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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.
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] In this scheme, the operating system retrieves data from secondary storage in same-size blocks called pages.
The 80286 added an MMU that supports segmentation, but not paging. When segmentation is enabled by turning on protected mode, the segment number acts as an index into a table of segment descriptors; a segment descriptor contains a base physical address, a segment length, a presence bit to indicate whether the segment is currently in memory ...
The general operation of the segmentation unit is otherwise unchanged. The paging unit may be enabled or disabled; if disabled, operation is the same as on the 80286. If the paging unit is enabled, addresses in a segment are now virtual addresses, rather than physical addresses as they were on the 80286.
There is normally one hash table, contiguous in physical memory, shared by all processes. A per-process identifier is used to disambiguate the pages of different processes from each other. It is somewhat slow to remove the page table entries of a given process; the OS may avoid reusing per-process identifier values to delay facing this.
Flat memory model or linear memory model refers to a memory addressing paradigm in which "memory appears to the program as a single contiguous address space." [1] The CPU can directly (and linearly) address all of the available memory locations without having to resort to any sort of bank switching, memory segmentation or paging schemes.
In computer operating systems, demand paging (as opposed to anticipatory paging) is a method of virtual memory management. In a system that uses demand paging, the operating system copies a disk page into physical memory only when an attempt is made to access it and that page is not already in memory (i.e., if a page fault occurs).
Therefore, the total time for paging is near 8 ms (= 8,000 μs). 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.