Doh! How did I miss that? I stand corrected. My apologies.
> Date: Mon, 29 Nov 2010 18:08:01 +0000 > From: [email protected] > Subject: Re: IEFBR14 > To: [email protected] > > 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:[email protected]] On Behalf > Of J R > Sent: Monday, November 29, 2010 10:44 AM > To: [email protected] > 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: [email protected] > > Subject: Re: IEFBR14 > > To: [email protected] > > > > 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:[email protected]] On Behalf > > Of Shmuel Metz (Seymour J.) > > Sent: Monday, November 29, 2010 7:25 AM > > To: [email protected] > > Subject: Re: IEFBR14 > > > > In <listserv%[email protected]>, on 11/28/2010 > > at 10:57 PM, Avram Friedman <[email protected]> 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) ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
