The OpenVMS Frequently Asked Questions (FAQ)

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3.6.2 Books and Tutorials?

Some of the OpenVMS books that are now or that have been available from the Elsevier Digital Press imprint

are listed in Table 3-3:

Table 3-3 DP Books
Title and Author ISBN
Getting Started with OpenVMS
Michael D. Duffy
Getting Started with OpenVMS System Management, 2nd Edition
David Donald Miller, et al
Introduction to OpenVMS, 5th Edition
Lesley Ogilvie Rice
Introduction to OpenVMS
David W Bynon
OpenVMS Alpha Internals: Scheduling and Process Control 1-55558-156-0
OpenVMS AXP Internals and Data Structures: Version 1.5 1-55558-120-X
OpenVMS System Management Guide
Baldwin, et al
The OpenVMS User's Guide, Second Edition
Patrick Holmay
Using DECwindows Motif for OpenVMS
Margie Sherlock
VAX/VMS Internals and Data Structures: Version 5.2 1-55558-059-9
Writing Real Programs in DCL, Second Edition
Hoffman and Anagnostopoulos
Writing OpenVMS Alpha Device Drivers in C
Sherlock and Szubowicz

Within the above table, no attempt is made to track which books are currently in print, or are currently out of print.

For various featured OpenVMS books, also please see the books link at the OpenVMS website:

For a bibliography of various OpenVMS books, please see:

3.7 What OpenVMS mailing lists and forums are available?

Various OpenVMS mailing lists are available, with some of the available lists detailed in Table 3-4, as are the various discussion forums in Table 3-5.

Table 3-4 OpenVMS Mailing Lists
Subscription Interest Area
OpenVMS Freeware archive announcement list 1
Two-way echo of vmsnet.internals 1
OpenVMS Alpha Internals discussions 1
BLISS discussions 1
Process Software MultiNet mailing list (news gateway) 1
Process Software TCPware mailing list (news gateway) 1
Process Software PMDF mailing list (news gateway) 1
The Software Resources International (SRI) CHARON-VAX VAX emulator package 1
Info-Zip's Zip & UnZip discussion list 1
RADIUS-VMS, a RADIUS server for OpenVMS discussion forum 1
Internet Service Providers (ISPs) running OpenVMS 1
Users of Mark Daniel's WASD web server for OpenVMS VAX and Alpha exists. Information about this list server and details on how to subscribe to the list are available at the referenced website.
VMS Forum

1This is the subscription address. Usually, you will want to send a mail message with no subject line, and a SUBSCRIBE or HELP command in the body of the mail message.

Various OpenVMS discussion forums are available, with some of the available lists detailed in Table 3-5.

Table 3-5 OpenVMS Discussion Forums
Description Forum Location
The comp.os.vms newsgroup
HP ITRC Forums
Hunter Goatley: The HG OpenVMS Message Board
Computing.Net: OpenVMS Message Area
Tek-Tips: DEC (HP/Compaq): OpenVMS Forum
OpenVMS.Org forums
OpenVMSHobbyist.Org forums
Encompasserve (DECUSserve) Notes Conferences

3.8 What is this Ask The Wizard website I've heard about?

The HP OpenVMS Ask The Wizard (ATW) website was an informal area discussing OpenVMS, containing questions and answers on a wide variety of topics.

For additional information on the OpenVMS Ask The Wizard (ATW) area and for a pointer to the available ATW archive, please see Section 3.8. ATW has been superceded (for new questions) by the ITRC discussion forums; the area remains available for reference.

To access a cited topic directly, use the URL filename WIZ_topic-number.HTML, or use the topic search engine. Cited topics are shown in parentheses, and act as unique topic addresses. These should not be confused with the relative topic numbers shown at the site. For example, the topic (1020) can be accessed directly using the URL filename wiz_1020.html, at the web site that the following URL resolves into:

A zip archive (named containing all of the available topics and questions can be downloaded from the above URL. The zip archive is completely regenerated when/if existing topics posted out to the ATW website are updated. Copies of this archive also generally ship out on the OpenVMS Freeware, as well.

New (informal) questions and discussions are now being directed away from the ATW area to the ITRC area, and specifically to the ITRC discussion forums:

3.9 Where can I find the latest C run-time library manuals?

The C run-time library (RTL) reference documentation has been moved from the C language documentation set to the OpenVMS documentation set. For the most recent version of the C RTL documentation and the OpenVMS standard C library, please see the OpenVMS manuals.

In addition to the user-mode C RTL, there is a second kernel-mode RTL accessable to drivers on OpenVMS Alpha and OpenVMS I64. For details on this second library and on the duplicate symbol errors that can be triggered when this library is referenced during an incorrectly-specified LINK command, please see Section 10.22.1. For general information on this kernel RTL, see the Digital Press book Writing OpenVMS Device Drivers in C. For details, please see the associated OpenVMS source listings distribution.

Chapter 4
Time and Timekeeping

If you are searching for something here, please consider using the text-format FAQ.

This chapter discusses time, timekeeping, system time synchronization, clock skew and clock drift, implications of using SUBMIT/AFTER=TOMORROW, and other time-related topics.

4.1 A brief history of OpenVMS Timekeeping, please?

Why does OpenVMS regards November 17, 1858 as the beginning of time...

The modified Julian date adopted by the Smithsonian Astrophysical Observatory (SAO) for satellite tracking is Julian Day 2400000.5, which turns out to be midnight on November 17, 1858.

