Re: Duration vs. Ada.Real_Time

[Marin David Condic <mcondic.auntie.spam@acm.org>]

Ah! We appear to have here a good illustration of the difference between
accuracy and precision. Duration'Small represents the precision which
objects of type Duration can represent. The LSB has to be at least
20mSec. However, if the clock has accuracy no better than 1Sec, the LSBs
are meaningless. The precision exceeds the accuracy of the actual
device. (my misunderstanding of the original question.)

I would think that a reasonable implementation of Ada for realtime
systems would want to insure that the precision had some relationship to
the accuracy of the clock available. Obviously, for workstations/PCs
with a non-realtime OS, etc., you can't exactly insist that the platform
be changed for the language. But stating, for example, that the
precision of a time representation can go down to atto-seconds can
easily mislead one to believe that the measurement of time is going to
be something close to that precision.

Clearly, one needs to know two things: What is the accuracy of the
clock(s) available to me through the hardware/OS? (Talk to the hardware
manufacturer.) And what is the relationship between my hardware and the
Ada implementation I have? (Talk to the compiler vendor.) Id est, does
it do me any good to say "delay 0.020;" with a given compiler and target
system?

MDC

Robert Dewar wrote:

> This may be confusing, the question was not about precision
> of Duration, it was about resolution of the timer
>
> The value of Duration'Small is only an upper bound for
> resolution, there is nothing in the RM that forbids an
> implementation where clock only updates every second (and
> indeed do not be surprised if some Unix implementations
> are like this).

--
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Marin David Condic - Quadrus Corporation - http://www.quadruscorp.com/
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