Marten Kemp <[EMAIL PROTECTED]> writes: > The recent thread about virtual memory sparked a (kind of) > idle question: why did the implementation in the S/370 > have a two-level scheme (segment and page)? My original > thought was that it facilitated definition of discontiguous > parts of an address space.
Well, mostly it is because smaller systems don't have enough real memory to hold a one level page table. The segment/page system allows page tables to be paged out, with the invalid bit in the segment table. VAX uses a two level system where page tables are paged. There is kernel space, which isn't paged and holds the first level tables referencing pagable second level tables. z/Archtecture has three levels. -- glen