Page 172 - DCAP103_Principle of operating system

Unit 5: Memory Management

5.5.5 Shared Pages Notes
Another advantage of paging is the possibility of sharing common code. This consideration is
particularly important in a time-sharing environment. Consider a system that supports 40 users,
each of whom executes a text editor. If the text editor consists of 150 KB of code and 50 KB of
data space, we would need 8,000 KB to support the 40 users. If the code is reentrant code, Here
we see a three-page editor-each page of size 50 KB; the large page size is used to simplify the
figure-being shared among three processes. Each process has its own data page. Reentrant code
(or pure code) is non-self-modifying code. If the code is reentrant, then it never changes during
execution. Thus, two or more processes can execute the same code at the same time. Each process
has its own copy of registers and data storage to hold the data for the process execution. The
data for two different processes will, of course, vary for each process.

Figure 5.19: Sharing of Code in a Paging Environment

Only one copy of the editor needs to be kept in physical memory. Each user’s page table maps
onto the same physical copy of the editor, but data pages are mapped onto different frames.
Thus, to support 40 users, we need only one copy of the editor (150 KB), plus 40 copies of the
50 KB of data space per user. The total space required is now 2,150 KB, instead of 8,000 KB-a
significant savings. Other heavily used programs can also be shared-compilers, window systems,
run-time libraries, database systems, and so on. To be sharable, the code must be reentrant. The
read-only nature of shared code should not be left to the correctness of the code; the operating
system should enforce this property have difficulty implementing shared memory. Shared
memory is usually implemented as multiple virtual addresses (one for each process sharing
the memory) that are mapped to one physical address. This standard method cannot be used,
however, as there is only one virtual page entry for every physical page, so one physical page
cannot have two (or more) shared virtual addresses. Organizing memory according to pages
provides numerous other benefits in addition to allowing several processes to share the same
physical pages.

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