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A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in the page table. It is the smallest unit of data for memory management in a virtual memory operating system. Virtual memory allows a page that does not currently reside in main memory to be addressed and used. If a program tries to access a location in such a page, an exception called a page fault is generated. The hardware or operating system is notified and loads the required page from the auxiliary store (hard disk) automatically. A program addressing the memory has no knowledge of a page fault or a process following it. Thus a program can address more (virtual) RAM than physically exists in the computer. Virtual memory is a scheme that gives users the illusion of working with a large block of contiguous memory space (perhaps even larger than real memory), when in actuality most of their work is on auxiliary storage (disk). Fixed-size blocks (pages) or variable-size blocks of the job are read into main memory as needed. A transfer of pages between main memory and an auxiliary store, such as a hard disk drive, is referred to as paging or swapping.〔 〕 == Page size trade-off == Page size is usually determined by processor architecture. Traditionally, pages in a system had uniform size, for example 4096 bytes. However, processor designs often allow two or more, sometimes simultaneous, page sizes due to the benefits and penalties. There are several points that can factor into choosing the best page size. ; Page size versus page table size : A system with a smaller page size uses more pages, requiring a page table that occupies more space. For example, if a 232 virtual address space is mapped to 4 KiB (212 bytes) pages, the number of virtual pages is 220 = (232 / 212). However, if the page size is increased to 32 KiB (215 bytes), only 217 pages are required. A multi-level paging algorithm can decrease the memory cost of allocating a large page table for each process by further dividing the page table up into smaller tables, effectively paging the page table. ; Page size versus TLB usage : Since every access to memory must be mapped from virtual to physical address, reading the page table every time can be quite costly. Therefore, a very fast kind of cache, the Translation Lookaside Buffer (TLB), is often used. The TLB is of limited size, and when it cannot satisfy a given request (a ''TLB miss'') the page tables must be searched manually (either in hardware or software, depending on the architecture) for the correct mapping. Larger page sizes mean that a TLB cache of the same size can keep track of larger amounts of memory, which avoids the costly TLB misses. ; Internal fragmentation of pages : Rarely do processes require the use of an exact number of pages. As a result, the last page will likely only be partially full, wasting some amount of memory. Larger page sizes increase the potential for wasted memory this way, as more potentially unused portions of memory are loaded into main memory. Smaller page sizes ensure a closer match to the actual amount of memory required in an allocation. : As an example, assume the page size is 1024 KiB. If a process allocates 1025 KiB, two pages must be used, resulting in 1023 KiB of unused space (where one page fully consumes 1024 KiB and the other only 1 KiB). ; Page size versus disk access : When transferring from a rotational disk, much of the delay is caused by seek time, the time it takes to correctly position the read/write heads above the disk platters. Because of this, large sequential transfers are more efficient than several smaller transfers. Transferring the same amount of data from disk to memory often requires less time with larger pages than with smaller pages. == Determining the page size in a program == Most operating systems allow programs to discover the page size at runtime. This allows programs to use memory more efficiently by aligning allocations to this size and reducing overall internal fragmentation of pages. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Page (computer memory)」の詳細全文を読む スポンサード リンク
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