[Simh] Recent bug fixes

[Bob Supnik]

Message: 5
Date: Fri, 06 Sep 2013 17:40:19 +0200
From: Johnny Billquist<bqt at softjar.se>
To:simh at trailing-edge.com
Subject: Re: [Simh] Various PDP11 fixes and enhancements
Message-ID:<5229F763.8090807 at softjar.se>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 2013-09-06 17:13, Bob Supnik wrote:

> 2. RS03/RS04 - third Massbus adapter with RS03/RS04 fixed head disk.
> Only tested with RT11, as M+ and RSTS/E support requires a unique
> SYSGENs. Feel free to hack on it.

Is this something specific to a third massbus, or just a case of the
RS03/RS04 devices as such? I mean, on M+ you can have all type of
devices on the same massbus.

>>> The Massbus implementation links an RH to a single device type.
>>> This is an artifact of the simulator, not of real life.
>>> Three RH adapters allows for 8 RP/RM units, 8 TU units, and 8 RS units.

> 3. PDP11 CPU - MMR1 no longer tracks changes to the PC. The issue that
> it should not track changes in floating point instructions is still open.

I wasn't aware of any discussions. Could you, or someone sum it up? Is
it a question of whether MMR1 should reflect changes to the general
registers if a FP instruction is aborted? If so, I'm pretty sure they
should be. I think I have read it somewhere in some documentation, and
could try and locate it, if that really is the question.

Also, MMR1 might reflect changes to the PC as well, as far as I can read
documentation... But it's "complicated".

	Johnny

>>> Looking at the J-11 microcode:
>>> MMR1 does not track FP changes. This is because in early systems, like
>>> the 11/45, it didn't have enough bits to record an 8 byte register change.
>>> The J-11 goes through great pains to undo register changes before it
>>> springs a trap or memory management abort.
>>> Likewise, the J-11 never tracks PC changes in MMR1. The instruction
>>> stream is read through a special microinstruction, because the hardware
>>> always prefetches the next instruction word. Even modes 47 and 57, which
>>> make no sense, don't update MMR1.

/Bob Supnik