MEMORY MANAGEMENT UNIT

Domain	Definition
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 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 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). Source: The Free On-line Dictionary of Computing.
Source: compiled by the editor from various references; see credits.

Hexadecimal (or equivalents, 770AD-1900s) (references)

4D 45 4D 4F 52 59 4D 41 4E 41 47 45 4D 45 4E 54 55 4E 49 54

Leonardo da Vinci (1452-1519; backwards) (references)

Binary Code (1918-1938, probably earlier) (references)

01001101 01000101 01001101 01001111 01010010 01011001 00100000 01001101 01000001 01001110 01000001 01000111 01000101 01001101 01000101 01001110 01010100 00100000 01010101 01001110 01001001 01010100

HTML Code (1990) (references)

&#77 &#69 &#77 &#79 &#82 &#89 &#32 &#77 &#65 &#78 &#65 &#71 &#69 &#77 &#69 &#78 &#84 &#32 &#85 &#78 &#73 &#84

ISO 10646 (1991-1993) (references)

004D 0045 004D 004F 0052 0059 004D 0041 004E 0041 0047 0045 004D 0045 004E 0054 0055 004E 0049 0054

Encryption (beginner's substitution cypher): (references)

Domain	Title
Books	Mc68851 Paged Memory Management Unit User's Manual (reference) (more book examples)
Source: compiled by the editor from various references; see credits.

MEMORY MANAGEMENT UNIT

Specialty Definition: MEMORY MANAGEMENT UNIT

Computing

Crosswords: MEMORY MANAGEMENT UNIT

Commercial Usage: MEMORY MANAGEMENT UNIT

Books

Expression: MEMORY MANAGEMENT UNIT

Alternative Orthography: MEMORY MANAGEMENT UNIT

4D 45 4D 4F 52 59 4D 41 4E 41 47 45 4D 45 4E 54 55 4E 49 54

01001101 01000101 01001101 01001111 01010010 01011001 00100000 01001101 01000001 01001110 01000001 01000111 01000101 01001101 01000101 01001110 01010100 00100000 01010101 01001110 01001001 01010100

&#77 &#69 &#77 &#79 &#82 &#89 &#32 &#77 &#65 &#78 &#65 &#71 &#69 &#77 &#69 &#78 &#84 &#32 &#85 &#78 &#73 &#84

004D 0045 004D 004F 0052 0059 004D 0041 004E 0041 0047 0045 004D 0045 004E 0054 0055 004E 0049 0054

473947495259247354835413947394854255484354

INDEX

Expression using "MEMORY MANAGEMENT UNIT": paged Memory Management Unit. Additional references.
Source: compiled by the editor from various references; see credits.