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From: nw@palladium.UUCP (Neil Webber)
Newsgroups: comp.sys.m68k
Subject: ADDA.L (SP)+,SP  (68020)
Message-ID: <407@palladium.UUCP>
Date: Mon, 30-Nov-87 12:10:24 EST
Article-I.D.: palladiu.407
Posted: Mon Nov 30 12:10:24 1987
Date-Received: Sat, 5-Dec-87 02:46:20 EST
Organization: Epoch Systems, Marlboro MA
Lines: 29
Keywords: MC68020

Is the following instruction sequence an "undefined" operation?

        ADDA.L  (SP)+,SP

The intent is to add the value at the top of the stack, plus 4, to
the stack pointer.

I've tried it, and it works, but I'm wondering if Motorola prefers to
consider this "undefined" operation.

The closest thing I can find to a discussion of this is under MOVES,
where it says (p B-117, 68020 User's Manual, Second Edition):

        MOVES.x An,(An)+
            or
        MOVES.x An,-(An)

        where An is the same address register for both source and destination
        operand is an undefined operation.

Is this meant to be a generic warning or only for MOVES?  By the way, if
someone from Motorola is reading this article, you should note that the
actual wording in the book is quite mangled ... it needs correction.

Thanks in advance.

-- 
Neil Webber / Epoch Systems, Marlboro MA / (617) 481-3717
        {harvard!cfisun, linus!alliant}!palladium!nw