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What's the rationale here?

We're trying to get the address of a common region.

COMRG

L? ? ?1,20
SVC? ?33

LR? ? R5,R1
USING COMREG,R5

The first thing it does is load a value from absolute address 20.

And then it does an SVC 33 - which provides a level of
abstraction I think. But you've just defeated that abstraction
by accessing an absolute address. If you're willing to start
at an absolute address, you may as well have your control
blocks chained off that. Perhaps this isn't considered to be
a control block - it is specific to this program and there
could be a chain of such programs.

Or alternatively - use the abstraction - SVC 33 can go and
inspect low memory itself?

Or is that not how the PSA works when there are multiple processors
or something like that?

Thanks. Paul.