Re: Time problem
- From: Joe Bloggs <JBloggs@xxxxxxxx>
- Date: Fri, 02 Dec 2005 07:00:34 -0800
On Thu, 01 Dec 2005 14:19:42 -0700, Kevin Handy <kth@xxxxxxx> wrote:
>I just set the time correctly, and after 1/2 hour it is off by
>5 minutes. Doing a bare "set time" seems to reset the clock to
>the correct time.
http://h71000.www7.hp.com/faq/vmsfaq_005.html
..1.1.2 Alpha hardware time-keeping details...
4.1.1.2.1 Battery-Backed Watch (BB_WATCH) Chip
This is battery backed up hardware timing circuitry used to keep the
correct time of year during rebooting, power failures, and system
shutdown. This clock keeps track of date and time in 24 hour binary
format.
The BB_WATCH time is used to initialize the running system time during
bootstrap, and the BB_WATCH time is read when the SET TIME command is
issued with no parameters; when the running system time is reset to
the value stored in the BB_WATCH. The running system time is written
into the BB_WATCH when the SET TIME command is issued with a
parameter.
The specification for maximum clock drift in the Alpha hardware clock
is 50 parts per million (ppm), that is less than ±0.000050 seconds of
drift per second, less than ±0.000050 days of drift per day, or less
than ±0.000050 years of drift per year, etc. (eg: An error of one
second over a day-long interval is roughly 11ppm, or
1000000/(24*60*60).) Put another way, this is .005%, which is around
130 seconds per month or 26 minutes per year.
The software-maintained system time can drift more than this,
primarily due to other system activity. Typical causes of drift
include extensive high-IPL code (soft memory errors, heavy activity at
device IPLs, etc) that are causing the processing of the clock
interrupts to be blocked.
4.1.1.2.2 EXE$GQ_SYSTIME
This is the OpenVMS Alpha system time cell. This cell contains the
number of 100ns intervals since November 17, 1858 00:00:00.00. This
cell is incremented by 100000 every 10ms by an hardware interval
timer.
4.1.1.2.3 EXE$GQ_SAVED_HWCLOCK
This cell is used by OpenVMS Alpha to keep track of the last time and
date that EXE$GQ_SYSTIME was adjusted. It keeps the same time format
as EXE$GQ_SYSTIME. The value in this cell gets updated in memory and
on disk, every time EXE$GQ_SYSTIME gets adjusted.
* The system parameters SETTIME and TIMEPROMPTWAIT determine how
the system time will be set.
* If SETTIME = 0
then EXE$INIT_HWCLOCK reads the hardware clock to set the system
time.
o IF TIMEPROMPTWAIT > 0
THEN the value of TIMEPROMPTWAIT determines how long the
user is prompted to enter the time and date. If time expires and no
time has been entered the system acts as if TIMEPROMPTWAIT = 0.
o IF TIMEPROMPTWAIT = 0
THEN the system time is calculated from the contents of
EXE$GQ_SAVED_HWCLOCK + 1.
o IF TIMEPROMPTWAIT < 0
THEN the user is prompted for the time and date and unable
to continue until the information is entered.
Unlike the VAX, the Alpha hardware clock tracks the full date and
time, not just the time of year. This means it is possible to boot
from the CD-ROM media without entering the time at the CD-ROM
bootstrap. (This provided that the time and date have been
initialized, of course.)
.
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