6809 assembly knowledge needed

From: ruud.baltissen_at_apg.nl
Date: Wed, 6 Jan 2010 11:40:02 +0100
Message-ID: <3FAF2D74FB701448B997B2CB7A0076C20F4C0F3E@winsv116.office01.internalcorp.net>
Hallo allemaal,
In between other projects I try to improve my "MP-ASM", Multi Processor
Assembler. Momentary it supports the 6502 and various variants, the Z80
and the 6800. I'm busy with the 6809 (used in the SuperPET so 'on
topic') and ran into an addressing mode I don't understand: xx,PCR or
[xx,PCR]. It's called "Constant Offset from PC". It is equivalent to
xx,X / [xx,X], which is more or less equivalent to xx,X / (xx,X) for the
6502 (because xx can be 16 bits as well). 
What I don't understand is how it can be used. I mean, if I want to load
something I would use:

	LDA Label1
	LDA [Label2]

How can I use the Program Counter as index if its content is only known
after compilation? The only thing I can think of is that you can program
a piece of code that loads something from a table and both code and
table have to be relocatable on the fly.

Any revelation, info or URL is welcome, TIA!

    / __|__
   / /  |_/     Groetjes, Ruud
   \ \__|_\
    \___|       URL: Ruud.C64.org

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Received on 2010-01-06 11:00:03

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