How does entire physical address space is mapped in virtual address space?

Question

I have read that in XV6, in every process virtual address space entire physical address space is mapped. How is that even possible?

Answer 1

I have read that in XV6, in every process virtual address space entire physical address space is mapped. How is that even possible?

Typically virtual address spaces are split into 2 areas - user-space (belonging to the current process) and kernel-space (same for all processes). Kernel-space is then split into smaller areas (kernel's code & data, areas for memory mapped devices, etc). If the remaining amount of kernel space is large enough to hold the entire physical address space, then there's no problem (but this is extremely unlikely).

However; this probably isn't what XV6 actually does. Far more likely is that it only maps physical RAM into kernel space (and does not map the entire physical address space at all).

For some real numbers; assume you have a computer with 128 MiB of RAM and a 32-bit CPU with 4 GiB of physical address space (where most of the physical address space is unused and isn't RAM); and you also have 256 MiB of kernel space left over. In that case, 128 MiB of RAM would fit in the remaining 256 MiB of kernel space (but the whole 4 GiB of the physical address space will not).

Of course this is still likely to fail on real systems (whenever there's too much RAM, and especially for 32-bit CPUs). Fortunately XV6 is only for "educational purposes" and isn't expected to actually work on real computers, so that's not a major problem.