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Re: ALTs on inputs and outputs


> I understand that at one time the merits of ALTs on
> output channels as well as input channels in Occam
> was debated.  CSP is even handed on the issue,
> can anyone explain why Occam isn't? -jc

A matter of ease of implementation. 

There is no problem with an ALT containing an output sending to a simple
input, when the ALT commits to that channel there is something at the other
end to receive the communication.

BUT when a channel appears as an output in one ALT and an input in another
ALT (instant deadlock if they are the same ALT!) there is a problem of
commitment.  The outputting ALT may have chosen that channel while the
inputting ALT has committed to another channel and we get into all sorts
of problems. We can do this only if we are able to withdraw from a
commitment - and that is possible, but a slow and possibly messy business.

HOWEVER, having said that I find I have a need for output guards in an ALT
driving input guards in another ALT for a network routing switch design.
So, I WILL be providing them in my occam to hardware compiler. It would be
nice if JCSP could provide them too :-)

| Barry M Cook, BSc, PhD, CEng, MBCS                                         |
| Senior Lecturer,                           Department of Computer Science, |
| Chartered Information Systems Engineer.    Keele University,               |
|                                            Keele,                          |
| Phone: +44 1782 583411                     Staffordshire,                  |
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| email: barry@xxxxxxxxxxxxxx                UK.                             |