paged memory management unit

Memory Management Unit
MMU
Paged Memory Management Unit

   <hardware, memory management> (MMU, "Paged Memory Management
   Unit", PMMU) A {hardware} device or circuit that supports
   {virtual memory} and {paging} by translating {virtual
   addresses} into {physical addresses}.

   The virtual {address space} (the range of addresses used by
   the processor) is divided into {pages}, whose size is 2^N,
   usually a few {kilobytes}.  The bottom N {bits} of the address
   (the offset within a page) are left unchanged.  The upper
   address bits are the (virtual) {page number}.  The MMU
   contains a {page table} which is indexed (possibly
   associatively) by the page number.  Each page table entry
   (PTE) gives the physical page number corresponding to the
   virtual one.  This is combined with the page offset to give
   the complete physical address.

   A PTE may also include information about whether the page has
   been written to, when it was last used (for a {least recently
   used} {replacement algorithm}), what kind of processes ({user
   mode}, {supervisor mode}) may read and write it, and whether
   it should be {cache}d.

   It is possible that no physical memory ({RAM}) has been
   allocated to a given virtual page, in which case the MMU will
   signal a "{page fault}" to the {CPU}.  The {operating system}
   will then try to find a spare page of RAM and set up a new PTE
   to map it to the requested virtual address.  If no RAM is free
   it may be necessary to choose an existing page, using some
   {replacement algorithm}, and save it to disk (this is known as
   "{paging}").  There may also be a shortage of PTEs, in which
   case the OS will have to free one for the new mapping.

   In a {multitasking} system all processes compete for the use
   of memory and of the MMU.  Some {memory management}
   architectures allow each process to have its own area or
   configuration of the page table, with a mechanism to switch
   between different mappings on a process switch.  This means
   that all processes can have the same virtual address space
   rather than require load-time relocation.

   An MMU also solves the problem of {fragmentation} of memory.
   After blocks of memory have been allocated and freed, the free
   memory may become fragmented (discontinuous) so that the
   largest contiguous block of free memory may be much smaller
   than the total amount.  With {virtual memory}, a contiguous
   range of virtual addresses can be mapped to several
   non-contiguous blocks of physical memory.

   In early designs memory management was performed by a separate
   {integrated circuit} such as the {MC 68851} used with the
   {Motorola 68020} {CPU} in the {Macintosh II} or the {Z8015}
   used with the {Zilog Z80} family of processors.  Later CPUs
   such as the {Motorola 68030} and the {ZILOG Z280} have MMUs on
   the same IC as the CPU.

   (1999-05-24)