[Coco] CoCo 3 MMU test for all

jdaggett at gate.net jdaggett at gate.net
Mon Jan 21 20:57:53 EST 2008

On 21 Jan 2008 at 18:18, Darren A. wrote:

> It may very well not be noise. The common read, as you say, is coming
> from the PEEK command. Robert G. has confirmed that a different result
> can be obatined using assembly instructions. The question remains as
> to whether or not the value of the instruction bytes are playing any
> role in this matter.
> Darren

And I believe that it is. The last instruction in the basic code before the LDB 
,X is encountered is a BSR. After which an RTS is encountered. The last by 
te  read back into the 6809 is the PCR low byte. Which happens to be $52 
(0101 0010). That is the last item on the data buss prior to the GIME chip 
register load into the 6809. Oddly enough bits 6 an 7 are 01. Concact that 
with xx11 1000 and you get  0111 1000  or $78 and not $38.



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