On comparing simple paging and simple segmentation on the one hand, with fixed and dynamic partitioning on the other creates foundation for a fundamental breakthrough in memory management. Two characteristics of paging and segmentation are the keys to this breakthrough.
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 an out of main memory such that it occupies different regions of main memory at different times during the course of execution.
A process may be broken up into a number of pieces and these pieces need not be contiguously located in main memory during the execution. The combination of dynamic run time address translation and the use of a page or segment table permit this.
If these two characteristics are present, then it is not necessary that all pages or all the segments of a process be in memory during execution. If a piece that holds the next instruction to be fetched and the piece that holds the next data location to be accessed are in main memory, then at least for a time execution may proceed.
There are two implications, the second more startling than the first, and both lead to improved system utility. More process may be maintained in main memory. Because we are going to load some of the pieces of any particular process, there is room for more processes. This leads to more efficient use of the processor because it is more likely that at least one of the more numerous processes will be in a Ready state at any particular state.
It is possible for a process to be larger than all the main memory. One of the most fundamental restrictions in programming is lifted. A programmer must be acutely aware of how much memory is available. If the program being written is too large, the programmer must devise ways to structure the program into pieces that can be loaded separately in some sort of overlay strategy. With virtual memory based on paging or segmentation, that job is left to the operating system and the hardware. As far as the programmer is concerned, they are dealing with a huge memory associated with disk storage. The operating system automatically loads pieces into main memory as required.
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A process executes only in main memory that memory is referred to as real memory.
