What is virtual memory?

Lately I have been working on the virtual memory system. Now most of you out there won’t know what virtual memory is, so here is a brief explanation.

Virtual memory is the mechanism used to make all tasks think they’re the only task running on that system at any given time, unless communication through memory is desired, in which the operating system makes that possible through a mechanism called shared memory.

That’s virtual memory in one full sentence. If you still don’t get it, that’s perfectly understandable. So here is an explanation which is a tiny bit longer. It explains the goals of virtual memory and explains different approaches to solving the very same problem.

When writing the code for any particular task on any system it’s a nuisance, to say the least, to have to keep the possible existence of other tasks on that system into account. The sole role of the operating system is to help other tasks run, and so what it does, is try to give the entire memory space, to each and every task.

While the whole memory space won’t be possible due to technical restrictions (the kernel itself must be somewhere in memory as well), quite a lot can be made available to user space (that’s where user tasks run).

One of the ways to get this all to work is to have each task ask the kernel for more memory each time they need it, while keeping all the data available to all the other processes. Sometimes, on architectures which don’t support memory protection, this is the only way to go forward.

Another way is to have the processor translate the virtual addresses into physical ones while the kernel concerns itself with the allocation of physical pages (a page is a chunk of memory of a predefined size). When a process isn’t allowed to access a particular physical page, it is simply not mapped to any virtual address at the moment that task is running (again, besides the kernel, but this region of memory has other ways to protect itself).

The advantage of the latter approach is that in this model it isn’t possible for one task to modify critical data or code in the other task. When it is desired to have two tasks sharing a region of space, that can be accomplished by mapping virtual pages in both tasks to the very same physical pages.