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Overlays are not a method of paging RAM to disk but merely of minimizing the program's RAM use. Subsequent architectures used memory segmentation, and individual program segments became the units exchanged between disk and RAM. A segment was the program's entire code segment or data segment, or sometimes other large data structures.
A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in a page table.It is the smallest unit of data for memory management in an operating system that uses virtual memory.
Eventually, memory fragmentation may lead to complete loss of (application-usable) free memory. Memory fragmentation is a kernel programming level problem. During real-time computing of applications, fragmentation levels can reach as high as 99%, and may lead to system crashes or other instabilities.
A 68451 MMU, which could be used with the Motorola 68010. A memory management unit (MMU), sometimes called paged memory management unit (PMMU), [1] is a computer hardware unit that examines all memory references on the memory bus, translating these requests, known as virtual memory addresses, into physical addresses in main memory.
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.
File system fragmentation is more problematic with consumer-grade hard disk drives because of the increasing disparity between sequential access speed and rotational latency (and to a lesser extent seek time) on which file systems are usually placed. [8] Thus, fragmentation is an important problem in file system research and design.
If the paging unit is enabled, addresses in a segment are now virtual addresses, rather than physical addresses as they were on the 80286. That is, the segment starting address, the offset, and the final 32-bit address the segmentation unit derived by adding the two are all virtual (or logical) addresses when the paging unit is enabled.
A page table is a data structure used by a virtual memory system in a computer to store mappings between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing process , while physical addresses are used by the hardware, or more specifically, by the random-access memory (RAM) subsystem.