Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wasatch!cs.utexas.edu!uunet!munnari!otc!metro!basser!wolfen!uowcsa!ian
From: ian@uowcsa.cs.uow.oz (Ian Gorton)
Newsgroups: comp.sys.transputer
Subject: Re: Parallel output on TDS2 for the PC
Summary: A solution
Keywords: Occam
Message-ID: <737@uowcsa.cs.uow.oz>
Date: 1 May 89 01:38:44 GMT
References: <1989Apr27.120625.5348@csuchico.uucp>
Organization: Uni of Wollongong, NSW, Australia
Lines: 52

In article <1989Apr27.120625.5348@csuchico.uucp>, tempest@csuchico.uucp (Kenneth Lui) writes:
> I'm just getting the feel of Occam 2 on TDS2.  I"ve written some simple
> SEQtype programs and some ALTtype programs.  I'm having some problems
> with PAR structures.  Specifically, I want to write a section of code that
          [deleted]
> I used the following code fragment:
> 	...
> 	PAR
> 	  SEQ
> 	    write.full.string(screen,"String from 'a'")
> 	    newline(screen)
> 	  SEQ
> 	    write.full.string(screen,"String from 'b'")
> 	    newline(screen)
> 	...
> 
> Needless to say, it doesn't work.  All I got was a big blank from the VDT,
> except one carriagereturn or linefeed.  I looked through the example

Yeah, the reason is that you're breaking one of the fundamental laws of
occam channels. You are attempting to write to the screen channel 
from TWO processes simultaneously. This is not allowed, as each channel
may only have one process outputting to it, and one inputting from it.

A library routine is your solution, it's documented in the 
PROGRAMMING INTERFACE section of the TDS manuals. The library process you 
need is called 'scrstream.multiplexor': it essentially multiplexes output
from a number of channels onto the 'real' screen channel. This sort of 
process is frequently required in occam programs.

So, to solve your problem you need,
   [2]CHAN OF ANY scr:
   CHAN OF INT stop:   -- to terminate multiplexor
   PAR
     scrstream.multiplexor(scr, screen, stop)
     SEQ
       PAR
         SEQ
           write.full.string(scr[0], " whatever")
           newline(scr[0])
         SEQ
           write.full.string(scr[1], "something else")
           newline(scr[0])
       stop ! 22  -- send any value to terminate multiplexor

The added complications come about because it's necessary to terminate 
the multiplexor process, and the signal to do this ( stop ! 22 ) must
be sent after EVERYTHING has been sent to the screen.

It should work! Don't forget to include the libraries.

Good luck,      Ian Gorton