[time-nuts] FreeBSD, NetBSD, or Minix-III?
Poul-Henning Kamp
phk at phk.freebsd.dk
Sun May 17 05:00:06 UTC 2009
In message <20090517031525.292E7BCE6 at ip-64-139-1-69.sjc.megapath.net>, Hal Murr
ay writes:
>This is one of the reasons why I was looking for a low-cost FPGA on PCI board
>with some way to get a couple of external inputs.
>
>Things get interesting if your hardware splits a 64 bit read into 2 32 bit
>transfers. Many years ago, all Intel chips did that. I don't know about
>today.
You don't need a 64bit counter in the first place.
You are going to have a thread around anyway, which adjusts your clock based
on the NTP PLL's output, that thread can maintain your "reference time"
so that a 32 bit counter will never overflow.
Which, I might add, is _exactly_ how FreeBSD does it :-)
Poul-Henning
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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