[Coco] Assembler Question???

jdaggett at gate.net jdaggett at gate.net
Wed Sep 2 15:22:27 EDT 2009

On 2 Sep 2009 at 13:45, uhmgawa wrote:

> jdaggett at gate.net wrote:
> > To the list:
> > 
> > I have started looking at two FPGA cores for the 6809 in altering one of them to handle 
> > 6309 opcodes and stumbled across a potential  issue. As far as I can remember the 
> > Motorola Freeware Assemblers treat subtraction as adding the 2's compliment of the 
> > subtrahend. Also I seem to remember that is how the internal workings of the MC6809 
> > is. There is actually no subtractor but the assemly code has the 2's compliment of the 
> > subtrahend and does addition.
> I don't believe that is the case as a subtract
> operation sets the carry to the inverse of that
> resulting from an add operation.  So in this
> respect subtraction of x and addition of -x
> aren't trivially interchangeable.
> Internally the ALU may well negate the subtrahend,
> perform an addition, and invert the carry bit,
> with this modification of an underlying add operation
> being keyed by a subtraction opcode.
> -- 
> uhmgawa at third-harmonic.com

Yes with subtraction you treat the carry differently. I understand that. What I needed to 
know was when the subtrahend is converted to 2's compliment, at assembly or internally 
in the ALU. I now know that it is internal and all is well. 


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