Re: Page sizes

From	Andrew Smallshaw <andrews@sdf.org>
Newsgroups	comp.arch.embedded
Subject	Re: Page sizes
Date	2020-07-04 06:11 +0000
Organization	A noiseless patient Spider
Message-ID	<slrn3fn4rg07d7.rts.andrews@sdf.org> (permalink)
References	<rd6iuc$fkg$1@dont-email.me>

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On 2020-06-27, Don Y <blockedofcourse@foo.invalid> wrote:
> Are there any processors/PMMUs for which the following would be true (nonzero)?
>
> (pagesize - 1) & pagesize

I'm a little late into this discussion but the first points that
came to mind don't seem to have been addressed elsewhere in the
thread.  My first thought was of systems with software TLB replacement
- ultimately the system developer has greater lattitude in how the
pages get laid out but then I realised that can only be at the
hierarchy level, i.e. so many pages of pages, so many pages of
pages of pages etc.  The fundamental size of a page has to be as
defined by the hardware (either one or a set of options) to determine
which bits get translated.  

That itself hints that the expression must be a contradiction, if
page sizes where not 2^n then the low order bits would not remain
constant in translation and you wouldn't be able to identify a page
simply be looking at the high order bits - you'd need to introduce
comparisons, additions and subtractions.  I suppose it could be
done but you'd wasting an awful lot of silicon and a lot of time
on each access for no clear reason, so in the absence of an actual
example I would discount such an architecture as purely theoretical.

On the other hand, be wary of the implicit assumption.  What is
being described?  This is C-style syntax that implies sizes are
being considered in units of char.  The above considers things at
the word level.  If the word length is not 2^n I would say all bets
are off.

-- 
Andrew Smallshaw
andrews@sdf.org

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Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-26 21:45 -0700
  Re: Page sizes Bernd Linsel <bl1@gmx.com> - 2020-06-27 08:33 +0200
    Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-27 00:32 -0700
      Re: Page sizes Bernd Linsel <bl1@gmx.com> - 2020-06-27 12:21 +0200
        Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-27 15:12 -0700
        Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-06-27 18:14 -0400
      Re: Page sizes David Brown <david.brown@hesbynett.no> - 2020-06-27 17:21 +0200
        Re: Page sizes Clifford Heath <no.spam@please.net> - 2020-06-28 15:30 +1000
      Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-06-27 18:02 -0400
  Re: Page sizes upsidedown@downunder.com - 2020-06-27 11:11 +0300
  Re: Page sizes John Speth <johnspeth@yahoo.com> - 2020-06-27 07:59 -0700
  Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-06-27 17:37 -0400
    Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-27 15:10 -0700
      Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-06-27 18:35 -0400
        Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-27 16:36 -0700
          Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-06-28 01:01 -0400
            Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-27 23:50 -0700
              Re: Page sizes upsidedown@downunder.com - 2020-06-28 13:27 +0300
                Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-28 11:49 -0700
              Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-06-30 00:05 -0400
                Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-29 23:24 -0700
                Re: Page sizes upsidedown@downunder.com - 2020-06-30 11:00 +0300
                Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-30 12:23 -0700
                Re: Page sizes upsidedown@downunder.com - 2020-07-01 13:31 +0300
                Re: Page sizes Dimiter_Popoff <dp@tgi-sci.com> - 2020-07-01 19:44 +0300
                Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-06-30 05:45 -0400
                Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-30 04:13 -0700
                Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-06-30 12:49 -0400
                Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-30 12:44 -0700
                Re: Page sizes George Neuner <gneuner2@comcast.net> - 2020-07-02 03:14 -0400
        Re: Page sizes upsidedown@downunder.com - 2020-06-28 09:24 +0300
          Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-28 00:11 -0700
            Re: Page sizes upsidedown@downunder.com - 2020-06-28 15:59 +0300
              Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-28 12:04 -0700
                Re: Page sizes upsidedown@downunder.com - 2020-06-30 06:43 +0300
                Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-29 23:47 -0700
          Re: Page sizes Rick C <gnuarm.deletethisbit@gmail.com> - 2020-06-28 01:18 -0700
            Re: Page sizes upsidedown@downunder.com - 2020-06-28 16:29 +0300
              Re: Page sizes Don Y <blockedofcourse@foo.invalid> - 2020-06-28 12:18 -0700
  Re: Page sizes Richard Damon <Richard@Damon-Family.org> - 2020-06-27 18:27 -0400
  Re: Page sizes Andrew Smallshaw <andrews@sdf.org> - 2020-07-04 06:11 +0000

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