نظم تشغيل 2- المحاضرة التاسعة

Virtual memory:
Virtual memory is a technique that allows the execution of processes that are not completely in
memory. One major advantage of this scheme is that programs can be larger than physical memory.
Further,
1- Virtual memory abstracts main memory into an extremely large, uniform array of storage,
separating logical memory as viewed by the user from physical memory.
2- This technique frees programmers from the concerns of memory-storage limitations.
3- Virtual memory also allows processes to share files easily and to implement shared memory.
4- In addition, it provides an efficient mechanism for process creation.
5- Virtual memory is not easy to implement, however, and may substantially decrease performance
if it is used carelessly.
6- Involves the separation of logical memory as perceived by users from physical memory. This
separation allows an extremely large virtual memory to be provided for programmers when only
a smaller physical memory is available.
7- Virtual memory makes the task of programming much easier, because the programmer no longer
needs to worry about the amount of physical memory.
Background
An examination of real programs shows us that, in many cases, the entire program is not needed. For
instance, consider the following:
1- Programs often have code to handle unusual error conditions. Since these errors seldom, if ever,
occur in practice, this code is almost never executed.
2- Arrays, lists, and tables are often allocated more memory than they actually need. An array may
be declared 100 by 100 elements, even though it is seldom larger than 10 by 10 elements. An
assembler symbol table may have room for 3,000 symbols, although the average program has less
than 200 symbols.
3- Certain options and features of a program may be used rarely. For instance, the routines on U.S.
government computers that balance the budget are only rarely used. Even in those cases where
the entire program is needed, it may not all be needed at the same time (such is the case with
overlays, for example).
The ability to execute a program that is only partially in memory would confer many benefits:
1- A program would no longer be constrained by the amount of physical memory that is available.
Users would be able to write programs for an extremely large virtual address space, simplifying
the programming task.
2- Because each user program could take less physical memory, more programs could be run at the
same time, with a corresponding increase in CPU utilization and throughput but with no increase
in response time or turnaround time.
3- Less I/O would be needed to load or swap each user program into memory, so each user program
would run faster. Thus, running a program that is not entirely in memory would benefit both the
system and the user