On Tue, 28 Jan 1997, Christoph Ewering wrote:

> Well, I don't think that this is silly. think it is silly to count the
> interrupts and when i reach 60 to add a second to the systemclock.

That's not an accurate representation of how it works.  For a more
in-depth explanation, I refer you to `The Design and Implementation of
4.4BSD' sections 3.4 and 3.6.  I'll summarize, but please understand that
it's just that:  a summary.

 - On each clock `tick', we increment the system time by 1000000/HZ
   microseconds.[*]

 - Scheduling and other kernel tasks (e.g. timeouts), as well
   as per-process timers (real, profiling, virtual) need resolution finer
   than a per-second tick.

 - Some user programs require a monotonically increasing time of day.

[*] Actually, we add a variable that is initially set to 1000000/HZ, but
this is close enough for this part of the discussion.  See the reference
to adjtime() in the last paragraph.

> So why don't i take 60 interrupts and then look what is in the RTC.
> You can share processtime with this interrupt, but i don't understand why
> use it to calculate the time. 

The system time is a side effect of the other kernel activities going on. 
It's not `calculated', but rather just a counter that gets incremented by
1000000/HZ microseconds on each tick. In addition to the RTC providing
only 1 second resolution, which is clearly insufficient resolution for
several user-level programs, your suggestion has two other problems: 

 - Access to the RTC is expensive, and

 - We will see significant `jitter' in the time-of-day clock if we happen
   to miss interrupts.

The latter will manifest itself as a clock that suddenly jumps forward as
the RTC time is used to update the system time (assuming that's even
practical, i.e. assuming RTC access takes an insignificant amount of
time).  The adjtime() call is mentioned in the previously cited text as a
solution to this problem, but understand that adjtime() _depends_ on being
able to modify the amount of time that the clock is incremented by at each
`tick,' something that is not possible with the RTC.

Hope this makes the issue a little more clear.

--scott