[Coco] Re: DS-69B Reverse Engineering

Ries, Rich [S&FS] Rich.Ries at Honeywell.com
Mon Aug 7 08:42:36 EDT 2006

Sorry, I get the CoCo Digest at work, so I just saw these messages... 

>>   7F9D    8607       8:     LDA   #7
>>   7F9F    B7FF71            STA   $FF71   -> PIA register on the
>>   7FA2    10AC9DDEF4        CMPY  ( 57076,PC)   Mysterious section
>>                                    We  haven't been using Y, and
don't seem to later
>>   7FA7    12                NOP
>>   7FA8    12                NOP
>>   7FA9    12                NOP
>>   7FAA   1012               ????  The disassembler chokes on this,
and it does
>>                                    appear to be an illegal opcode (on
the 6809).
>>   7FAC   E661               LDB   1,S   Now we start making sense
>>   7FAE    B6FF22            LDA   $FF22   I think we're enabling an
IRQ here

How's this for a wild and/or wacky theory:
(1) There should be a time delay between storing the data to the PIA
register, and enabling the IRQ, and the instructions are harmless
"thumb-twiddling" instructions, and
(2) These specific instructions were used as a "copyright marker" -- if
anyone else came up with something like a DS-69, AND THE CODE INCLUDED
THIS MYSTERIOUS SECTION, there'd be a very good chance of winning a
lawsuit against the copyright-breaking second company.


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