Just found this in the V1.13 Migration manual:
*Description:* In parmlib member DIAGxx, you can specify the virtual
location of the LCCA control block (mapped by the IHALCCA macro) or the
location of the */_PCCA_/* control block (mapped by the IHAPCCA macro)
to be used when a CPU is brought online. Before z/OS V1R12, if the
IHALCCA or IHAPCCA structures specified on the CBLOC VIRTUAL24 or
VIRTUAL31 keyword of DIAGxx did not specify either the 24-bit
(VIRTUAL24) or 31-bit (VIRTUAL31) location, the default location for the
LCCA or */_PCCA_/* was in 24-bit virtual storage.
Beginning with z/OS V1R12, the default location for the LCCA and
*/_PCCA_/* is now in 31-bit virtual storage if you do not specify
VIRTUAL24 or VIRTUAL31 for the structures IHALCCA and IHAPCCA. If only
one standard processor is online during the IPL, the LCCA and */_PCCA_/*
of the IPL CPU will be in 24-bit storage regardless of the
specification. If there is more than one processor, and DIAGxx permits
it, you'll have the LCCA and */_PCCA_/* in 31-bit storage.
--
Peter Hunkeler
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