[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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