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Re: What are the various alternate uses for the PC's LSB ?

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On 5/12/2011 5:01 PM, whygee wrote:
> Paul A. Clayton wrote:
>> Linked Load/Store Conditional (with different names) are also used by
>> Alpha (as you surmised), ARM, and Power(PC).
>> It has the additional semantic of failing if there is interference
>> with another thread (it is a very limited form of transactional memory).
> Yes, it's definitely a RISC technique worth digging.
> Simple, elegant, flexible, inspiring.
>
> yg

And really only works properly on a snoopy bus.

And even on a snoopy bus can have problems with fairness.  If you aren't 
careful with how you do bus arbitration, one thread can block forever. 
Note: don't assume all processors are running at the same clock speed.

If you want to guarantee fairness forward progress on a machine that 
doesn't have a snoopy bus, you have to implement more mechanisms.  So, 
in the end, LL/SC isn't very RISCy at all.

Let alone a machine where you want to export the atomic RMW to the 
interconnect, so that you can do NYU Ultracomputer style fetch and op 
combining.  Or, heck, even the simple form of RMW coalescing that most 
GPUs provide.

However, LL/SC is a mechanism that allows a lot of different instruction 
sequences to be made atomic.  You want atomic-increment-with-saturation 
at [0,255] ... ?  LL/SC can do that for you.

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Re: What are the various alternate uses for the PC's LSB ? "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-05-12 08:49 -0700
  Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-13 02:01 +0200
    Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-12 20:32 -0700
      Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-13 12:47 +0200
        Re: What are the various alternate uses for the PC's LSB ? Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2011-05-13 08:44 -0700
          Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-13 19:53 +0200
            Re: What are the various alternate uses for the PC's LSB ? Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2011-05-13 11:36 -0700
              Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-13 22:26 +0200
            Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-14 17:17 -0700
              Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-16 14:22 +0200
                Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-16 07:38 -0700
          Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-14 17:14 -0700
      Re: What are the various alternate uses for the PC's LSB ? rpw3@rpw3.org (Rob Warnock) - 2011-05-13 07:45 -0500

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