Path: utzoo!utgpu!watmath!clyde!att!pacbell!hoptoad!peora!rtmvax!bilver!wbeebe
From: wbeebe@bilver.UUCP (bill beebe)
Newsgroups: comp.os.minix
Subject: Re: MINIX for the 386
Message-ID: <341@bilver.UUCP>
Date: 23 Dec 88 04:39:39 GMT
References: <1836@ast.cs.vu.nl>
Reply-To: wbeebe@bilver.UUCP (bill beebe)
Organization: W. J. Vermillion, Winter Park, FL
Lines: 28

In article <1836@ast.cs.vu.nl> ast@cs.vu.nl (Andy Tanenbaum) writes:
>
>The subject of the 386 has come up several times recently...
>... I would forget 16 of the 386's 48 address bits
>and just concentrate on making a virtual memory system that uses a single
>32-bit segment.  In effect, this simulates the 68020, and should make porting
>to 68020 (68030) systems straightforward.

The mode your talking about is called Flat, or Motorola, mode. In that mode
the base address in the descriptor is set to zero, and the limit is set
to all binary ones. The granularity bit is set so that the limit is in
4K byte pages, not single bytes. What this buys is the creation of only
one set of descriptors that are loaded when the kernel first boots, and
remain untouched after that. With just a *little* work, the descriptor for
the stack can be segragated from the rest of the data and code descriptors
so that some protection from stack over-run (and subsequent code/data
corruption) can be enforced in the hardware. There are a few examples on
how to set the 80386 family up in flat mode.

>The 286 has such an awful architecture that I am not inclined to cater to it
>at all, despite the fact that my own machine is a 286 and our department has
>a room full of them.

I agree. The 80386DX and 80386SX provide solutions to the problems that
drive mad men crazy when they try to program 80286 PM.


I might also remind you that the 80486 is Real Close Now :-).