Stan writes:
> I don't think you want a [powerfail] variable ... that implies
> a polling strategy,
> which is (almost) always something you want to avoid on a multi-user
> computer.
 
The reason for using a CI variable is that it provides more than just a yes/no
decision, and does so while there's time to act on it. Polling in this case is
satisfactory because most of the time, application-level code wouldn't care
whether power was about to fail. It's only when the code is about to start a
logically atomic transaction that it needs to know. If the application knows
well in advance of a possible shutdown, it can warn the user to expect a
problem and let the user take whatever steps are appropriate (like taking an
early lunch).
 
> Instead, MPE/iX has a mechanism for registering processes that want to
> be informed about powerfail and shutdown requests.
 
This would work if there were some guarantee of the amount of power remaining
before a shutdown. I don't think it fills the need if it's called just before
the SA5300, since there'd be no time (and probably no facility) for user
interaction. A user-level process will want to perform terminal, network or
disc I/O in response to this notification; if it's done immediately before the
SA5300, there won't be time for I/Os to complete. Also, if there are 500+
instances of an application running and all must be notified, there will
suddenly be 500 processes in the READY queue. This seems like an uncomfortable
situation.
 
However, the interrupt mechanism would be nice for batch processing. A batch
process can presumably save its state and terminate in response to the
interrupt.
 
-- Bruce
 
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