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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Ada.Execution_Time
Date: Tue, 28 Dec 2010 14:14:57 +0000
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> And conversely, the catastrophic accuracy of the VxWorks real-time
> clock service does not hinder its usability for real-time application.

Catastrophic?

The Radstone PPC7A cards (to take one example) have two facilities: (a)
the PowerPC decrementer, run off a crystal with some not-too-good quoted
accuracy (50 ppm, I think), and (b) a "real time clock".

The RTC would be much better termed a time-of-day clock, since what it
provides is the date and time to 1 second precision. It also needs
battery backup, not easy to justify on naval systems (partly because it
adversely affects the shelf life of the boards, partly because navies
don't like noxious chemicals in their equipment).

We never used the RTC.

The decrementer is the facility used by VxWorks, and hence by GNAT under
VxWorks, to support time; both Ada.Calendar and Ada.Real_Time (we are
still at Ada 95 so I have no idea about Ada.Execution_Time). We run with
clock interrupts at 1 ms and (so far as we can tell from using bus
analysers) the interrupts behave perfectly reliably.

For a higher-resolution view of time we've extended Ada.Calendar, using
the PowerPC's mftb (Move From Time Base) instruction to measure sub-tick
intervals (down to 40 ns).