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  Memory Management Unit
       
           (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 cached.
       
          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)
       
       

  memory management unit /mˈɛməɹi mˈanɪdʒmənt jˈuːnɪt/ (MMU /ˌɛmˌɛmjˈuː/)
  Speicherverwaltungseinheit  [comp.]
        "memory management units"  - Speicherverwaltungseinheiten