@George, thanks. I'm looking at the from a software development point of
view, not a sysprog point of view.
So if a shop is using an ETS and does not schedule the change then the TOD
clock will be steered into reflecting the leap seconds, right? The TOD clock
will slow down until it falls back to the correct time, right?
For a "Time Control Parameter Change event" where does the leap second get
reflected? Still in the hardware TOD clock? It effectively stops for one
second? Or in CVTLSO?
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On
Behalf Of George Kozakos
Sent: Wednesday, October 12, 2016 1:29 PM
Subject: Re: How set CVTLSO?
> Ah! I am starting to understand. Leap Second steering is accomplished
> the PTFF instruction and is independent of CVTLSO. PTFF appears to
> the physical clock.
> So ... steering and CVTLSO are essentially alternatives, right? CVTLSO
> should not include any leap seconds that were previously (or were
> be) "steered" -- is that right? And if a shop is using STP it is
> probably not modifying CVTLSO: CVTLSO is probably either zero, or at least
> I getting this right?
No, you have it wrong. Steering has nothing to do with leap seconds. STP
checks the external time source (ETS) at regular intervals and makes
adjustments via steering to keep accurate with the ETS.
It just so happens that if you don't schedule the leap second at the
appropriate time via the STP panel, when the leap second occurs, the STP UTC
time will be 1 second ahead of the ETS and so steering will occur to correct
If you do schedule the leap second, then STP generates a "Time Control
Parameter Change event" external interrupt that gets processed by z/OS to
make the leap second adjustment. If it is a positive change, z/OS spins on
all CPUs for the amount of the positive leap second change to ensure there
are no duplicate UTC time stamps and updates CVTLSO. No steering is required
as STP UTC time remained accurate with the ETS.
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