On Mon, 21 Apr 2014 11:44:01 -0500, Paul Gilmartin wrote: >On Mon, 21 Apr 2014 07:01:50 -0500, Tom Marchant wrote: > >>Real storage with real addresses < 2 GiB are below the bar. >>Real storage with real addresses > 2 GiB are above the bar. >> >ITYM ">= 2 GiB"
Right. >But what do you call real storage with a real address = 3 GiB or >thereabouts? "Nothing special." The real address is above 2 GiB. >I understand that for historic reasons, pertaining >only to z/OS, not other OSes, 2 GiB <= virtual address < 4GiB >is prohibited, Prohibited? I think that is too strong a word. IARV64 would not return storage with a virtual address in that range. >and GETMAIN (STORAGE?) for even 64-bit >addressable storage The service for obtaining storage above the bar is IARV64. >will not return storage in that area. Would not return storage in that virtual range. The real storage backing a request could be in that range. Now that range and more (to 32 GiB) is reserved for the use of Java. And there is an undocumented parameter on IARV64, USE2GTO32G, to allow storage to be returned in that virtual storage range. >But why? The excuse that I heard was that it was to avoid confusion. Something about the chance That a program with a 64-bit address that used it improperly, clearing bit 32, would incorrectly address the wrong location. My opinion is that it has caused far more confusion. I have seen code that examined an address, and if it found that some of bits 0-31 were non-zero, while bit 32 was one, it declared the address to be invalid. >What harm would befall if it did? None, IMO. And we wouldn't have people who believe to this day that the bar is "2 gig think". >>A GETMAIN request can specify whether the storage is to be backed by real >>storage < 16 M, <2 G, or anywhere. >> >Why should the application even care? Most don't. There are a small number of reasons for having real storage located below the line or below the bar. Actually, when LOC is specified, the real storage to back it is obtained from anywhere until the storage is page-fixed, then LOC is honored. >Doesn't virtual storage appear >the same regardless where it's physically backed? Yes. -- Tom Marchant ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
