Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!hubcap!ncrcae!ncr-sd!crash!pnet01!jca
From: jca@pnet01.cts.com (John C. Archambeau)
Newsgroups: comp.os.minix
Subject: Re: The future of MINIX
Message-ID: <4593@crash.cts.com>
Date: 10 Jul 89 04:36:12 GMT
Sender: news@crash.cts.com
Organization: People-Net [pnet01], El Cajon CA
Lines: 21

Might as well post this to everybody since this question will most likely be
asked again.  Yes, the Evans' 286 protect mode kernel should run on a 386 or
386SX machine.  Intel designed the 80x86 to be downward compatable.  So if you
have a 386 or 386SX you should be able to run 8086 and 80286 code with NO
MODIFICATIONS whatsoever.  Everything is there in the chip.  The reason that
this works is that the unused bits in the 24-bit descriptor table are set to
zero, so when the 386 runs into a descriptor table with the high byte set to
zero, it's clicking in the back of its mind "ah ha!  this person wants me to
emulate a brain damaged 286" and does so appropriately.  I might be wrong of
where in the descriptor table the bits are set to zero, but I know all unused
bits are set to zero.  See "The 80386/80387 Architecture" for details on 286
emulation on a 386.  One of the authors is Issacson who is also the author of
the A86 assembler.

 /*--------------------------------------------------------------------------*
  * Flames: /dev/null (on my Minix partition)
  *--------------------------------------------------------------------------*
  * ARPA  : crash!pnet01!jca@nosc.mil
  * INET  : jca@pnet01.cts.com
  * UUCP  : {nosc ucsd hplabs!hd-sdd}!crash!pnet01!jca
  *--------------------------------------------------------------------------*/