Re: [time-nuts] FreeBSD, NetBSD, or Minix-III?

[Lux, James P]

On 5/16/09 10:00 PM, "Poul-Henning Kamp" <p...@phk.freebsd.dk> wrote:

> In message <20090517031525.292e7b...@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 might want a 64 bit counter, if you want to keep time across processor
resets.

In which case, if you're saddled with 32 bit (or 8 bit!) reads, you have to
do multiple reads, so that by the end of the process, you can assure
yourself it's consistent.

E.g read high, read low, read high, read low
So you can check low #1 against low #2, and figure out if you had a roll
over.



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