Sorry for the incomplete post. I had a finger check... >Sorry for my re-visit to this thread cause today I read something about >FIXED and DREF virtual storage area and feel that something should be >added. > >For FIXED area, it's no longer true: when you GETMAIN, all pages will be >backed up by real frames because FIXED area is supposed to serve disabled >user.
Remember, I wrote I'll discuss the standard problem state program case... There are many more features and special cases. Fixed storage being one of them. Fixed storage is not only to support diabled users but much more often used in the ubiquituos I/O processing. The channel subsystem (the I/O part of System z hardware) does not use DAT. Channel commands transfer data blocks from and to real storage to and from I/O devices, resp. Before the I/O can be initiated, MVS's I/O supervisor code has to make sure the virtual storage allocated for the I/O buffers is not being paged out while the channel subsystem is working on the I/O request. Therefore, the pages will be fixed before the I/O supervisor passes the I/O request to the channel subsystem and will be unfixed thereafter. >And in my opinion, that's why DREF is superior to FIXED: it's backed up >by real frames only when you access it for the first time (like pageable >area). And MVS has special facility to make it suitable for disabled user. DREF was introduced when real storage still was a scarce resource but when systems already had quite some expanded storage. DREF storage was pageable storage but it could only be paged to expanded storage and never to aux storage. Thus a page fault could be resolved without doing I/O and could be done synchroneously (a page move). This allowed code running in disbled state to relieve some fixed storage by using DREF instead. >Of course like FIXED area, after first reference DREF pages will be fixed >there unless released or swapped out (for private area). No, its pageable storage. Since z/OS does not support expanded storage, DREF is much like fixed. (I do not know the difference in its handling, if any, off hand). -- Peter Hunkeler Credit Suisse ---------------------------------------------------------------------- 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
