Johnny Luo wrote: > Hi, > > MVS(Multiple Virtual Storage) is the basic concept for z/os.However,after > entering > the mainframe world for eight months I still cannot understand it > thoroughly，especially > for ' multiple address spaces'. > > To make things simple,I'll assume 31-bit addressing only. > > 1,MVS allow different task to have their own address space.While the number > of address > space increases in system,the total amount of virtual storage can become > very big. > However,as I know,virtual storage is supported by real storage and auxiliary > storage.With > 31-bit addressing,the max size of real storage or auxiliary storage is 2G. > So,the max size of virtual storage=max real storage+max auxiliary > storage=4G > Does this mean that the total size of all the address spaces cannot > exceed 4G although > in theory each address space can have a max size of 2G?
Johnny, Your assumption here is wrong. The maximum size of virtual is 2G per address space. The limit on aux is much larger: paging data sets (what you call "aux") are disk files that back up the pages not currently in real storage. So with lots of page data sets you can have lots of address spaces, each with a max of 2G virtual. Only the pages currently in use are kept in real storage; the other pages are either backed up on DASD or exist only in potential (if you only need 200M for your work, you will not have 2G of empty virtual taking up space on paging DASD). > > 2,Take a TSO user's address space for example.Suppose its size is > 5M.Whenthe user logon > to tso,the address space is established which means 5M virtual storage space > is allocated for > this address space.So this 5M space is being mapped using real storage plus > auxiliary storage > even if the user doesn't do any job after logon? No. See above. If user is idle for a while, their pages are written to DASD, thus not taking any real storage space; after some period of inactivity, they may be swapped out, meaning almost everything of there's is on DASD, not real. When the user gets active, pages are swapped or paged back into real only as needed (demand paging). Hope this helps. Kind regards, -Steve Comstock ---------------------------------------------------------------------- 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
