> 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.
Actually,
Region First Table
Region Second Table
Region Third Table
Segment Table
Page Table
Richard Corak
