From: Warren Toomey <wkt@henry.cs.adfa.oz.au>
Message-Id: <199804030100.LAA13088@henry.cs.adfa.oz.au>
Subject: Re: Bug in Bob Supnik's Emulator?
To: sms@moe.2bsd.com (Steven M. Schultz)
Date: Fri, 3 Apr 1998 11:00:34 +1000 (EST)
Cc: pups@minnie.cs.adfa.oz.au
In-Reply-To: <199804030050.QAA07798@moe.2bsd.com> from "Steven M. Schultz" at "Apr 2, 98 04:50:26 pm"
Reply-To: wkt@cs.adfa.oz.au
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In article by Steven M. Schultz:
[re bugs in Bob Sunik's PDP emulator]

> 	My speculation is that there's a MMU emulation bug somewhere.  'vi' is
> 	a overlaid split I/D program.  Overlays in 2.11BSD are done via
> 	'page flipping' (altering MMU registers).  Also 2.11 uses the 'expand
> 	downward' bit on the stack (as well as relying on MMR3 - i think that's
> 	the one - for instruction restart after growing the stack).  If there's
> 	a subtle gotcha in the MMU emulation that will cause problems 
> 	eventually.  2.11 is not alone in using the ED bit and instruction
> 	restart - if the problem is MMU related it could show up under other
> 	systems (V7).   It would be interesting to know if 'vi' encountered
> 	problems on V7 but V7 doesn't have usermode overlays so getting 'vi'
> 	to run would be very problematic.
> 
> 	Steven

The 2bsd distribution in the archive comes with an early non-overlayed vi
which compiles on V7. However, I haven't got it to work correctly yet. I
suspect that the /etc/termcap entry I was using is not recognised by this
early version of termlib.

This is all irrelevant to the emulator bug, BTW.
Steven, have you mentioned your hypothesis to Bob?

	Warren