I do not think will work.  Your blocking flag needs to be read and set 
with an atomic operation (ie. single instruction).  See:

Mutual Exclusion: http://en.wikipedia.org/wiki/Mutual_exclusion

Atomicity: http://en.wikipedia.org/wiki/Atomicity_%28programming%29

and Concurrency: http://en.wikipedia.org/wiki/Consistency_model

M90 clear flags
...

M91 P1 Semaphore Down (wait for blocking semaphore #1 and acquire it)
    ...
    ...
M92 P1 Semaphore Up (release blocking flag #1)

As a note, this can also be thought of in terms of Mutexs or Locks, but 
it must be an atomic operation.

   EBo --

On Mon, 10 Sep 2012 15:11:04 +0200, andy pugh wrote:
> Ignoring (for a moment) the means of communication, could the
> handshaking be as simple as a few more M-codes?
>
> M90 clear flags
> M91 P1 wait for blocking flag 1 to clear
> M92 P2 set blocking flag 2
> M93 P2 release blocking flag 2

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