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Lexikon

Memory Management Unit

e, memory management> (MMU, "Paged Memory Management Unit", PMMU) A ef="module.php?name=Lexikon&file=search&eid=1&query=hardware">hardware device or circuit that supports ef="module.php?name=Lexikon&file=search&eid=1&query=virtual memory">virtual memory and ef="module.php?name=Lexikon&file=search&eid=1&query=paging">paging by translating {virtual addresses} into {physical addresses}. The virtual ef="module.php?name=Lexikon&file=search&eid=1&query=address space">address space (the range of addresses used by the processor) is divided into ef="module.php?name=Lexikon&file=search&eid=1&query=pages">pages, whose size is 2^N, usually a few ef="module.php?name=Lexikon&file=search&eid=1&query=kilobytes">kilobytes. The bottom N ef="module.php?name=Lexikon&file=search&eid=1&query=bit">bits of the address (the offset within a page) are left unchanged. The upper address bits are the (virtual) ef="module.php?name=Lexikon&file=search&eid=1&query=page number">page number. The MMU contains a ef="module.php?name=Lexikon&file=search&eid=1&query=page table">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 ef="module.php?name=Lexikon&file=search&eid=1&query=cache">cached. It is possible that no physical memory (ef="module.php?name=Lexikon&file=search&eid=1&query=RAM">RAM) has been allocated to a given virtual page, in which case the MMU will signal a "ef="module.php?name=Lexikon&file=search&eid=1&query=page fault">page fault" to the ef="module.php?name=Lexikon&file=search&eid=1&query=CPU">CPU. The ef="module.php?name=Lexikon&file=search&eid=1&query=operating system">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 ef="module.php?name=Lexikon&file=search&eid=1&query=replacement algorithm">replacement algorithm, and save it to disk (this is known as "ef="module.php?name=Lexikon&file=search&eid=1&query=paging">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 ef="module.php?name=Lexikon&file=search&eid=1&query=multitasking">multitasking system all processes compete for the use of memory and of the MMU. Some ef="module.php?name=Lexikon&file=search&eid=1&query=memory management">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 ef="module.php?name=Lexikon&file=search&eid=1&query=fragmentation">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 ef="module.php?name=Lexikon&file=search&eid=1&query=virtual memory">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 ef="module.php?name=Lexikon&file=search&eid=1&query=integrated circuit">integrated circuit such as the ef="module.php?name=Lexikon&file=search&eid=1&query=MC 68851">MC 68851 used with the ef="module.php?name=Lexikon&file=search&eid=1&query=Motorola 68020">Motorola 68020 ef="module.php?name=Lexikon&file=search&eid=1&query=CPU">CPU in the ef="module.php?name=Lexikon&file=search&eid=1&query=Macintosh II">Macintosh II or the ef="module.php?name=Lexikon&file=search&eid=1&query=Z8015">Z8015 used with the ef="module.php?name=Lexikon&file=search&eid=1&query=Zilog Z80">Zilog Z80 family of processors. Later CPUs such as the ef="module.php?name=Lexikon&file=search&eid=1&query=Motorola 68030">Motorola 68030 and the ef="module.php?name=Lexikon&file=search&eid=1&query=ZILOG Z280">ZILOG Z280 have MMUs on the same IC as the CPU. (1999-05-24)

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