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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Any leap year issues caused by Ada yesterday?
Date: Tue, 06 Mar 2012 16:46:35 +0000
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Date: 2012-03-06T16:46:35+00:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> Under VxWorks you can read the TSC without assembly, there is a library
> function for that (pentiumTscGet64).
>
>    type Timestamp is new Unsigned_64;
>    procedure pentiumTscGet64 (Clock : out Timestamp);
>    pragma Import (C, pentiumTscGet64, "pentiumTscGet64");
>
> should do the work.

Not sure if there was an equivalent for PPC.

> The actual problem is to get the multiplier, the BIOS time, and keeping the
> TSC synchronized with the system clock. Funnily Wind River did all that for
> Pentium IV. But then they were too lazy to support it on more recent x86
> processors. The most troublesome thing about VxWorks is that Wind River
> adds and removes its parts at will. There is no such thing as backward
> compatibility whatsoever.

The board manufacturer defined the multiplier for us, so no problems.

> As for multicore/sleep mode issues, AFAIK Intel fixed that, i.e. the TSC
> frequency is never changed. I don't know anything about MacOS, but probably
> they deploy the same lousy schema of getting time from the PIT timer or
> something like that, so the problems.

I think I may have misunderstood the evidence here. Trying it again, the
TSC runs at pretty close to the nominal 2.4 GHz, but it was unreasonable
of me to try to *measure* it by looping for a second and seeing how much
the TSC changed, and then be surprised at errors of the order of a few
milliseconds. But, as you say, you have to calibrate the TSC somehow.