Os II Lec8-Paging and segmentation

PAGING
Both unequal fixed-size and variable-size partitions are inefficient in the use of memory; the former results in internal fragmentation, the latter in external fragmentation. Suppose, however, that main memory is partitioned into equal fixed-size chunks that are relatively small, and that each process is also divided into small fixed-size chunks of the same size. Then the chunks of a process, known as pages, could be assigned to available chunks of memory, known as frames, or page frames. We show in this section that the wasted space in memory for each process is due to internal fragmentation consisting of only a fraction of the last page of a process. There is no external fragmentation.
Figure 7.9 illustrates the use of pages and frames. At a given point in time, some of the frames in memory are in use and some are free. A list of free frames is maintained by the operating system. Process A, stored on disk, consists of four pages. When it comes time to load this process, the operating system finds four free frames and loads the four pages of process A into the four frames (Figure 7.9b). Process B, consisting of three pages, and process C, consisting of four pages, are subsequently loaded. Then process B is suspended and is swapped out of main memory. Later, all of the processes in main memory are blocked, and the operating system needs to bring in a new process, process D, which consists of five pages.
Now suppose, as in this example, that there are not sufficient unused contiguous frames to hold the process. Does this prevent the operating system from loading D? The answer is no, because we can once again use the concept of logical address. A simple base address register will no longer suffice. Rather, the operating system maintains a page table for each process. The page table shows the frame location for each page of the process. Within the program, each logical address consists of a page number and an offset within the page. Recall that in the case of simple partition, a logical address is the location of a word relative to the beginning of the program; the processor translates that into a physical address. With paging, the logical-to-physical address translation is still done by processor hardware. Now