On Fri, 26 Feb 2016 23:03:59 -0600, Tom Marchant wrote: >On Fri, 26 Feb 2016 22:48:43 -0600, Mike Schwab wrote: > >>On Fri, Feb 26, 2016 at 10:37 PM, Tom Marchant >><[email protected]> wrote: >><deleted> >>> In addition, when an instruction references a storage >>> location whose upper bits match the prefix register, those bits are >>> replaced with 0. >>> This last function of prefixing was not implemented on System/360 or the >>> earliest >>> system/370 models. >>> >><deleted> >>> -- >>> Tom Marchant >>About 1985-1989, I remember our mainframe taking a lot of crashes from >>jumps to low address space on a 2 processor pre-XA machine. Was some >>of this caused by this function? > >I doubt it. Do you have a dump? > >IIRC, the address in the prefix register is the address of a page in SQA, and >that any >reference to that SQA page would result in accessing absolute page zero.
BTW, without this reverse prefixing, that SQA page would contain the zero page for a processor, giving you twice the opportunity to overlay real page zero. And now that I think of it, I'm not at all sure that MVS maps any virtual address to that real address. -- Tom Marchant ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
