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In a system using segmentation, computer memory addresses consist of a segment id and an offset within the segment. [3] A hardware memory management unit (MMU) is responsible for translating the segment and offset into a physical address, and for performing checks to make sure the translation can be done and that the reference to that segment and offset is permitted.
Paging allows the cumulative total of virtual address spaces to exceed physical main memory. A process can store data in memory-mapped files on memory-backed file systems, such as the tmpfs file system or file systems on a RAM drive, and map files into and out of the address space as needed.
Since the base is set to 0 in all cases and the limit 4 GiB, the segmentation unit does not affect the addresses the program issues before they arrive at the paging unit. (This, of course, refers to 80386 and later processors, as the earlier x86 processors do not have a paging unit.) Current Linux also uses GS to point to thread-local storage.
A similar mechanism is used for memory-mapped files, which are mapped to virtual memory and loaded to physical memory on demand. When physical memory is not full this is a simple operation; the page is written back into physical memory, the page table and TLB are updated, and the instruction is restarted.
Similarly, a page frame is the smallest fixed-length contiguous block of physical memory into which memory pages are mapped by the operating system. [1] [2] [3] A transfer of pages between main memory and an auxiliary store, such as a hard disk drive, is referred to as paging or swapping. [4]
In computing, mmap(2) is a POSIX-compliant Unix system call that maps files or devices into memory. It is a method of memory-mapped file I/O. It implements demand paging because file contents are not immediately read from disk and initially use no physical RAM at all.
If the memory access time is 0.2 μs, then the page fault would make the operation about 40,000 times slower. Performance optimization of programs or operating systems often involves reducing the number of page faults. Two primary focuses of the optimization are reducing overall memory usage and improving memory locality.
A terminal pager, paging program or simply pager is a computer program used to view (but not modify) the contents of a text file moving down the file one line or one screen at a time. Some, but not all, pagers allow movement up a file. [ 1 ]