SAO started tracking satellites with an 8K (nonvirtual) 36-bit IBM 704 in 1957 when Sputnik went into orbit. The Julian day was 2435839 on January 1, 1957. This is 11225377 octal, which was too big to fit into an 18-bit field. With only 8K of memory, the 14 bits left over by keeping the Julian date in its own 36-bit word would have been wasted. SAO also needed the fraction of the current day (for which 18 bits gave enough accuracy), so it was decided to keep the number of days in the left 18 bits and the fraction of a day in the right 18 bits of one word.

Eighteen bits allows the truncated Julian Day (the SAO day) to grow as large as 262143, which from November 17, 1858, allowed for 7 centuries. Possibly, the date could only grow as large as 131071 (using 17 bits), but this still covers 3 centuries and leaves the possibility of representing negative time. The 1858 date preceded the oldest star catalogue in use at SAO, which also avoided having to use negative time in any of the satellite tracking calculations.

The original Julian Day (JD) is used by astronomers and expressed in days since noon January 1, 4713 B.C. This measure of time was introduced by Joseph Scaliger in the 16th century. It is named in honor of his father, Julius Caesar Scaliger (note that this Julian Day is different from the Julian calendar that is named for the Roman Emperor Julius Caesar!).

Why 4713 BC? Scaliger traced three time cycles and found that they were all in the first year of their cyle in 4713 B.C. The three cycles are 15, 19, and 28 years long. By multiplying these three numbers (15 * 19 * 28 = 7980), he was able to represent any date from 4713 B.C. through 3267 A.D.

The starting year was before any historical event known to him. In fact, the Jewish calendar marks the start of the world as 3761 B.C. Today his numbering scheme is still used by astronomers to avoid the difficulties of converting the months of different calendars in use during different eras.

The following web sites:

are all good time-related resources, with some general and some specific to OpenVMS.

4.1.1 Details of the OpenVMS system time-keeping? VAX hardware time-keeping details... TOY clock

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 only keeps track of months, days, and time. The time is kept relative to January 1st, at 00:00:00.00 of the year the clock was initiailized.

The VAX Time-Of-Year (TOY) clock (used to save the time over a reboot or power failure) is specified as having an accuracy of 0.0025%. This is a drift of roughly 65 seconds per month.

The VAX Interval Time is used to keep the running time, and this has a specified accuracy of .01%. This is a drift of approximately 8.64 seconds per day.

Any high-IPL activity can interfere with the IPL 22 or IPL 24 (this depends on the VAX implementation) clock interrupts---activities such as extensive device driver interrupts or memory errors are known to slow the clock. EXE$GQ_SYSTIME

This is the OpenVMS VAX system time cell. This cell contains the number of 100ns intervals since a known reference. This cell is incremented by 100000 every 10ms by an hardware interval timer. EXE$GQ_TODCBASE

This cell contains the time and date the system time was last adjusted by EXE$SETTIME. It uses the same format as EXE$GQ_SYSTIME. On adjustment of the system time a copy of EXE$GQ_SYSTIME is stored in this cell in both memory and on disk. This cell is used to get the year for the system time. EXE$GL_TODR

This cell contains the time and date the system time was last adjusted by EXE$SETTIME. It uses the same format as the time of year clock. On adjustment of the system time this cell gets saved back to both memory and disk. The contents of this cell are used to test the validity of the TOY clock.

The system parameters SETTIME and TIMEPROMPTWAIT determine how the system time will be set.

When booting a CD-ROM containing an OpenVMS VAX system, the system will typically be deliberately configured prompt the user to input the time -- this is necessary in order to boot with the correct time.

If either TIMEPROMPTWAIT or SETTIME are set to zero, OpenVMS VAX will use the TOY clock to get the time of year, and the year will be fetched from the distribution medium. The value of the year on the distribution medium (saved within the SYS.EXE image) will most likely be that of when the kit was mastered, and cannot be changed. Unless the current year happens to be the same year as that on the distribution, most likely the year will be incorrect. (Further, with the calculation of Leap Year also being dependent on the current year, there is a possibility that the date could be incorrect, as well.) Alpha hardware time-keeping details... 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. 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. 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.

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.)

IA-64 (Itanium) hardware time-keeping details to be added... Why does VAX need a SET TIME at least once a year?

Because the VAX Time Of Year (TOY) has a resolution of 497 days, the VAX system time is stored using both the TOY and the OpenVMS VAX system image SYS.EXE. Because of the use of the combination of the TOY and SYS.EXE, you need to issue a SET TIME command (with the time parameter specified) at least once between January 1st and about April 11th of each year, and whenever you change system images (due to booting another OpenVMS VAX system, booting the standalone BACKUP image, an ECO that replaces SYS.EXE, etc).

The SET TIME command (with the current time as a parameter) is automatically issued during various standard OpenVMS procedures such as SHUTDOWN, and it can also obviously be issued directly by a suitably privileged user. Issuing the SET TIME command (with a parameter) resets the value stored in the TOY, and (if necessary) also updates the portion of the time (the current year) saved in the SYS.EXE system image.

This VAX TOY limit is the reason why OpenVMS VAX installation kits and standalone BACKUP explicitly prompt for the time during bootstrap, and why the time value can "get weird" if the system crashes outside the 497 day window (if no SET TIME was issued to update the saved values), and why the time value can "get weird" if a different SYS$SYSTEM:SYS.EXE is used (alternate system disk, standalone BACKUP, etc).

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