[Simh] PDP-11 -- MFPI, etc, with memory mgmt off

[Bob Supnik]

Seems like an appropriate question to me.

The answer is no. If the MMU is not operating, the MT/FPI/D instructions 
operate on physical addresses and are limited to 16bit addresses.

The PSW mode bits (in particular, previous mode) are operative for 
choosing which stack pointer is used if the operand specification is R6. 
So MTPI SP will store the operand popped from the stack in the stack 
pointer selected by PSW<prev_mode>. The address calculation on the stack 
is purely physical, ie, 16 bits.

If the MMU is not enabled, programs can only see memory addresses 000000 
- 157777 and the IO page. DMA devices can see (on the Unibus or the 
original Qbus) 18 bits of memory addresses or (on the Q22 bus or a 
Unibus system with an IO map) 22 bits of memory addresses.

/Bob Supnik

On 9/21/2019 8:24 PM, simh-request at trailing-edge.com wrote:
> Message: 2
> Date: Sat, 21 Sep 2019 19:49:39 -0400
> From: "dave porter"<dave_list_addr at verizon.net>
> To:<simh at trailing-edge.com>
> Subject: [Simh] PDP-11 -- MFPI, etc, with memory mgmt off
> Message-ID: <98F67D8C56794670847F6C44F7EE968F at street>
> Content-Type: text/plain; format=flowed; charset="UTF-8";
> 	reply-type=original
>
> I hope you'll excuse a non-SIMH question on this list.
>
> Over in the retrocomputing forum on Stack Exchange,
> I'm involved in a conversation that says that the
> 11/73 move to/from previous-mode space instructions
> MTPI, MTPD, MFPI, MFPD, will, when the MMU is
> not enabled, use the previous-mode bits from the
> PS directly as bits 16,17 of the physical address.
>
> I've never heard of such a thing and I cannot find
> anything written that really says what these
> instructions do when running unmapped.
>
> Anyone know about this?  (Mr. Supnik?)
>
> If I shouldn't have asked this non-SIMH question
> on this list, just say so and I won't do it again.
> Thanks.
>