Discussion:
PLO register specifications?
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Charles Mills
2017-10-10 18:04:56 UTC
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The syntax of PLO is of course PLO R1,D2(B2),R3,D4(B4).

For many of the operation codes (5, e.g.) R1 and R3 (in the syntax, not GPRs
1 and 3) are unused. What is the recommended practice for what registers to
specify? It seems odd to me that the PoOp is silent. GPR 0 would be the
obvious choice, but it is specifically dis-recommended for R3 under certain
circumstances. I kind of have a funny gut feeling about specifying a
particular GPR other than 0.

What do people do generally? Or am I missing something in the documentation?

Charles

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Greg Dyck
2017-10-10 18:39:47 UTC
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Post by Charles Mills
For many of the operation codes (5, e.g.) R1 and R3 (in the syntax, not GPRs
1 and 3) are unused. What is the recommended practice for what registers to
specify? It seems odd to me that the PoOp is silent. GPR 0 would be the
obvious choice, but it is specifically dis-recommended for R3 under certain
circumstances. I kind of have a funny gut feeling about specifying a
particular GPR other than 0.
It's acceptable (and normal) to specify 0 as the null specification for
operand R3 when operand R3 is not used by a function, such as as with
PLO function 5, etc.

Regards,
Greg
Rob Scott
2017-10-11 11:54:50 UTC
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Charles

I would recommend hiding naked PLO behind a macro and give the user something more friendly to code.

In my own PLO-esque macros, I used R14, R15 and R2 - although the macro caller could obviously either respecify them via some sort of WORKREGS=(Rx,Ry,Rz) keyword or save and restore regs from working storage.

Rob

-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-***@LISTSERV.UA.EDU] On Behalf Of Charles Mills
Sent: Tuesday, October 10, 2017 7:06 PM
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Subject: PLO register specifications?

The syntax of PLO is of course PLO R1,D2(B2),R3,D4(B4).

For many of the operation codes (5, e.g.) R1 and R3 (in the syntax, not GPRs
1 and 3) are unused. What is the recommended practice for what registers to specify? It seems odd to me that the PoOp is silent. GPR 0 would be the obvious choice, but it is specifically dis-recommended for R3 under certain circumstances. I kind of have a funny gut feeling about specifying a particular GPR other than 0.

What do people do generally? Or am I missing something in the documentation?

Charles

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Peter Relson
2017-10-11 12:14:01 UTC
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Post by Charles Mills
What is the recommended practice for what
registers to specify?
Surely there is no "recommended" practice. You use whatever you feel like
that meets your needs. If something is not allowed, that is what the PoOp
would explain. And possibly would also explain things that might not work
under some circumstances.

Answers to your next question "What do people do generally?" could shed
light on common usage.

Peter Relson
z/OS Core Technology Design


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