Virtual memory

Flogging a dead horse perhaps, as I believe the discussion is simply a matter of differing definitions of the phrase "virtual memory", but let's try a different approach: Suppose we have a machine which at the instruction level provides 32-bit addresses, that is, it presents a 32-bit address space (4GB) to the user. However, there is only 1 MB of physical RAM memory in the machine. Without trying to go through the whole history of OS development (leaving out things such as overlays, shared libs, etc.), and without accounting for memory required by the kernel/OS for simplicity, consider several scenarios: - 1. User addresses map directly to physical addresses. User address 0 is physical address 0. The user address space is obviously limited to 1MB or less of memory and the user will be aware of such if an attempt to address beyond that is made. - 2. We add an MMU to map addresses. Multiple user address spaces may exist and can be mapped to different areas in physical memory. User address 0 may or may not ref physical address 0. A user is still aware of a limited address space dictated by the 1MB of physical memory. The total of the user address spaces cannot exceed 1MB. - 3. We add swapping-to-disk. The system as a whole is no longer limited to 1MB of memory, there may be multiple user address spaces totalling more than 1MB of memory. Each user however, is still limited to a max of 1MB of address space, i.e. each user is still aware of the limited physical memory. - 4. We add address-faulting, demand-paging - whatever one wants to call it. The user address space is no longer limited by physical memory. The user can hit any address in their 32-bit space and (magically) find a valid memory location there. More physical memory can be added (or removed) 'underneath' the user(s) without their awareness. By the nomenclature I grew up with or suffer under, the term "virtual memory" only applies in scenario 4, although "virtual addresses" could be said to have been introduced in scenario 2. Or, scenario 4 was my understanding of the *commonly-agreed-upon application* of the phrase "virtual memory", although I would not argue that in a very general sense it may be applied to scenario 2 or 3.