Yes. There is still variety, though. On PowerPC, for example, there is the concept of segments. On 32bit systems, the address space is divided into 16 segments

Yes. There is still variety, though. On PowerPC, for example, there is the concept of segments. On 32bit systems, the address space is divided into 16 segments of 256MB each. Each of them has a privileged register that can be setup in different ways (e.g. half could be per-process, half machine-wide). The 4 bits from the segment ID eventually map to a 24 bit value (or trigger a protection violation if e.g. the page is kernel-only), so that the original 32bit address becomes a 32 - 4 + 24 = 52 bit virtual address. The MMU, then will look up the higher order bits in the address to convert from 52bit virtual addresses to actual 32bit physical addresses. The virtual address space is larger because in theory you could have processes with completely disjoint addressing (up to 1 million of 4GB each, i.e. 4PB). In the end, you still only have 32 bits to address memory chips, though. :-) On 64 bit systems, there are many more segments, 236. Instead of having (64 * billion) registers, there is a single register pointing to a hash table in memory that the CPU will walk through. Addresses are translated from 64->80->64 bits in a similar way.