Indirect Bit

Message-ID	<4F84FB1E.6CD1FC88@sonic.net> (permalink)
Date	2012-04-10 19:31 -0800
From	Mark Thorson <nospam@sonic.net>
Organization	none
Newsgroups	comp.arch
Subject	Indirect Bit

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The PDP-10 and Nova had an indirect bit, in which a pointer
had both an address and a bit to indicate whether the object
being addressed was data or another pointer.  On the PDP-10,
the pointer was word length (36 bits) but addresses were only
18 bits, so there was plenty of room for the indirect bit,
four register select bits (used for a base register), and
a bunch of unused bits.  On the Nova which was 16 bits,
bit 0 was used, which took away the ability to address odd
bytes (or it could have been used to double the address
space, which would have meant a lot on the Nova).

These machines had the peculiar feature that an instruction
could execute for an indefinite number of cycles until the
last pointer was found (indirect bit = 0).  This allowed the
CPU to run through linked lists without wasting instruction
bandwidth and registers reading in the pointers so they can
address memory.

Now, with 64-bit architectures, there's a bunch of address
bits nobody needs.  How about taking A63 and using it as
an indirect bit?  That could extend the x86 architecture
in a way that's software-compatible with the existing
architecture.

This gets really interesting if it's handled in the memory
controller.  When the CPU issues an address with A63 = 1,
it falls through to the memory controller, which then reads
the pointer from memory (or cache), checks the state of A63
on that pointer, and either fetches the data to return to
the CPU or fetches another pointer.  Of course, the pointers
must contain virtual addresses, so the MC will have to have
its own MMU and TLB.

This offloads all of the messing around with pointers from
the CPU bus, registers, cache, etc.  If you do a lot of
linked lists, this is an enormous win.

What if you don't use linked lists?  Lots of APIs are
accessed through data structures for which the application
has a handle (pointer to a pointer).  For example, a
communication channel might be controlled through a handle
to a data structure that holds the channel's control and
status bits and anything else it needs, like a pointer
to a callback function.  Driver libraries and layered
communication protocols frequently use data structures
that collect together all of the API stuff for a device
or service.

This form of access differs from the linked list in that
you are only going two levels deep and you are applying an
offset to the second level.  That is, the object you want
to access probably isn't the zeroth item in the data
structure, so you need to apply an offset from the pointer.
How does software specify this offset?

Unless you're doing something stupid that wastes tons of
address space, you don't need more than about 40 virtual
address bits.  In the future, no von Neumann architecture
machine will ever need more than 52 address bits for a user
application.  If A51:A0 is the linear address, A52 is the
user/kernel bit, and A63 is the indirect bit, that leaves
10 bits available to use for the offset into the data
structure.  That's enough to address a 1K data structure
with byte granularity.  All of the API data structures
I've seen are much smaller than that, but if 64-bit word
granularity is acceptable, the offset can be shifted by
three bit positions to address 8K.

These two forms of access can be served by one mechanism.
The VA comes out of the CPU, then the MC cycles through one
or more pointers and applies the offset (A62:A53) to the
last pointer.  Even linked lists can benefit from the
offset, because the data in the referenced item probably
resides at an offset from the pointer to that item.

So why isn't this already being implemented?  It relieves
the biggest bottlenecks in the system -- CPU cache space,
CPU instruction bandwidth, and CPU-cache bus bandwidth.
Maybe it doesn't simulate well on typical applications,
but typical applications aren't available that take
advantage of it.  You'd need to modify the compiler and
recompile them to use the indirect bit and offset.

