Editing Memory management unit (section)

=== VAX ===
[[VAX]] pages are 512 bytes,<ref name="vax-ref-manual">{{Cite book |url=http://bitsavers.org/pdf/dec/vax/archSpec/EY-3459E-DP_VAX_Architecture_Reference_Manual_1987.pdf |chapter=Chapter 4 Memory Management |title=VAX Architecture Reference Manual |date=1987 |publisher=[[Digital Equipment Corporation]] |id=EY-3459E-DP |isbn=0-932376-86-X}}</ref>{{rp|page=199}} which is very small. An OS may treat multiple pages as if they were a single larger page. For example, [[Linux]] on VAX groups eight pages together. Thus, the system is viewed as having {{nowrap|4 KB}} pages. The VAX divides memory into four fixed-purpose regions, each {{nowrap|1 [[Gigabyte|GB]]}} in size. They are:<ref name="vax-ref-manual" />{{rp|pages=200-201}}
;P0 space: Used for general-purpose per-process memory such as heaps.
;P1 space: (Or control space) which is also per-process and is typically used for supervisor, executive, [[kernel (operating system)|kernel]], user [[Stack (abstract data type)|stacks]] and other per-process control structures managed by the operating system.
;S0 space: (Or system space) which is global to all processes and stores operating system code and data, whether paged or not, including pagetables.
;S1 space: Which is unused and "Reserved to [[Digital Equipment Corporation|Digital]]".<ref name="vax-ref-manual" />{{rp|pages=200-201}}

Page tables are big linear arrays.<ref name="vax-ref-manual" />{{rp|pages=209-215}} Normally, this would be very wasteful when addresses are used at both ends of the possible range, but the page tables for P0 and P1 space are stored in the paged S0 space.<ref name="vax-ref-manual" />{{rp|pages=211-212}} Thus, there is effectively a two-level [[Tree (data structure)|tree]], allowing applications to have sparse memory layout without wasting a lot of space on unused page table entries. Unlike page table entries in most MMUs, page table entries in the VAX MMU lack an [[accessed bit]].<ref name="vax-ref-manual" />{{rp|pages=203-205}} OSes which implement paging must find some way to emulate the accessed bit if they are to operate efficiently. Typically, the OS will periodically unmap pages so that page-not-present faults can be used to let the OS set an accessed bit.