From: Gabriel Paubert <paubert@iram.es>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Tom Rini <trini@kernel.crashing.org>,
"Randy.Dunlap" <rddunlap@osdl.org>,
randy.dunlap@verizon.net,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] move SWAP option in menu
Date: Fri, 7 Mar 2003 11:03:31 +0100 [thread overview]
Message-ID: <20030307100330.GA4758@iram.es> (raw)
In-Reply-To: <1046984808.18158.115.camel@irongate.swansea.linux.org.uk>
On Thu, Mar 06, 2003 at 09:06:49PM +0000, Alan Cox wrote:
> On Thu, 2003-03-06 at 19:33, Gabriel Paubert wrote:
> > I'd be very surprised if it were possible to have swap on a MMU-less
> > machine (no virtual memory, page faults, etc.). Except for this nitpick,
> > the patch looks fine, but my knowledge of MM is close to zero (and
> > also of the new config language, but I'll have to learn it soon).
>
> You can, and people have had swapping long before virtual memory.
Indeed, I was unclear. A long time ago some OS I used distinguished between
swapping (getting rid of a whole process' address space) and paging. The
former one you can implement on any machine (with restrictions), the second
one needs an MMU and that's what CONFIG_SWAP means AFAICT.
> Most ucLinux platforms can't swap because they can't dynamically relocate code.
I believe that dynamically relocating code is fairly easy (PIC may help too),
but data is not: how do you relocate pointers of a swapped out process when
you swap it in at a different address?
I have fuzzy memories of a system in which you had a pair of
privileged registers (base and limit) which allowed you to implement
swapping and moving programs around in physical memories: all addresses
were checked against the limit and the base was added to perform
physical accesses. I might be wrong: it was about 20 years ago and I've
used so many different systems since then. But there is no such
mechanism on a 68000 for example (you could add it externally) and
it has its own problems (no easy way of sharing library code).
(Yes we're drifting way off-topic.)
> Linux 8086 can swap because it can use CS/DS updates to relocate code/data.
Unless I miss a subtle trick, that's using the segment registers as a
poor man's MMU. You can share library code with far calls but you can't
use "far" data pointers, can you?
> The way it worked on older systems is that you never run a program which
> isnt entirely in memory. With that constraint you know it won't suddenely
> want data you don't have.
Oh yes, I've used such systems a loooong time ago. But I can't remember
the details well enough.
Gabriel.
next prev parent reply other threads:[~2003-03-07 9:58 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-03-05 4:36 [PATCH] move SWAP option in menu Randy.Dunlap
2003-03-05 18:17 ` Gabriel Paubert
2003-03-05 21:14 ` Randy.Dunlap
2003-03-06 18:43 ` Tom Rini
2003-03-06 18:49 ` Christoph Hellwig
2003-03-06 18:54 ` Tom Rini
2003-03-06 18:56 ` Christoph Hellwig
2003-03-06 19:33 ` Gabriel Paubert
2003-03-06 21:06 ` Alan Cox
2003-03-07 10:03 ` Gabriel Paubert [this message]
2003-03-07 12:41 ` Alan Cox
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20030307100330.GA4758@iram.es \
--to=paubert@iram.es \
--cc=alan@lxorguk.ukuu.org.uk \
--cc=linux-kernel@vger.kernel.org \
--cc=randy.dunlap@verizon.net \
--cc=rddunlap@osdl.org \
--cc=trini@kernel.crashing.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).