> -----Original Message----- > From: IBM Mainframe Discussion List > [mailto:[EMAIL PROTECTED] On Behalf Of Binyamin Dissen > Sent: Wednesday, May 30, 2007 1:33 PM > To: [email protected] > Subject: Re: Is there a cost in switching AMODEs? > > > On Wed, 30 May 2007 10:39:48 -0700 Edward Jaffe > <[EMAIL PROTECTED]> > wrote: > > :>Binyamin Dissen wrote: > :>> :>In my experience, excessive switching of addressing > mode is quite > :>> :>expensive. I suspect it's because the entire pipeline > must be flushed. > > :>> I didn't see that in the POPs. > > :>AFAIK, the Principles of Operation does not discuss > processor pipeline > :>considerations for any instruction. There is some cursory > information > :>here and there regarding instruction prefetch. That's about it. > > Why would an AMODE change require a pipline flush? It doesn't > branch anywhere. > The current pipline should be valid. > > -- > Binyamin Dissen <[EMAIL PROTECTED]>
I am not a hardware person, but with the pipeline, instructions are executed physically out-of-order albeit logically in-order. If you switch AMODEs, a down stream instruction such as a LOAD may need to be reexecuted because the logical address changed (i.e. the 64 bit address became a 31 bit or vice versa) and the correct data is now needed? Or at least the correct logical address. Or even just a simple LA might have a different result, depending on the AMODE. Just my guess that flushing the pipeline is an "easy" way to ensure that downstream instructions' partial results are redone "just in case". -- John McKown Senior Systems Programmer HealthMarkets Keeping the Promise of Affordable Coverage Administrative Services Group Information Technology The information contained in this e-mail message may be privileged and/or confidential. It is for intended addressee(s) only. If you are not the intended recipient, you are hereby notified that any disclosure, reproduction, distribution or other use of this communication is strictly prohibited and could, in certain circumstances, be a criminal offense. If you have received this e-mail in error, please notify the sender by reply and delete this message without copying or disclosing it. ---------------------------------------------------------------------- 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
