Re: Hardware Clock
Re: Hardware Clock
- Subject: Re: Hardware Clock
- From: Ed Wynne <email@hidden>
- Date: Sat, 8 Jul 2006 13:59:20 -0400
On Jul 8, 2006, at 7:00 AM, Gerriet M. Denkmann wrote:
On 08.07.2006, at 08:48, Ed Wynne wrote:
On Jul 8, 2006, at 1:38 AM, Gerriet M. Denkmann wrote:
The problem: whenever (B) is initialized from (A) a random value
in the range -0.5 ... +0.5 seconds is added to the time.
To test: make the time (B) equal to the correct wall clock time.
Sleep. Awake. Compare time to wall clock. See difference of
-0.5 ... +0.5 seconds.
The hardware clock is probably only second accurate and its likely
the OS only samples it once, instead of polling for the next
second increment. This would save about .5s (on average) from the
wakeup time.
Yes; further proof for this is the fact that the system time after
awakening always starts with a full second (no fractional part).
But: if the OS can poll the hardware clock, why can't I?
You could, if you knew how... and were sufficiently motivated.
Apple does not provide an API or document how to do this. On OS X
you'll need a kext to talk to the SMU for current machines, the PMU
on slightly older machines, and god knows what on even older
machines. It also used to be different for laptops vs desktops, I
don't know if it still is. So how do you do it? Get the appropriate
linux kernel source and go spelunking, they've figured most of it out.
-Ed
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