15 Jul
2016
15 Jul
'16
11:51 a.m.
When browsing the Tops-10 sources I stumbled onto this (d36com.mac):
NMXTIM:
IFN FTOPS10,<
MOVE T1,DATE## ;GET CURRENT UDT (IN DAYS,,FRACTION)
MOVE T2,TIME## ;GET CURRENT TIME (IN JIFFIES SINCE MIDNIGHT)
LSH T1,-^D17 ;TRUNCATE TO NUMBER OF HALF DAYS.
SUBI T1,124210_1 ;CONVERT DAYS SINCE 1858 TO DAYS SINCE 1977
MOVE T3,[^D60*^D60*^D12]
IMUL T3,TICSEC## ;SECONDS SINCE HALF-DAY
TRNE T1,1 ;IS THIS THE SECOND HALF OF A DAY?
SUB T2,T3 ;YES, RECORD SECONDS SINCE HALF-DAY
IDIV T2,TICSEC## ;CONVERT JIFFIES INTO SECONDS.
IMULI T3,^D1000 ;CONVERT TO NUMBER OF MILLISECONDS*TICSEC
IDIV T3,TICSEC## ;CONVERT TO NUMBER OF MILLISECONDS.
SKIPL T2 ;MAKE SURE WE HAVE A POSITIVE NUMBER OF SECONDS
TDNE T1,[XWD -1,600000] ;MAKE SURE NO DATE OVERFLOW
BUG.(HLT,COM911,D36COM,SOFT,<The date is past 9 November 2021>,,<
Cause: The 2 byte julian half-day field in an event message is limited
to 9 november 2021. The routine above has calculated the julian
half-day, and has found that it overflowed.
I doubt very much that the date itself has really gone past 2021.
Probably someone smashed an AC or the routine to get the time
from the monitor is returning junk.
)
RET
;END IFN FTOPS10
This made me search for some documentation, and I found aa-k181a-tk
which said, amongst other things:
EVENT TIME Is the source node date and time of event
processing. Consists of:
+----------+--------+-------------+
! JULIAN ! SECOND ! MILLISECOND !
! HALF DAY ! ! !
+----------+--------+-------------+
where:
JULIAN HALF DAY (2) : B = Number of half days
since 1 Jan 1977 and
before 9 Nov 2621
(0-32767). For example,
the morning of Jan 1,
1977 is 0.
SECOND (2) : B = Second within current
half day (0-43199).
MILLISECOND (2) : B = Millisecond within
current second (0-999).
If not supported, high
order bit is set,
remainder are clear, and
field is not printed
when formatted for
output.
Some observations/questions:
* The code in Tops-10 actually checks for the half day value beeing
not more than 16 bits, i.e. it will not trigger until 2066. This
means that I will consinder it a typical case of SEP when it does.
* I have no idea what will happen when the date goes past 9-Nov-2021,
i.e. when the sixteen bit field gets the sign bit set and possibly
some software somewhere goes belly up. Maybe we should try to do a
controlled test?
* Will this be/become a problem? What to do about it?
--Johnny