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Path: utzoo!mnetor!uunet!husc6!mit-eddie!uw-beaver!tektronix!sequent!treat
From: treat@sequent.UUCP (Scott Tetrick)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Switching from Protected to Real Mode
Message-ID: <2850@sequent.UUCP>
Date: Tue, 25-Aug-87 10:55:28 EDT
Article-I.D.: sequent.2850
Posted: Tue Aug 25 10:55:28 1987
Date-Received: Fri, 28-Aug-87 02:09:25 EDT
References: <1387@imagen.UUCP> <3813@well.UUCP>
Organization: Sequent Computer Systems, Beaverton, OR
Lines: 10
Keywords: 286
Summary: Getting from Protected Mode to Real mode on an 80286

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. 

It is important to note that it is NOT the INT 3 that returns the 80286
to real mode, but the hardware of the AT.  Whenever a double fault occurs,
the 80286 performs a SHUTDOWN bus cycle.  This is detected by the hardware
of the AT, and generates a RESET to the processor.  Memory contents are
still valid from before the reset.  The prefetch queue is flushed on a reset.