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 > > ---------------------------------------------------------------------- > 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 ---------------------------------------------------------------------- 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 ---------------------------------------------------------------------- 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
