[1] : The Free On-line Dictionary of Computing (27 SEP 03)

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
     
     
see also:
hardware virtual memory paging virtual addresses physical addresses address space 
pages kilobytes bit page number page table 
least recently used replacement algorithm user mode supervisor mode cache 
ram page fault cpu operating system multitasking 
memory management fragmentation integrated circuit mc 68851 motorola 68020 
macintosh ii z8015 zilog z80 motorola 68030 zilog z280 

Fractional Unit >> F Words
Fractional Unit, definition of term: Fractional Unit
fractional+unit_pag1.html

Flash Memory >> F Words
Flash Memory, definition of term: Flash Memory
flash+memory_pag1.html

Line Management >> L Words
Line Management, definition of term: Line Management
line+management_pag1.html

Loudness Unit >> L Words
Loudness Unit, definition of term: Loudness Unit
loudness+unit_pag1.html

Main Memory >> M Words
Main Memory, definition of term: Main Memory
main+memory_pag1.html

Extended Memory >> E Words
Extended Memory, definition of term: Extended Memory
extended+memory_pag1.html

Expanded Memory >> E Words
Expanded Memory, definition of term: Expanded Memory
expanded+memory_pag1.html

Control Unit >> C Words
Control Unit, definition of term: Control Unit
control+unit_pag1.html

Angstrom Unit >> A Words
Angstrom Unit, definition of term: Angstrom Unit
angstrom+unit_pag1.html

Edo Memory >> E Words
Edo Memory, definition of term: Edo Memory
edo+memory_pag1.html

Page 1 2 Next