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In computer operating systems, memory paging (or swapping on some Unix-like systems) is a memory management scheme by which a computer stores and retrieves data from secondary storage [a] for use in main memory. [1]
Memory protection is a way to control memory access rights on a computer, and is a part of most modern instruction set architectures and operating systems.The main purpose of memory protection is to prevent a process from accessing memory that has not been allocated to it.
In computing, a page fault is an exception that the memory management unit (MMU) raises when a process accesses a memory page without proper preparations. Accessing the page requires a mapping to be added to the process's virtual address space.
However, many modern operating systems implement their memory access-control schemes via paging instead of segmentation, so it is often the case that invalid memory references in operating systems such as Windows are reported via page faults instead of general protection faults. Operating systems typically provide an abstraction layer (such as ...
In addition, in most architectures the page table holds an "access" bit and a "dirty" bit for each page in the page table. The CPU sets the access bit when the process reads or writes memory in that page. The CPU sets the dirty bit when the process writes memory in that page. The operating system can modify the access and dirty bits.
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.
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Thrashing occurs when there are too many pages in memory, and each page refers to another page. Real memory reduces its capacity to contain all the pages, so it uses 'virtual memory'. When each page in execution demands that page that is not currently in real memory (RAM) it places some pages on virtual memory and adjusts the required page on RAM.