Why do we need virtual memory?

Question

So my understanding is that every process has its own virtual memory space ranging from 0x0 to 0xFF....F. These virtual addresses correspond to addresses in physical memory (RAM). Why is this level of abstraction helpful? Why not just use the direct addresses?

I understand why paging is beneficial, but not virtual memory.

Answer 1

There are many reasons to do this:

• If you have a compiled binary, each function has a fixed address in memory and the assembly instructions to call functions have that address hardcoded. If virtual memory didn't exist, two programs couldn't be loaded into memory and run at the same time, because they'd potentially need to have different functions at the same physical address.

• If two or more programs are running at the same time (or are being context-switched between) and use direct addresses, a memory error in one program (for example, reading a bad pointer) could destroy memory being used by the other process, taking down multiple programs due to a single crash.

• On a similar note, there's a security issue where a process could read sensitive data in another program by guessing what physical address it would be located at and just reading it directly.

• If you try to combat the two above issues by paging out all the memory for one process when switching to a second process, you incur a massive performance hit because you might have to page out all of memory.

• Depending on the hardware, some memory addresses might be reserved for physical devices (for example, video RAM, external devices, etc.) If programs are compiled without knowing that those addresses are significant, they might physically break plugged-in devices by reading and writing to their memory. Worse, if that memory is read-only or write-only, the program might write bits to an address expecting them to stay there and then read back different values.

Hope this helps!