Source : Free On-Line Dictionary of Computing

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 {bit}s 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)