4.8 MULTI-USER PAGING

Up to this point, we have looked at the effect of paging on a single process and on the global paging characteristics. This section expands the picture by examining how paging operates in an environment of multiple users. Each process has its own pair of page tables for PO and P1. Figure 4.21 shows two processes, and for simplicity's sake, each process is shown with a single PO page table, Some of the pages are valid and in memory, and are shaded in the figure. Some pages are not valid. Remember that there must be one PTE for every page in the image even if that page is not currently in memory. As mentioned, there is no reason the process' pages residing in physical memory should be assigned with any pattern, so the frames are shown scattered in memory.

A fortunate side effect of paging is that the two processes cannot possibly interfere with one another. This means that there is no possibility for COSMOS to accidentally or deliberately access the frames used by REGI because the two virtual processes are kept physically separated in memory by their page tables. To see why, examine Figure 4.21 again. Notice what happens when Process REGI runs off the end of page 3 in memory. Does it start executing instructions in the frame owned by Process COSMOS? At first glance, it might seem so because the two frames are adjacent in memory. But the continuity of a process is governed...