, 24-Aug-87 21:22:28 EDT
Article-I.D.: b.86
Posted: Mon Aug 24 21:22:28 1987
Date-Received: Wed, 26-Aug-87 00:44:54 EDT
References: <1387@imagen.UUCP> <3813@well.UUCP>
Organization: Carnegie-Mellon University, CS/RI
Lines: 25
Keywords: 286

In article <3813@well.UUCP> johnl@well.UUCP (John A. Limpert) writes:
>From what I have heard, they load the IDTR (interrupt descriptor
>table register) with 0 and execute an INT 3 instruction.  This causes
>multiple faults and drops the 80286 back into real mode.  I am not
>sure what state this leaves you concerning segments and the prefetch
>queue.  I picked this info up from someone who had gone to one of
>the OS/2 seminars.

The way I understand it, if another fault occurs after the 286 starts 
processing a double fault (actually any multiple faults occurring on the same
instruction), the 286 goes into the shutdown state.  The AT has hardware to
issue a reset when the 286 indicates it has entered shutdown.  Could this be
why the above works?  If so, it wouldn't have any advantage over simply
toggling the reset line through a MOV AL,0FEh/OUT 64h,AL sequence (and of
course you would still have to set the magic flag in CMOS RAM).

[the final fault is of course the missing interrupt descriptor table when
the 286 tries to invoke the double fault exception handler]

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