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Re: x2AnyChannels do not allow alting

Peter wrote:

> In CSP, the PAR operator is defined in terms of external choice (sort of
> ALTing).  In occam, we all know the following equivalencees:
>   PAR                    ==             ALT
>     in0 ? x0                              in0 ? x0
>     in1 ? x1                                in1 ? x1
> 				          in1 ? x1
> 					    in0 ? x0

I didn't!

Maybe it's a question of what "==" means, see below.

Admitting that my CSP isn't all it should be, I may need my thinking
straightening out (please help), but I've been interpreting this differently.

My interpretation ...

The ALT case indicates that we either do SEQ( in0?x0; in1?x1 ) or
SEQ( in1?x1; in0?x0 ) which give the same result as each other and as
concurrent execution of both inputs. So, the ALT form is a valid
IMPLEMENTATION of PAR, but not the only one - true concurrency is also

When converting the PAR case to hardware I really do allow all paths to be
concurrent - it's easy, it's natural, it boosts performance, ... Although I
am happy that sequentialisation is needed on a single processor, this is
surely not a good equivalence for multi-processor systems, is it?

I'd prefer the == to be "may be implemented by" rather than "is the same
as". Maybe my problem is that I'm just misinterpreting the meaning of "=="
as used above and it does mean "gives the same result as" ("equals" is
seriously overloaded in mathematics/computing!).


| Barry M Cook, BSc, PhD, CEng, MBCS                                         |
| Senior Lecturer,                           Department of Computer Science, |
| Chartered Information Systems Engineer.    Keele University,               |
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