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From: aland@infmx.UUCP (Dr. Scump)
Newsgroups: comp.arch,comp.misc
Subject: Re: LoadAll?
Summary: protected mode -> real mode on '286:  can do!
Message-ID: <3015@infmx.UUCP>
Date: 8 Jan 90 08:07:15 GMT
References: <597@unmvax.unm.edu> <25493@cup.portal.com> <25528@cup.portal.com>
Reply-To: aland@infmx.UUCP (alan denney)
Distribution: usa
Organization: INFORMIX Professional Services ("Peace thru Normalization")
Lines: 27

In article <25528@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson) writes:
>What Tim implied, but didn't say explicitly, was that switching to protected
>mode on a 286 is a one-way trapdoor.  Can't get back to real-address mode.
>This is why MS-DOS on the 286 must run in real mode (8086 compatibility mode)
>because it needs to run 8086/8 binaries.

You *can* switch back to real mode on the 286 -- it's possible, though
not trivial.  The basic strategy is to save off the registers, etc.
and then trigger a reset of the '286 via the keyboard controller (which
makes the 286 start up in real mode).  Then, restore the registers,
etc. and away you go!  (over-simplified, but this is the gist of it).
PS/2s have some other method of resetting the 8086 to real mode, but
I'm not sure how it is done.

This is how protected-mode DOS extenders work, e.g. DOS/16M (which
is used in Informix's DOS products and in Lotus 1-2-3 Release 3.0).

As you indicated, the 386 has instructions to switch between each of
the four(?) modes: real, protected (286), native, and virtual 8086.

--
  Alan S. Denney  @  Informix Software, Inc.    "We're homeward bound
       {pyramid|uunet}!infmx!aland               ('tis a damn fine sound!)
 --------------------------------------------    with a good ship, taut & free
  Disclaimer:  These opinions are mine alone.    We don't give a damn, 
  If I am caught or killed, the secretary        when we drink our rum
  will disavow any knowledge of my actions.      with the girls of old Maui."