[Coco] Assembler Question???

Robert Gault robert.gault at worldnet.att.net
Wed Sep 2 13:00:00 EDT 2009

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. 
> If this is so then, the ALU becomes simpler when doing a FPGA core.
> james 

Can't find anything regarding the guts of the 6809 or 6309 in the data 
sheets with respect to how subtraction is accomplished. Assembly code 
can use either SUBA #-1 or ADDA #$FF to get the same answer but that 
does not indicate what happens in the CPU core.

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