On Sat, 6 Dec 2025 22:39:51 -0500 Tony Harminc <[email protected]> wrote:
:>On Sat, 6 Dec 2025 at 15:53, Binyamin Dissen < :>[email protected]> wrote: :> :>> Does the cache get flushed if, for example, SAM31 is issued when currently :>> in :>> 31 bit mode or if SAC 512 is issued when in AR mode? :>I think the cases are different. :>For SAC, the PofO says "a serialization and checkpoint-synchronization :>function *is *performed before the operation begins and again after the :>operation is completed", and there is no "model-dependant" wording. I don't :>see that they can fast-path (i.e. skip) these in the "SAC-to-current" case, :>because the lack of the checkpoint-synchronization might in theory be :>detectable by the program. :>SAM, on the other hand, doesn't seem to do anything very expensive in any :>case. There is no cache flush or checkpoint-synch or whatever you want to :>call it - there is just checking of current PSW addressing bits (which is :>surely cheap) to avoid interpreting the NSI address in a too narrow :>addressing mode and effectively doing a wild branch. They could skip that :>without having to say so in the "SAM-to-current" case because it's :>undetectable. But by the same token, it's useless. Maybe they do, and maybe :>they don't, but I'd be very surprised if they bothered. :>What is the cost of an SAM just to be safe? :>It looks very cheap to me. Unlike SAC. :>In passing, I've seen software that issues SAC 0 / SAC 512 many many times :>more often than it needs to, i.e. it really doesn't bother to keep track of :>the current mode. The world didn't fall apart performance-wise. :>Needless to say, this is just my interpretation of the PofO, which is :>probably no better than yours. Perhaps "cache" was the incorrect term. I was referring to the instructions executed ahead of the PSW expecting everything to be the same and disposed of if a branch is unexpectedly taken/not taken. If SAM changes the addressing mode the instructions pre-processed that use addressing must be discarded. -- Binyamin Dissen <[email protected]> http://www.dissensoftware.com Director, Dissen Software, Bar & Grill - Israel ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
