Guardian Procedure Calls Reference Manual (G06.25+)

Guardian Procedure Calls (H-K)
Guardian Procedure Calls Reference Manual—522629-013
7-53
JULIANTIMESTAMP Procedure
Procedures that work with a 64-bit Julian timestamp are COMPUTETIMESTAMP,
CONVERTTIMESTAMP, INTERPRETTIMESTAMP, JULIANTIMESTAMP, and
SETSYSTEMCLOCK. Where possible, it is recommended that applications use
these procedures rather than the procedures that work with 48-bit timestamps.
•
A 48-bit timestamp is a quantity equal to the number of 10-millisecond units since
00:00, 31 December 1974. The 48-bit timestamp always represents local civil time
(wall clock time); consequently, this value is affected by standard time/daylight
saving time changes and time zone differences.
Procedures that work with a 48-bit timestamp are: CONTIME, TIME, and
TIMESTAMP.
•
Process creation time is initialized by calling TIMESTAMP, which returns the local
civil time in centiseconds (0.01 second = 10 milliseconds) since midnight (00:00)
on 31 December 1974, in an array of three words. Only the two low-order words
are saved in the process control block (PCB); this is sufficient to make the
unnamed process ID unique.
•
The RCLK instruction ($READCLOCK in TAL) is another source of timestamps. It
returns a 64-bit timestamp representing the local civil time in microseconds since
midnight (00:00) on 31 December 1974. Note that this is not a Julian timestamp
and therefore it is not transferable across HP systems. Applications should avoid
using the RCLK instruction except where necessary.
•
Process timing uses 64-bit elapsed time counters with microsecond resolution;
these are also not Julian timestamps.
•
There is no way to generalize about internal timing using 64-bit Julian timestamps
or 48-bit timestamps. Each section of the operating system manages time using
the method most appropriate for its application.
•
All time and calendar units in this discussion are defined in The Astronomical
Almanac published annually by the U.S. Naval Observatory and the Royal
Greenwich Observatory.
•
The value returned by JULIANTIMESTAMP(3), a count of the number of
microseconds since COLDLOAD of this processor is not affected by SETTIME.
Therefore the calculation:
JULIANTIMESTAMP(0) - JULIANTIMESTAMP(1)
(current time) - (cold load time)
will not always match what is returned by JULIANTIMESTAMP(3).
Example
MY^TIME := JULIANTIMESTAMP; ! returns the current GMT
Note. Because processor clocks are not synchronized to the nearest microsecond, values
obtained from different processors might not agree.