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Date:      Wed, 23 Oct 1996 08:59:25 +0200
From:      Nadav Eiron <nadav@barcode.co.il>
To:        johnson@charming.nrtc.northrop.com
Cc:        questions@FreeBSD.ORG
Subject:   Re: Virtual memory question:  map first page?
Message-ID:  <326DC24D.5EAA@barcode.co.il>
References:  <9610230515.AA02752@charming.nrtc.northrop.com>

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johnson@charming.nrtc.northrop.com wrote:
> 
> I noticed that in imgact_aout.c the text segment gets mapped starting at
> linear address 4096.
> 
> Is it possible to map the first page of linear memory?  What I would like
> to be able to do is set up a process in which linear addresses starting
> at 0 are mapped.

Most OS's don't map the first page of memory. The reason is that many
bugs are caught that way. If you dereference a NULL pointer you'll find
yourself  in the first page of memory. Not mapping that would mean that
you'll get a signal for that. Why would you care so much for the first
page of virtual address space?

> 
> When I modified imgact_aout.c to map the first linear page of memory as
> an experiment, and then ran a specially cooked executable to exercise that
> code, the machine immediately hung.

Don't know, but it's not a good idea.

> 
> Is there something special on the first linear page of a user process?
> 
> Thanks,
> 
> Greg Johnson
> johnson@nrtc.northrop.com

Nadav



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