Date: Wed, 8 Sep 1999 11:15:48 +1000 (EST)
From: Dave Horsfall <dave@fgh.geac.com.au>
To: PDP Unix Preservation Society <pups@minnie.cs.adfa.edu.au>
Subject: Re: Diff between 11/20 and 11/45?
In-Reply-To: <199909070604.XAA04723@mpl.ucsd.edu>
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On Mon, 6 Sep 1999, Carl Lowenstein wrote:

> The first genuine user-mode difference that I remember coming across was
> an incompatibility in the result of
> 
> 	MOV SP, -(SP)

Anything involving the same register as src and dst in this way was,
err, different...

And I have an annotation that the JSR does not behave as documented.
Unlike page 91, the sequence is not (tmp) <- (dst) / v(SP) <- reg /
reg <- PC / PC <- (tmp).  The first ISP code is not present i.e. the
SP is decremented first, not saved, and the last is PC <- (dst).

> It isn't really clear to me why one would want to use this particular
> instruction, however it turned out to hang both BASIC and FOCAL at the
> time.  A zero-length patch wasn't too hard to figure out.

Some sort of frame pointer linking, on an architecture that didn't
have separate frame pointers (like the Vax)?

-- 
Dave Horsfall VK2KFU  dave@geac.com.au  Ph: +61 2 9978-7493 Fx: +61 2 9978-7422
Geac Computers P/L (FGH Division) 2/57 Christie St, St Leonards 2065, Australia