🚀 go-pugleaf

The CVTxxx stuff is MVS specific and won't work, e.g., on VM.

The TOD Clock is explained quite well in Principles of Operation in
Chapter 4, "Control".  The basic rules are that on a single CPU the TOD
clock will always appear to increase -- successive STCK instructions
will store ever-increasing values -- and on a multiprocessor system the
TOD clock will always yield a *unique* value even if two CPUs issue
STCK instructions "simultaneously".

The "recommended standard epoch" of January 1, 1900, 0 a.m. GMT is
essentially universal.  The high-order ("sign") bit of the TOD clock
turned to a '1' on the fateful day of May 11, 1971, on which day it
became apparent who was (incorrectly) using signed arithmetic (the TOD
clock is unsigned).  If you STCK and this high-order bit is a '1', then
your clock is most likely set to the recommended standard epoch.

The high-order word measures Mega (i.e., 2**20) microseconds, which is
the basis for the seemingly bizarre resolution of 1.048576 seconds.  As
mentioned in the PrincOps, this is more than enough resolution for
determining the current time of day (since the system TOD clock is
typically set from the operator's wristwatch).  For measuring elapsed
time intervals, you should use both words of the clock for maximum
resolution, with the caveat that different processor models have
different actual resolutions.

      John Pershing
      IBM Research, Yorktown Heights