Re: Global Warming - Forth as answer

From	rickman <gnuarm@gmail.com>
Newsgroups	comp.lang.forth
Subject	Re: Global Warming - Forth as answer
Date	2013-05-16 14:03 -0400
Organization	A noiseless patient Spider
Message-ID	<kn36qa$o4b$1@dont-email.me> (permalink)
References	(4 earlier) <2013May15.170815@mips.complang.tuwien.ac.at> <kn08r4$nc$1@dont-email.me> <2013May15.172242@mips.complang.tuwien.ac.at> <kn0o96$u8u$1@dont-email.me> <2013May16.145356@mips.complang.tuwien.ac.at>

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On 5/16/2013 8:53 AM, Anton Ertl wrote:
> rickman<gnuarm@gmail.com>  writes:
>> On 5/15/2013 11:22 AM, Anton Ertl wrote:
>> How is OVER_+ reversible exactly?  You can recover the data you started
>> with using OVER -, but is that really the same as saying it is
>> reversible?
>
> Yes, that's the idea.  You can run the computation in forward or
> reverse direction.

But that is not running the computation in the reverse direction, that 
is recovering the prior state of the machine by a new computation.

>> I suppose OVER_+ would have to be
>> atomic such as ADD A,B can be reversed by SUB A,B, but I don't think
>> this is really what the concept is about.
>
> It is, at least as far as programming is concerned; you need to
> organize your program such that you can input the results, run it in
> reverse mode, and can produce the parameters of the program.  I dimly
> remember that the C-ish language used by the course instructor to show
> things allowed "a+=b", but not "c = a+b".

Exactly, because c = a+b clobbers the data in c which is lost forever. 
I guess if the data can be recovered by *any* means, it is not lost, so 
maybe OVER + is reversible.  The data is changed, but information is not 
lost.  The same would be true by adding a constant, the constant can be 
subtracted to recover the original data.  Except that data is clobbered 
in the "unseen" registers used to implement the OVER.  What value did it 
have originally.  Even if you don't care, it is information, so it is 
lost and the instruction is irreversible no matter what follows it.

So my final conclusion is that OVER_+ is not reversible unless it is 
atomic and is actually the same as A <= A + B, not an OVER followed by a +.

-- 

Rick

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Global Warming - Forth as answer visualforth@rocketmail.com - 2013-05-14 15:21 -0700
  Re: Global Warming - Forth as answer hughaguilar96@yahoo.com - 2013-05-14 18:40 -0700
    Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-14 21:00 -0700
      Re: Global Warming - Forth as answer visualforth@rocketmail.com - 2013-05-15 22:24 -0700
        Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-19 19:25 -0700
  Re: Global Warming - Forth as answer albert@spenarnc.xs4all.nl (Albert van der Horst) - 2013-05-15 10:05 +0000
    Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-15 12:42 +0000
      Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-15 10:56 -0400
        Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-15 15:08 +0000
          Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-15 11:19 -0400
            Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-15 15:22 +0000
              Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-15 15:42 -0400
                Re: Global Warming - Forth as answer visualforth@rocketmail.com - 2013-05-15 13:45 -0700
                Re: Global Warming - Forth as answer Bernd Paysan <bernd.paysan@gmx.de> - 2013-05-15 23:38 +0200
                Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-16 12:53 +0000
                Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-16 14:03 -0400
                Re: Global Warming - Forth as answer anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-17 12:41 +0000
        Re: Global Warming - Forth as answer albert@spenarnc.xs4all.nl (Albert van der Horst) - 2013-05-17 13:17 +0000
          Re: Global Warming - Forth as answer dirk.bruehl@usa.net - 2013-05-17 08:36 -0700
            Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-19 19:12 -0700
            Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-20 00:06 -0700
    Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-15 10:47 -0400
      Re: Global Warming - Forth as answer albert@spenarnc.xs4all.nl (Albert van der Horst) - 2013-05-17 13:23 +0000
        Re: Global Warming - Forth as answer rickman <gnuarm@gmail.com> - 2013-05-17 09:59 -0400
    Re: Global Warming - Forth as answer Steve <nospam275@gmail.com> - 2013-05-19 18:56 -0700
  Re: Global Warming - Forth as answer oh2aun@gmail.com - 2013-05-22 00:50 -0700

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