OS II Lec9-Virtual Memory and Thrashing

Virtual memory
Virtual memory
A storage allocation scheme in which secondary memory can be addressed as though it were part of main memory. The addresses a program may use to reference memory are distinguished from the addresses the memory system uses to identify physical storage sites, and program-generated addresses are translated automatically to the corresponding machine addresses. The size of virtual storage is limited by the ad- dressing scheme of the computer system and by the amount of secondary memory available and not by the actual number of main storage locations.
Comparing simple paging and simple segmentation, on the one hand, with fixed and dynamic partitioning, on the other, we see the foundation for a fundamental break- through in memory management. Two characteristics of paging and segmentation are the keys to this breakthrough:
1. All memory references within a process are logical addresses that are dynamically translated into physical addresses at run time. This means that a process may be swapped in and out of main memory such that it occupies different regions of main memory at different times during the course of execution. ?
2. A process may be broken up into a number of pieces (pages or segments) and these pieces need not be contiguously located in main memory during execution. The combination of dynamic run-time address translation and the use of a page or segment table permits this. ?Now we come to the breakthrough. If the preceding two characteristics are present, then it is not necessary that all of the pages or all of the segments of a process be in main memory during execution. If the piece (segment or page) that holds the next instruction to be fetched and the piece that holds the next data location to be ac- cessed are in main memory, then at least for a time execution may proceed.