Question

Question from OS

In a system that uses paging, a process cannot access memory that belongs to another process. Why? Why is it theoretically possible with segmentation but not paging?

Answer 1

An address on a paging system is a logical page number and an offset. The physical page is found by searching a table based on the logical page number to produce a physical page number. Because the operating system controls the contents of this table, it can limit a process to accessing only those physical pages allocated to the process. There is no way for a process to refer to a page it does not own because the page will not be in the page table. To allow such access, an operating system simply needs to allow entries for non-process memory to be added to the process page table. This is useful when two or more processes need to exchange data they just read and write to the same physical addresses (which may be at varying logical addresses). This makes for very efficient inter process communication. With segmentation, you only need a small amount of information - the segment descriptor - to share, instead of changing protection on a large number of pages. In addition, you don't have to worry about mapping the code to the same address - the addresses within the code are relative to the segment, so it can be relocated more easily.