  Problems with VAX System Time? 
 The Question is:
 
We are experiencing a problem where the system clock gets changed somehow,
which impacts our real-time processing.  It is a VAX 6610 in a two node
cluster running VMS 5.5-2. When the system clock is reset,  the following
symptoms occur: 'monitor'ing funct
ions  (i.e.; mon proc/topc)  displays the box,  but no data.  control t does
not work.  DECnet errors.  Have you seen this behavior before?  Any input
would be beneficial.  Thanks.
 
 
 The Answer is:
 
  On OpenVMS VAX, the VAX Time Of Year (TOY) Clock is read during a system
  bootstrap and is then used to initialize the Time Of Day Register (TODR),
  and periodic IPL 22 or IPL 24 interrupts are used as "ticks" to update
  the value stored in the TODR register.  (The TODR is a 32 bit field, and
  the format of the value stored has a resolution of approximately 497 days.
  The year is stored in the system image, SYS.EXE.)  The value stored in the
  TODR is then used as the basis for the system time.  The result of all
  this effort assembles a quadword time value that is loaded into the system
  data cell EXE$GQ_SYSTIME, a value that is then updated at ten millisecond
  intervals.
 
  Tools such as TCP/IP ntp and DECnet-Plus (DECnet/OSI) DTSS can modify the
  system clocks (TOY and TODR) and the system time cell, as (obviously) can
  the SET TIME command.  Errant kernel-mode code could also modify the
  EXE$GQ_SYSTIME cell, or block clock interrupts.  Hardware problems
  (correctable memory errors, CPU problems, clock problems) have also been
  known to cause various sorts of time-related misbehaviour or -- as in the
  case of a failed backup battery -- the inavailability of the saved system
  time at system bootstrap.  Swapping system disks around (other OpenVMS
  systems, standalone BACKUP, etc) has also been known to scrozzle the time
  (over a reboot), as the year portion of the time is stored in the system
  image and the rest is stored in the TODR register.
 
  High-IPL code (eg: repeatedly handling a correctable memory error) can
  block the TODR ticks, resulting in temporal slowdowns.  Further, extreme
  system velocity has been proven to result in a temporal slowdown from
  the viewpoint of a stationary observer, though relativistic velocity
  deltas are not commonly encountered in most current computer rooms.
 
  Please contact the Compaq Customer Support Center for assistance in
  resolving this.
 
	--
 
  Note: There are various different implementations of timekeeping hardware
  on VAX systems, including various interval timers as well as differences
  in the IPL used for the clock interrupts.
 
  Note: To prevent overflow of the TODR and to update the year value stored
  in SYS.EXE, VAX systems should have a SET TIME (with no parameters) issued
  between the change of the year and 497 days from the begining of the
  previous year -- this is typically during the first three months of each
  calendar year.  A shut down and reboot will accomplish this task, as well.
 
