Newsgroups: comp.os.minix
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From: aswNS@hamp.hampshire.edu (Albert S Woodhull)
Subject: Re: Install Disks
Message-ID: <b147cb$173b2d.1d2@news.hampshire.edu>
Date: Tue, 21 Nov 1995 04:59:45 GMT
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Organization: Hampshire College, Amherst MA
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Re: "shadowing" in Minix:

Someone (I think the attributions got a little mixed up in this
thread) wrote:

: >  I have no idea why this interferes with Minix, but I DO know what ram 
: >shadowing is. RAM shadowing is a process on the 386 and above where a 
: >section of slower ROM is copied into a section of faster RAM and then the 
: >RAM is set "read only" (by a magical feat of V86 mode) and the ROM is 
... etc.

On Mon, 20 Nov 1995 22:09:03 GMT Sanjeev Sawni (ssawni@inforamp.net)
wrote in comp.os.minix:

: Here's something from the FAQ about shadowing -- the term as used
: under Minix:
: 17.  HOW IS THE FORK(2) SYSTEM  CALL  IMPLEMENTED  ON  THE  68K
: PROCESSORS WITHOUT A MEMORY MANAGEMENT UNIT?
:      This is done by *shadowing*. This is copying the memory around
: as long as  both  child  and parent processes do exist.  Although this
...

I think there is some confusion here. The second definition refers
only to the way fork was done on some Motorola CPUs. This is not used 
on any of the other implementations of Minix, and especially not on 
the implementations on Intel CPUs. The first definition partially
quoted above is almost surely what anyone using an Intel CPU system
means. 

There are comments on shadowing and #ifdef SHADOWINGs scattered
throughout the current version of the Minix source. They are best
ignored, since they all refer to the Motorola technique, but they are
vestiges of ports of earlier versions of Minix to Motorola platforms
and generally have nothing to do with Minix 1.7, which hasn't been
ported to any non-Intel platforms yet. The current plan is to clean 
all these vestiges out of the Minix source.

The kind of shadowing that *is* relevant to Intel systems is *not*
mentioned anywhere in the Minix source code, because it is enabled (or
not) by the ROM BIOS before Minix is started, and (as Kees explained
to me) Minix doesn't care about it.

If you weren't confused before I wrote this you may be now. 

--
Albert S. Woodhull, Hampshire College, Amherst, MA
awoodhull@hamp.hampshire.edu
woodhull@shaysnet.com
413-549-2962