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Thread

Indirect Bit Mark Thorson <nospam@sonic.net> - 2012-04-10 19:31 -0800
  Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-11 01:29 -0500
    Re: Indirect Bit sarr.blumson@alum.dartmouth.org - 2012-04-11 14:48 +0000
      Re: Indirect Bit mac <acolvin@efunct.com> - 2012-04-12 14:43 +0000
    Re: Indirect Bit Morten Reistad <first@last.name> - 2012-04-22 15:19 +0200
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-22 23:24 -0700
  Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-11 08:32 +0200
    Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-11 05:49 -0700
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:22 -0700
        Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-13 08:14 -0700
          Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-13 16:13 -0700
            Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-13 20:01 -0700
              Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-14 03:16 +0000
                Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-14 08:14 -0700
        Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-13 16:05 -0700
          Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-13 17:01 -0700
            Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-14 13:49 -0700
          Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-13 20:07 -0400
          Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-14 18:47 +0200
          Re: Indirect Bit Morten Reistad <first@last.name> - 2012-04-22 15:26 +0200
            Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-22 23:25 -0700
              Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-23 09:18 +0100
                Re: Indirect Bit EricP <ThatWouldBeTelling@thevillage.com> - 2012-04-23 19:35 -0400
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 08:37 +0100
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 09:42 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-24 01:58 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 11:26 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-24 05:02 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 13:58 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-24 10:07 -0700
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-24 04:30 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-24 13:01 +0100
  Re: Indirect Bit Stefan Monnier <monnier@iro.umontreal.ca> - 2012-04-11 10:03 -0400
  Re: Indirect Bit kenney@cix.compulink.co.uk - 2012-04-11 09:22 -0500
    Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-12 14:11 -0700
      Re: Indirect Bit Stefan Monnier <monnier@iro.umontreal.ca> - 2012-04-12 18:05 -0400
        Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 20:13 -0700
      Re: Indirect Bit Mark Thorson <nospam@sonic.net> - 2012-04-12 16:55 -0800
        Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-12 17:25 -0700
          Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-12 20:03 -0500
          Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:33 -0700
            Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-13 15:57 -0700
              Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-13 17:03 -0700
        Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 20:14 -0700
  Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-11 10:35 -0400
  Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-11 08:54 -0700
  Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 07:55 -0700
    Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-12 10:16 -0700
      Re: Indirect Bit jacko <jackokring@gmail.com> - 2012-04-12 11:07 -0700
        Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:34 -0700
      Re: Indirect Bit Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-04-12 11:35 -0700
        Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:41 -0700
          Re: Indirect Bit jacko <jackokring@gmail.com> - 2012-04-13 14:52 -0700
          Re: Indirect Bit Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-04-15 09:24 -0700
            Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-15 22:45 +0200
              Re: Indirect Bit Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-04-15 18:04 -0700
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-12 21:12 -0700
    Re: Indirect Bit timcaffrey@aol.com (Tim McCaffrey) - 2012-04-12 19:23 +0000
    Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-13 08:54 +0200
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-14 14:37 -0700
    Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-15 23:42 +0000
      Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-16 08:08 -0700
        Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-16 23:38 +0000
          Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-16 21:29 -0700
      Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-16 21:25 -0700
  Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-12 14:05 -0700
    Re: Indirect Bit ChrisQ <blackhole@devnull.com> - 2012-04-14 10:12 +0000
      Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-14 06:25 -0700
        Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-14 14:50 +0100
          Re: Indirect Bit ChrisQ <blackhole@devnull.com> - 2012-04-14 18:01 +0000
            Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-14 14:00 -0700
              Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-14 20:58 -0400
            Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-16 09:57 -0700
            Re: Indirect Bit timcaffrey@aol.com (Tim McCaffrey) - 2012-04-20 23:19 +0000
          Re: Indirect Bit "Paul A. Clayton" <paaronclayton@gmail.com> - 2012-04-16 08:50 -0700
            Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-16 18:09 +0100
              Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-16 23:19 +0000
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-17 09:20 +0100
                Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-17 12:49 +0200
                Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-17 12:40 +0000
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-18 12:01 +0100
                Re: Indirect Bit anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-04-17 12:34 +0000
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-17 09:10 -0700
                Re: Indirect Bit Andrew Reilly <areilly.---@bigpond.net.au> - 2012-04-17 22:05 +0000
                Re: Indirect Bit jgk@panix.com (Joe keane) - 2012-04-19 19:37 +0000
                Re: Indirect Bit torbenm@diku.dk (Torben Ægidius Mogensen) - 2012-04-20 11:10 +0200
                Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-20 11:24 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-20 15:01 +0100
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-20 15:03 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-20 08:01 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-20 16:35 +0100
                Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-18 11:10 -0700
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-18 15:37 -0700
          Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-16 11:21 -0700
            Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-16 19:36 +0100
              Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-18 11:17 -0700
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-18 15:33 -0700
                Re: Indirect Bit Andreas Eder <andreas_eder@gmx.net> - 2012-04-19 00:29 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-19 10:50 +0100
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-19 11:29 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-19 21:39 +0100
                Re: Indirect Bit EricP <ThatWouldBeTelling@thevillage.com> - 2012-04-20 15:14 -0400
                Re: Indirect Bit "Andy (Super) Glew" <andy@SPAM.comp-arch.net> - 2012-04-20 22:43 -0700
                Re: Indirect Bit EricP <ThatWouldBeTelling@thevillage.com> - 2012-04-21 17:14 -0400
                Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-21 19:47 -0500
                Re: Indirect Bit Nomen Nescio <nobody@dizum.com> - 2012-04-22 10:45 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-22 09:57 +0100
                Re: Indirect Bit Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-22 12:54 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-22 13:00 +0100
                Re: Indirect Bit Nomen Nescio <nobody@dizum.com> - 2012-04-22 13:28 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-22 13:04 +0100
                Re: Indirect Bit Fritz Wuehler <fritz@spamexpire-201204.rodent.frell.theremailer.net> - 2012-04-22 23:00 +0200
                Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-22 18:05 -0400
                Re: Indirect Bit glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-04-22 23:20 +0000
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-23 08:22 +0100
                Re: Indirect Bit Michael S <already5chosen@yahoo.com> - 2012-04-22 03:56 -0700
                Re: Indirect Bit EricP <ThatWouldBeTelling@thevillage.com> - 2012-04-22 12:28 -0400
                Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-22 12:56 -0500
                Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-22 13:15 -0700
                Re: Indirect Bit Nomen Nescio <nobody@dizum.com> - 2012-04-22 22:30 +0200
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-22 21:40 +0100
                Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-22 17:15 -0500
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-23 08:26 +0100
                Re: Indirect Bit Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2012-04-23 08:02 -0700
                Re: Indirect Bit nmm1@cam.ac.uk - 2012-04-23 17:02 +0100
                Re: Indirect Bit Fritz Wuehler <fritz@spamexpire-201204.rodent.frell.theremailer.net> - 2012-04-23 20:17 +0200
                Re: Indirect Bit Robert Wessel <robertwessel2@yahoo.com> - 2012-04-23 13:07 -0500
                Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-22 13:16 -0700
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-23 13:14 -0700
                Re: Indirect Bit Anne & Lynn Wheeler <lynn@garlic.com> - 2012-04-23 16:51 -0400
                Re: Indirect Bit Quadibloc <jsavard@ecn.ab.ca> - 2012-04-23 14:22 -0700
                Re: Indirect Bit MitchAlsup <MitchAlsup@aol.com> - 2012-04-21 11:17 -0700
        Re: Indirect Bit jacko <jackokring@gmail.com> - 2012-04-14 08:02 -0700

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