[TUHS] quick question on PDP-11 addressing

[John Levine via TUHS]

It appears that ron minnich via TUHS <rminnich at gmail.com> said:
>I was re-reading the KT-11 docs, and got somewhat confused, and wanted
>to verify my memory, that (absent Split I&D) user programs had virtual
>addresses in the range 0-0xffff, and kernels the same: 0-0xffff.

Yes. All PDP-11's were 16 bit machines and the addresses generated by programs
were 16 bits. We used octai so it was 0 to 0177777.

Different models had more or less elaborate schemes to map the 16 bit program
addresses into 18 or 22 bit physical addresses. Some models had two modes, user
and kernel, some had three, user, supervisor and kernel, with separate I and D
addressing in each.

For each of the two or six virtual address spaces there was an eight-entry map that
mapped 8K pages of program addresses to 8K pages of physical memory.  Unix systems
as far as I know always treated user programs as a single chunk, or with I/D a single
sharable chunk of I and another chunk of D.

Bonus kludgery: Unibus addresses were 18 bits so on 22 bit systems (11/44 and /70)
there was another address map from Unibus to physical memory for DMA devices.

R's,
John