Well, technically, on a Unibus, even with the Unibus map, a DMA device
still only sees an 18-bit address. Or am I confused? But if the map is
enabled, then it will end up being able to access all 4M of memory anyhow.
But your answer made me wonder. Are you saying that M[TF]P[DI] are not
using the previous mode bits in the PSW (when MMU is enabled)? Oh, maybe
you are saying that even if MMU is disabled, the selection of which R6
to use is still happening, based on PSW.
Johnny
On 2019-09-22 04:28, Bob Supnik wrote:
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-requ...@trailing-edge.com wrote:
Message: 2
Date: Sat, 21 Sep 2019 19:49:39 -0400
From: "dave porter"<dave_list_a...@verizon.net>
To:<simh@trailing-edge.com>
Subject: [Simh] PDP-11 -- MFPI, etc, with memory mgmt off
Message-ID: <98F67D8C56794670847F6C44F7EE968F@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.
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|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
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