Describe How virtual memory works?

Answer 1

Virtual memory is a special file on the harddisk that the operating system can swap pages of physical RAM back and forth with to allow more programs than fit in physical RAM to run on the machine at the same time. For example on the computer I am writing this on right now the installed physical RAM is only 1GB, but the virtual memory file is about 50GB. Thats a lot more resident programs and data than the machine could support without the virtual memory!

Answer 2

Virtual memory is software representation of RAM. New OSes do not allow direct access to RAM instead they create Virtual RAM.

Using software, OS can always identify place in real RAM knowing it is Virtual address. It allows to protect memory from unauthorized access.

Answer 3

It is easiest to start with a little computer history.

Back in the old days when there were only big computers (ie before PC's), even the big IBM machines had limited RAM - the reason being the astronomical cost; in those days, a big mainframe might have had about 256KB or 512KB (note the KB, Kilo bytes. 1000KB = 1MB). The company where I worked in the early 1970's ran payroll for 30,000 employees on a 16KB IBM 360 mainframe. A 256KB memory upgrade for a System 360 cost around a million dollars or so if I remember correctly.

Programs needed much more memory to run, but companies could not afford to buy enough. So the idea came along to use space on a fast disk drive (cheap but relatively slow) to extend the available RAM. Any data held in RAM which is not currently being used is 'paged' out to disk and the RAM is freed up to be used by some other program. The system keeps track of which bit of RAM was paged out to which bit of disk. When the data is needed again it is automatically moved back into an available space in RAM from disk.

Thus the system can operate as though it has perhaps ten times as much RAM as it really has, at the expense of running a bit slower. (With Windows the paging space is usually about 1 1/2 times the actual RAM so the total space available is close to double the actual RAM installed, this can be changed).

The amount of total RAM that is used for the process is known as 'Primary Storage or Memory' and the space on disk is known as 'Secondary Storage or Memory'. Some of the RAM does not take part in the paging process (after all the storage management programs must always be available to page things in!) and this is called 'Real Storage or Memory'.

The total RAM + disk memory available is 'Virtual Storage or Memory'.

The name Virtual Memory is applied both to the memory and also to the technique, you need to be aware of the context in a sentence to know which is being talked about.

Hope that helps.

Answer 4

disk