On Mon, 17 Feb 2014 08:02:40 -0800, Charles Mills wrote: > >I got to thinking it would be nice to have a "store different" instruction (or >make store behave this way automatically under the covers) which would >invalidate the cache only if what it were storing were different from what was >in memory already. > >-----Original Message----- >From: IBM Mainframe Discussion List [mailto:[email protected]] On >Behalf Of John McKown >Sent: Monday, February 17, 2014 6:37 AM > >Combining the thoughts engendered from about three replies, I wonder if >avoiding a branch as follows (on a processor which supports the >instructions) would perform better than branching. > >LT R0,CURRENT #LOAD CURRENT AND SET CC >SPM R1 #SAVE CC FROM LT >A R0,SUM #ADD SUM TO IT >IPM R1 #RESTORE CC FROM LT >STOC R0,SUM,NZ #STORE SUM ONLY IF CC OF LT WAS NZ > Doesn't one also want to avoid fetching the line into cache if it's not already there?
I once examined the circuit diagram of some 3rd-party add-on DRAM for a PDP-12 we had. The hardware compared the data to be stored with that already in memory and bypassed the write-back if identical. -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
