On 18 October 2016 at 13:15, Paul Gilmartin
<0000000433f07816-dmarc-requ...@listserv.ua.edu> wrote:
>   I find:
>     z/OS 2.2.0
>     z/OS MVS
>     z/OS MVS Programming: Assembler Services Guide
>     Understanding 31-bit addressing
>     Understanding the use of central storage
>     Central storage considerations for user programs
>     Load real address (LRA) instruction

As suggested, you need to look at the Principles of Operation. LRA is
a matter of hardware, not MVS. Of course the MVS environment
potentially affects some things, but LRA is entirely defined by the
hardware.

> Woe be unto anyone who did LRA; LA in AMODE 24  (But why?.  And it
> doesn't discuss AR mode.  Perhaps elsewhere, but my curiosity is exhausted.

POO.

> Which leaves a historical question.  Was AMODE 31 antedated by the
> availability of real storage >16 MiB,

Assuming you mean the hardware addressing mode, rather than the MVS
Assembler and Binder's notion of AMODE for modules, Yes. But in a
quirky way. In the pre-XA era, there was a time when 64 MB of real
storage could be addressed by DAT tables, but not by channels or
instructions running with DAT off.

> or was AMODE 64 antedated by the availability of real storage >2GiB?

How is this an "or"? You sound like a lawyer at bar: "My client did
not kill Mr X, or in the alternative, if he did kill Mr X he did it
with justification." Maybe I just need to apply De Morgan to your
sentence...

> How did LRA(G) operate in those eras?

LRA provided a 26-bit real address (padded on the left to 32 bits) in
the 64 MiB era.

> And in a virtual guest with shadow page tables, does LRA return the
> real real address or a virtual real address?

Always a virtual real address.

> I suppose it's all emulated by CP anyway, so it depends.

No, it doesn't depend. What sense would it make to return a real real
address to LRA? If you want a real real address you need to use a
conceptually different level of query; a meta-query, one might say.
Normally this would be a Diagnose.

> Or maybe SIE.  My head hurts.

With (XA+) or without (370 pre-XA) SIE, the result is the same. With
SIE it's all handled by the "hardware". Pre SIE, since LRA is
privileged, and the guest always runs in real problem state, CP
provides the answer.

Tony H.

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