Re: IEFBR14

Doh! How did I miss that?

I stand corrected. My apologies.



Date: Mon, 29 Nov 2010 18:08:01 +0000
From: BillF@xxxxxxxxxxxx
Subject: Re: IEFBR14
To: IBM-MAIN@xxxxxxxxxxx

Read the linked article again. He did mention XR.

" There were three possible instructions that could be used to zero R15: ``Clear Register R15'', ``Subtract Register R15,R15'', and ``Exclusive Or Register R15,R15''."

Bill Fairchild
Rocket Software

-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@xxxxxxxxxxx] On Behalf Of J R
Sent: Monday, November 29, 2010 10:44 AM
To: IBM-MAIN@xxxxxxxxxxx
Subject: Re: IEFBR14

He also omitted Exclusive Or (XR 15,15), unless that's what he meant by "Clear Register".


Date: Mon, 29 Nov 2010 16:08:33 +0000
From: BillF@xxxxxxxxxxxx
Subject: Re: IEFBR14
To: IBM-MAIN@xxxxxxxxxxx

He meant three possible instructions that only occupied two bytes of storage, I believe ("All three required the same memory and processing cycles. They were equal and interchangeable."). LA is a 4-byte instruction. A number of 4-byte instructions that were available way back when comes to mind: e.g., L R15,=F'0'; LM R15,R15,=F'0'.

Each byte of "core" storage in the 1960s was extremely scarce. He also omitted Subtract Logical Register 15,15, which is a 2-byte instruction and which executed slightly faster on a S/360 model 30 than Subtract Register.

But you are right; there is no Clear Register instruction.

Bill Fairchild
Rocket Software

-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@xxxxxxxxxxx] On Behalf Of Shmuel Metz (Seymour J.)
Sent: Monday, November 29, 2010 7:25 AM
To: IBM-MAIN@xxxxxxxxxxx
Subject: Re: IEFBR14

In <LISTSERV%201011282257541113.0458@xxxxxxxxxxx>, on 11/28/2010
at 10:57 PM, Avram Friedman <ibmsysprog@xxxxxxxxxxxxxx> said:

http://www.miketaylor.org.uk/tech/oreilly/more-iefbr14.html >From one
of the two IBM co-authors
Note not part of the original OS spec added as an after thought

Given the following quote, his memory is not reliable:

There were three possible instructions that could be used to
zero R15: ``Clear Register R15'', ``Subtract Register R15,R15'',
and ``Exclusive Or Register R15,R15''.

There is, of course, no "Clear Register" instruction. The third
obvious instruction is LA.

--
Shmuel (Seymour J.) Metz, SysProg and JOAT
ISO position; see <http://patriot.net/~shmuel/resume/brief.html>
We don't care. We don't have to care, we're Congress.
(S877: The Shut up and Eat Your spam act of 2003)

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