Re: 6809 assembly knowledge needed

You might want to drop in on the flex users group if you want some real-world 6809 programming expertise - http://www.evenson-consulting.com/flexusergroup/fug14.htm .

Jack


----- Original Message ----
> From: William Levak <wlevak@SDF.LONESTAR.ORG>
> To: cbm-hackers@musoftware.de
> Sent: Wed, January 6, 2010 5:01:33 PM
> Subject: Re: 6809 assembly knowledge needed
> 
> On Wed, 6 Jan 2010, ruud.baltissen@apg.nl wrote:
> 
> > 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.
> 
> The SuperPet 6809 Assembler manual does not say much about this addressing 
> mode other than it is a form of indexed adressing.  The only thing I can 
> think of is a jump table located inline in the code.  Potentially, for an 
> error jump after a subroutine call, using xx as an index into the jump 
> table.
> 
> 
> wlevak@sdf.lonestar.org
> SDF Public Access UNIX System - http://sdf.lonestar.org
> 
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