On Tue, 11 Oct 2016 10:39:20 -0700, Charles Mills wrote: >Ah! I am starting to understand. Leap Second steering is accomplished with >the PTFF instruction and is independent of CVTLSO. PTFF appears to slow down >the physical clock. > >So ... steering and CVTLSO are essentially alternatives, right? CVTLSO >should not include any leap seconds that were previously (or were about to >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. Am >I getting this right? > CVTLSO should be the difference between ETOD and UTC regardless of how prior leap second adjustments were accomplished.
If a shop chooses to keep CVTLSO zero, STP will steer ETOD to match UTC within a few hours after a leap second. If a shop chooses to use CVTLSO non-zero, STP (perhaps by non-disclosed techniques) will be signalled by the HMC at a leap second to make user tasks non-dispatchable for a second during which the CPU will add one second to CVTLSO. In principle, a shop could elect to run ETOD at TAI, always ten seconds ahead of IBM's recommendation and maintain CVTLSO at a corresponding ten second higher value. But why? It's not clear why the convention is to add CVTLDTO to ETOD but subtract CVTLSO. Perhaps a designer had a phobia of negative numbers. I believe either Peter Relson or John Eells has explained all this with more detail and probabaly more accurately than I in some previous June or December. -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
