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HP OpenVMS Systemsask the wizard |
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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.
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