[Coco] CoCo 3 MMU test for all

[jdaggett at gate.net]

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.

qed

james