| slovo | definícia |
paged
(encz) | paged,stránkovaný adj: Zdeněk Brož |
Paged
(gcide) | Page \Page\, v. t. [imp. & p. p. Paged (p[=a]jd); p. pr. & vb.
n. Paging (p[=a]"j[i^]ng).]
To mark or number the pages of, as a book or manuscript; to
furnish with folios.
[1913 Webster] |
paged
(foldoc) | paging
paged
A technique for increasing the memory space
available by moving infrequently-used parts of a program's
working memory from RAM to a secondary storage medium,
usually hard {disk. The unit of transfer is called a page.
A memory management unit (MMU) monitors accesses to memory
and splits each address into a page number (the most
significant bits) and an offset within that page (the lower
bits). It then looks up the page number in its page table.
The page may be marked as paged in or paged out. If it is
paged in then the memory access can proceed after translating
the virtual address to a physical address. If the
requested page is paged out then space must be made for it by
paging out some other page, i.e. copying it to disk. The
requested page is then located on the area of the disk
allocated for "swap space" and is read back into RAM. The
page table is updated to indicate that the page is paged in
and its physical address recorded.
The MMU also records whether a page has been modified since it
was last paged in. If it has not been modified then there is
no need to copy it back to disk and the space can be reused
immediately.
Paging allows the total memory requirements of all running
tasks (possibly just one) to exceed the amount of {physical
memory}, whereas swapping simply allows multiple processes
to run concurrently, so long as each process on its own fits
within physical memory.
(1996-11-22)
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| | podobné slovo | definícia |
rampaged
(encz) | rampaged,běsnil v: Zdeněk Brožrampaged,zuřil v: Zdeněk Brož |
unpaged
(encz) | unpaged,nestránkováno xkomczax |
Equipaged
(gcide) | Equipaged \Eq"ui*paged\, a.
Furnished with equipage.
[1913 Webster]
Well dressed, well bred.
Well equipaged, is ticket good enough. --Cowper.
[1913 Webster] |
Paged
(gcide) | Page \Page\, v. t. [imp. & p. p. Paged (p[=a]jd); p. pr. & vb.
n. Paging (p[=a]"j[i^]ng).]
To mark or number the pages of, as a book or manuscript; to
furnish with folios.
[1913 Webster] |
demand paged
(foldoc) | demand paging
demand paged
A kind of virtual memory where a page
of memory will be paged in if an attempt is made to access
it and it is not already present in main memory. This
normally involves a memory management unit which looks up
the virtual address in a page map to see if it is paged
in. If it is not then the operating system will page it in,
update the page map and restart the failed access. This
implies that the processor must be able to recover from and
restart a failed memory access or must be suspended while some
other mechanism is used to perform the paging.
Paging in a page may first require some other page to be moved
from main memory to disk ("paged out") to make room. If this
page has not been modified since it was paged in, it can
simply be reused without writing it back to disk. This is
determined from the "modified" or "dirty" flag bit in the
page map. A replacement algorithm or policy is used to
select the page to be paged out, often this is the {least
recently used} (LRU) algorithm.
Prepaging is generally more efficient than demand paging.
(1998-04-24)
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paged memory management unit
(foldoc) | Memory Management Unit
MMU
Paged 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)
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