> Ah! I am starting to understand. Leap Second steering is accomplished
> the PTFF instruction and is independent of CVTLSO. PTFF appears to slow
> the physical clock.
> So ... steering and CVTLSO are essentially alternatives, right? CVTLSO
> should not include any leap seconds that were previously (or were about
> 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 stable.
> I getting this right?
> Charles

No, you have it wrong. Steering has nothing to do with leap seconds. STP
the external time source (ETS) at regular intervals and makes adjustments
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
time will be 1 second ahead of the ETS and so steering will occur to
correct it.

If you do schedule the leap second, then STP generates a "Time Control
Change event" external interrupt that gets processed by z/OS to make the
second adjustment. If it is a positive change, z/OS spins on all CPUs for
amount of the positive leap second change to ensure there are no duplicate
time stamps and updates CVTLSO. No steering is required as STP UTC time
accurate with the ETS.

George Kozakos
z/OS Software Service, Level 2 Supervisor

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