Date: Tue, 7 Sep 1999 20:48:50 -0400
From: Tim Shoppa <SHOPPA@trailing-edge.com>
To: PUPS@minnie.cs.adfa.edu.au
Message-Id: <990907204850.202001b4@trailing-edge.com>
Subject: Re: Diff between 11/20 and 11/45?
Sender: owner-pups@minnie.cs.adfa.edu.au
Precedence: bulk

>	MOV SP, -(SP)
>
>It isn't really clear to me why one would want to use this particular
>instruction

"MOV SP" is often-used shorthand for "MOV some-non-zero-value", since no
sane implementation would ever have a zero in the SP.  So this would
put a non-zero value on top of the stack (perhaps as a flag, to be
cleared by CLR (SP) when ready) - at least on machines where this was legal!

On which machine does this fail, BTW?  On a 11/15, 11/20, 11/23, 11/35
or 11/40 this ought to work, decrementing SP by two before putting it on
the stack, and on the 11/03, 11/04, 11/05, 11/10, 11/34, and 11/45
SP is decremented by two before being put on the stack, according to my
notes.

-- 
 Tim Shoppa                        Email: shoppa@trailing-edge.com
 Trailing Edge Technology          WWW:   http://www.trailing-edge.com/
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