On PC, BIOS configures memory controller in such way as to hide boundaries of memory modules from OS. Resulting address space still contains some holes and stuf

On PC, BIOS configures memory controller in such way as to hide boundaries of memory modules from OS. Resulting address space still contains some holes and stuff that is not physical memory, but OS gets map of this from BIOS. Originally (on systems with 36pin SIMMs) this wasn't the case and you had to match memory modules such that they produce continuous block of addresses. In essence the situation is pretty much same as for user space program: you get big address space and list of memory regions that are mapped and usable.