Several years ago at SHARE, I did a presentation on various requests for new
architecture. Such an instruction to determine accessibility was at the top of
the list. So, I dragged out an architecture instruction template and designed
one on the spot: Given a base register, index register, and long displacement
using a standard RXY instruction format, set a condition code to indicate
whether the access caused an exception.
Oh, plus programming note 1: This instruction is absolutely useless!
Upon reflection, my proposed PN#1 was not totally accurate. If you happened
to be running in a non-virtual environment such as OS/MFT, things would work
just swell. But for any OS that uses paging to present a virtual address space
to the user, only the OS knows the answer to this question. So, you need to use
an OS service to make the determination.
As an alternative to setting up an ESPIE environment, you might want to
consider transactional execution (TX, available on the z12 and up). In addition
to its multiprocessing lock elision features, TX also provides an easy means
for an application to do speculative execution by evading certain
program-interruption conditions. For example:
TBEGIN TDB,2
JNZ ABORTED
... speculatively access data.
TEND
If the code branches to the ABORTED label, a transaction diagnostic block is
stored (except for CC1), and the program can determine what exception condition
caused the abort and whether recovery is likely without OS intervention. See
the TX section of Chapter 5 for more details.
