Editing Memory management unit (section)

===Sun MMU===
The original [[Sun-1]] is a [[single-board computer]] built around the [[Motorola 68000]] microprocessor and introduced in 1982. It includes the original Sun 1 memory management unit that provides address translation, memory protection, memory sharing and memory allocation for multiple processes running on the CPU.  All access of the CPU to private on-board RAM, external [[Multibus]] memory, on-board [[I/O]] and the Multibus I/O runs through the MMU, where address translation and protection are done in a uniform fashion. The MMU is implemented in hardware on the CPU board.

The MMU consists of a context register, a [[Memory segmentation|segment]] map and a page map.  Virtual addresses from the CPU are translated into intermediate addresses by the segment map, which in turn are translated into physical addresses by the page map.  The page size is {{nowrap|2 KB}} and the segment size is {{nowrap|32 KB}} which gives 16 pages per segment.  Up to 16 contexts can be mapped concurrently.  The maximum logical address space for a context is {{nowrap|1024 pages}} or {{nowrap|2 MB.}}  The maximum physical address that can be mapped simultaneously is also {{nowrap|2 MB.}}

The context register is important in a multitasking operating system because it allows the CPU to switch between processes without reloading all the translation state information.  The 4-bit context register can switch between 16 sections of the segment map under supervisor control, which allows 16 contexts to be mapped concurrently.  Each context has its own virtual address space.  Sharing of virtual address space and inter-context communications can be provided by writing the same values in to the segment or page maps of different contexts.  Additional contexts can be handled by treating the segment map as a context cache and replacing out-of-date contexts on a least-recently used basis.

The context register makes no distinction between user and supervisor states.  Interrupts and traps do not switch contexts, which requires that all valid interrupt vectors always be mapped in page 0 of context, as well as the valid supervisor stack.<ref>Sun 68000 Board User's Manual, Sun Microsystems, Inc, February 1983, Revision B</ref>

The [[Sun-2]] workstations are similar; they are built around the [[Motorola 68010]] microprocessor and have a similar memory management unit, with {{nowrap|2 KB}} pages and {{nowrap|32 KB}} segments.  The context register has a 3-bit system context used in supervisor state and a 3-bit user context used in user state.<ref>{{cite web|url=http://bitsavers.org/pdf/sun/sun2/Sun-2_Architecture_Manual_Dec83.pdf|title=Sun-2 Architecture Manual|publisher=[[Sun Microsystems]]|date=15 December 1983|pages=9–12}}</ref>

The [[Sun-3]] workstations, except for the Sun-3/80, Sun-3/460, Sun-3/470, and Sun-3/480, are built around the [[Motorola 68020]], and have a similar memory management unit.  The page size is increased to {{nowrap|8 KB}}.  (The later models are built around the [[Motorola 68030]] and use the 68030's on-chip MMU.)

The [[Sun-4]] workstations are built around various [[SPARC]] microprocessors, and have a memory management unit similar to that of the Sun-3 workstations.