虚拟地址空间和虚拟内存有什么区别？

Question

When a process is created by the user a virtual space address is created which is is the size 4g for a 32 bit os (0 to 2^32-1)? The process thinks its has the whole memory to use but the virtual addresses are converted to physical addresses and stored in the page table of the process. I am confused that what virtual memory is then? Is it same as virtual address space?

Answer 1

What is the difference between virtual address space and virtual memory?

The virtual address space is literally space (in the same way that a backyard shed is storage space).

Virtual memory is something you can put in the virtual space (in the same way that gardening tools are something that you could put in a backyard shed).

You could also leave part of the virtual address space empty/unused; or put something that does not behave like memory (eg a memory mapped device) in the virtual address space (in the same way that you could leave part of a backyard shed empty, or put things that aren't garden tools in the shed).

For a more complete example; let's say you have a 32-bit system and virtual address spaces are 4 GiB or 4096 MiB. This 4096 MiB of space may be split 3072 MiB of "user-space" that the process could use, and 1024 MiB of "kernel space" that is reserved for the kernel itself. In this case it might end up like:

• 1024 MiB of space used by the process for virtual memory

• 2048 MiB of space that isn't used by the process (and isn't virtual memory)

• 128 MiB of space used by the kernel for virtual memory

• 32 MiB of space used by the kernel for memory mapped devices (that isn't virtual memory)

• 864 MiB of space that is not used by the kernel (and isn't virtual memory)

In that case; you'd have a total of "1024 + 128 = 1152 MiB" of virtual memory (and "2048 + 32 + 864 = 2944 MiB" of space that isn't virtual memory).