In an answer by user RichF, it is claimed that the previous-address-space moves were able to access 'all of memory' without memory management being enabled.
The instructions MFPI (move from previous instruction space), MFPD (… previous data space), MTPI (move to previous instruction space), MTPD (… previous data space) facilitate implementation of OS kernel services; user space and kernel space are usually non-overlapping so the kernel can't immediately see user space. Memory management generally uses the current-mode (kernel/super/user) field from the processor status (PSW) to determine which map to use; these instructions use the previous-mode field instead.
I've never really thought about what the previous-mode moves would do if the MMU was not enabled. The answer I linked to above says that the previous-mode bits from the PSW are used directly as the high-order 2 bits of the 18-bit bus address (the low 16 bits are provided via the usual operand).
I've poked around the manuals I have at home, and on bitsavers, but I can't see any mention of this usage. The instruction definitions all assume the MMU is enabled. E.g., for MFPD (from the 11/70 handbook, even though it says 11/45 in the text):
This instruction is provided in order to allow interaddress space communication when the PDP-11/45 is using the Memory Management unit.
The address of the source operand is determined in the current address space. That is, the address is determined using the SP and memory pages determined by PS<15:14>. The address itself is then used in the previous D space (as determined by PS<13:12> to get the source operand. This operand is then pushed on to the current R6 stack.
OH, I'm supposed to have an actual question? OK: does anyone have documentation or actual experience to say what these 4 instructions do when memory management is not enabled? Was this 'off-label' use model-specific?