#@hardware, memory management@# (MMU, "Paged Memory Management Unit", PMMU) A hardware device or circuit that supports the processor) is divided into pages, whose size is 2^N, (the offset within a page) are left unchanged. The upper 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 It is possible that no physical memory ( RAM) has been allocated to a given virtual page, in which case the MMU will 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 " paging"). There may also be a shortage of PTEs, in which case the OS will have to free one for the new mapping. 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. 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 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 used with the Zilog Z80 family of processors. Later CPUs the same IC as the CPU. (1999-05-24)
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