[TUHS] quick question on PDP-11 addressing

[Clem Cole via TUHS]

below

On Sun, May 3, 2026 at 4:00 PM ron minnich via TUHS <tuhs at tuhs.org> wrote:

> I was re-reading the KT-11 docs,

A PDP-11 has 16-bit registers, so all addressing must be within a 64K
boundary.

There are a few versions of the MMU

   - KS11 [CSS option for 11/20 -
   http://gunkies.org/wiki/KS11_Memory_Protection_and_Relocation_option]
   - KT-11B [CSS option https://gunkies.org/wiki/KT11-B_Technical_Manual ]
   - KT-11C,CD  for the 11/45 class [45, 50, 55) [
   https://bitsavers.trailing-edge.com/pdf/dec/pdp11/1145/EK-KT11C-MM-005-KT11-C_CD_Memory_Management_Maintenance_Manual_1976.pdf
   ]\
   - For the 11/70, the MMU is builtin and the PARs are SW compatible with
   KT-11-C
   - and the KT-11D for the 40 class [40, 35]
   https://gunkies.org/wiki/KT11-D_Memory_Management_Unit
   - later 40 class processors (34, 34A and 60) use the KT-11A which is
   software compatible with the KT-11D

In all cases, regardless of operation mode (user, supervisor, kernel), the
CPU has a 16-bit address (virtual) space of 32K words (64K bytes).  The MMU
then maps those 16 bits into either an 18 or 22-bit physical address
space.  On a 40-class processor, that means a total of 64K bytes of address
space at a time, regardless of how the data in that address space is used.
On the 45 and 70 class processors have the funky I/D bit, which gives 2
separate 16-bit address spaces, instruction or data, so the MMU and OS can
provide 2 64K bytes chunks of memory (for a total of 128K bytes) to the CPU,
however, the '17th' which picks which of those two 64K byte hunks is being
used is inferred from either and instruction or data fetch.




> 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.
>
> Thanks, I know this has been discussed before, but I'm giving a talk
> at IWP9 next week and want to make sure I'm not missing something. The
> KT-11 doc left me uncertain.
>
> You gotta love those old DEC docs, complete with SSI logic diagrams ...
>