[Info-vax] OT: IA-128 ???

[Neil Rieck]

I think the CPU bit-size confusion comes from people confusing
"logical address" (what the application programmer sees when looking
at the GPRs) with "physical address". In PDP-11 for example, the OS
was aware of more memory than 16-bits but could not access that space
it directly. To access physical memory, the OS first needed to set up
offset registers in "the memory mapping logic" and "UNIBUS mapping
logic" to define the higher bits. Now when the associated application
fetched instructions or data, the 16-bit PC (program counter) or the
16-bit computed data address (remember this was CISC) would be mapped
to an 18-bit or 22-bit (depending upon the model) physical address.

Some of these ideas got blurred when memory mapping logic moved inside
the CPU package (out-of-sight, out-of-mind? For example, the 68030 was
just a 68020 with an integrated MMU). MMU registers were always there
but usually would only be available to people programming operating
systems or device drivers. Application programmers would remain
blissfully ignorant of them.

NSR