What's the difference between virtual address space and the actual address space of the computer?

Question

I thought that virtual address space was a section of RAM allocated to a specific process. But the book I'm reading says that 4 gbs is the standard limit of virtual address space. Isn't that the entire amount of RAM? If that is the case then I'm confused at what virtual address space is. Can anyone enlighten me?

Answer 1

That's the whole point of virtual addresses: The OS handles the physical memory, the process handles its own, virtual memory which is mapped to any memory the OS has available, not necessarily RAM.

On a 32 bit operating system the virtual address space (VAS) is, as you say, usually 4 GiB. 32 bits give you (2^32) addresses (0 ... (2^32)-1), each addressing one byte.

You could have more or less physical RAM and still have a 4-GiB-VAS for each and every process running. If you have less physical RAM, the OS would usually swap to harddrives.

The process doesn't need to know any of this, it can use the full VAS it is given by the OS and it's the OS' job to supply the physical memory.

(This is actually just a dumbed-down version of the Wikipedia article on VAS.)