Question

Virtual memory is a good concept currently used by modern operating systems. But I was stuck answering a question and was not sure enough about it. Here is the question:

Suppose there are only a few applications running on a machine, such that the
physical memory of system is more than the memory required by all the
applications. To support virtual memory, the OS needs to do a lot work. So if
the running applications all fit in the physical memory, is virtual memory
really needed?

(Furthermore, the applications running together will always fit in RAM.)

Answer 1

Even when the memory usage of all applications fits in physical memory, virtual memory is still useful. VM can provide these features:

• Privileged memory isolation (every app can’t touch the kernel or memory-mapped hardware devices)
• Interprocess memory isolation (one app can’t see another app’s memory)
• Static memory addresses (e.g. every app has main() at address 0x0800 0000)
• Lazy memory (e.g. pages in the stack are allocated and set to zero when first accessed)
• Redirected memory (e.g. memory-mapped files)
• Shared program code (if more than one instance of a program or library is running, its code only needs to be stored in memory once)