<hardware, memory management> (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)

Memory Management Unit

Paged Memory Management Unit

In computing, an input–output memory management unit (IOMMU) is a memory management unit (MMU) connecting a direct-memory-access–capable (DMA-capable) I/O bus to the main memory. Like a traditional MMU, which translates CPU-visible virtual addresses to physical addresses, the IOMMU maps device-visible virtual addresses (also called device addresses or I/O addresses in this context) to physical addresses.

In computer science, region-based memory management is a type of memory management in which each allocated object is assigned to a region. A region, also called a zone, arena, area, or memory context, is a collection of allocated objects that can be efficiently reallocated or deallocated all at once.

<memory management, storage> A collection of techniques for providing sufficient memory to one or more processes in a computer system, especially when the system does not have enough memory to satisfy all processes' requirements simultaneously. Techniques include swapping, paging and virtual memory. Memory management is usually performed mostly by a hardware memory management unit. (1995-01-23)

Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when no longer needed.

<storage> Memory above the first megabyte of address space in an IBM PC with an 80286 or later processor. Extended memory is not directly available in real mode, only through EMS, UMB, XMS, or HMA; only applications executing in protected mode can use extended memory directly. In this case, the extended memory is provided by a supervising protected-mode operating system such as Microsoft Windows. The processor makes this memory available through a system of global descriptor tables and local descriptor tables. The memory is "protected" in the sense that memory assigned a local descriptor cannot be accessed by another program without causing a hardware trap. This prevents programs running in protected mode from interfering with each other's memory. A protected-mode operating system such as Windows can also run real-mode programs and provide expanded memory to them. DOS Protected Mode Interface is Microsoft's prescribed method for an MS-DOS program to access extended memory under a multitasking environment. Having extended memory does not necessarily mean that you have more than one megabyte of memory since the reserved memory area may be partially empty. In fact, if your 386 or higher uses extended memory as expanded memory then that part is not in excess of 1Mb. See also conventional memory. (1996-01-10)

<software, storage> (XMM) The memory manager software implementing Extended Memory Specification, such as HIMEM or QEMM386. XMM's can usually also act as A20 handlers. (1996-01-10)

In operating systems, memory management is the function responsible for managing the computer's primary memory.