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From: DDA@CMU-CS-C.ARPA
Newsgroups: net.micro
Subject: 6809 LEAX B,X vs ABX
Message-ID: <13267@sri-arpa.UUCP>
Date: Wed, 2-Nov-83 14:33:54 EST
Article-I.D.: sri-arpa.13267
Posted: Wed Nov  2 14:33:54 1983
Date-Received: Sat, 5-Nov-83 20:37:30 EST
Lines: 17

From:  Drew Anderson <DDA@CMU-CS-C.ARPA>

In case anyone is interested, the difference is that ABX is unsigned
and LEAX B,X is signed.  Thus with $1000 in X and $F0 in B, ABX yields
$10F0 and LEAX B,X yields $0FF0 (at least according to my TI programmer!)
As such, when you are using a table with 129 to 256 items, you must point
X at the middle of the table when using the LEAX instruction.  I believe
that the main reason for this is doing something like a small negative
stack pointer move such as:
ADJSTK   LDA #BYTES  # BYTES TO RESERVE
         NEGA        MAKE NEGATIVE OFFSET
         LEAS A,S    RESERVE BYTES.

It should be noted that all LEA arguments are 2's complement.
			Drew Anderson
			DDA@cmu-cs-c
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