If the MMU is enabled, the instructions use PSW<prev_mode> to compute
the source/destination operand physical address (if memory) or the stack
pointer to use (if SP). If the MMU is off, PSW<prev_mode> only matters
if the source/destination operand is SP.
/Bob
On 9/22/2019 12:00 PM, simh-requ...@trailing-edge.com wrote:
Message: 4
Date: Sun, 22 Sep 2019 09:01:42 +0200
From: Johnny Billquist<b...@softjar.se>
To:simh@trailing-edge.com
Subject: Re: [Simh] PDP-11 -- MFPI, etc, with memory mgmt off
Message-ID:<158acaa4-22c1-bb9c-ecdf-3c2b5d273...@softjar.se>
Content-Type: text/plain; charset=utf-8; format=flowed
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
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