نظم تشغيل 2- المحاضرة السابعة

Paging
? Paging is a memory-management scheme that permits the physical address space of a
process to be noncontiguous. Paging avoids external fragmentation and the need for
compaction. It also solves the considerable problem of fitting memory chunks of varying
sizes onto the backing store; most memory management schemes used before the introduction
of paging suffered from this problem.
? The problem arises because, when some code fragments or data residing in main memory
need to be swapped out, space must be found on the backing store. The backing store has the
same fragmentation problems discussed in connection with main memory, but access is much
slower, so compaction is impossible. Because of its advantages over earlier methods, paging
in its various forms is used in most operating systems.
? The basic method for implementing paging involves breaking physical memory into fixed-sized
blocks called frames and breaking logical memory into blocks of the same size called pages.
? When a process is to be executed, its pages are loaded into any available memory frames from
the backing store. The backing store is divided into fixed-sized blocks that are of the same size
as the memory frames.
? The hardware support for paging is illustrated in figure below. Every address generated by the
CPU is divided into two parts: a page number (p) and a page offset (d). The page number is
used as an index into a page table.
? The page table contains the base address of each page in physical memory. This base address is
combined with the page offset to define the physical memory address that is sent to the memoryPaging
? Paging is a memory-management scheme that permits the physical address space of a
process to be noncontiguous. Paging avoids external fragmentation and the need for
compaction. It also solves the considerable problem of fitting memory chunks of varying
sizes onto the backing store; most memory management schemes used before the introduction
of paging suffered from this problem.
? The problem arises because, when some code fragments or data residing in main memory
need to be swapped out, space must be found on the backing store. The backing store has the
same fragmentation problems discussed in connection with main memory, but access is much
slower, so compaction is impossible. Because of its advantages over earlier methods, paging
in its various forms is used in most operating systems.
? The basic method for implementing paging involves breaking physical memory into fixed-sized
blocks called frames and breaking logical memory into blocks of the same size called pages.
? When a process is to be executed, its pages are loaded into any available memory frames from
the backing store. The backing store is divided into fixed-sized blocks that are of the same size
as the memory frames.
? The hardware support for paging is illustrated in figure below. Every address generated by the
CPU is divided into two parts: a page number (p) and a page offset (d). The page number is
used as an index into a page table.
? The page table contains the base address of each page in physical memory. This base address is
combined with the page offset to define the physical memory address that is sent to the memory