[Coco] Undefined bits, was:CoCo 3 MMU test for all

[Roger Taylor]

At 11:53 AM 1/21/2008, you wrote:
>The main reason that the two high bits return different values is 
>because those bit are floating on the data bus.
>On all reads, the data bus is in TRI-state and its up the i/o or 
>memory to place its data on the bus.  If no data is placed then the 
>CPU will read noise.
>Most of the time the noise on the data bus is whatever the data was 
>there from the cycle before.
>
>As you pointed out art, the LDA 0,X and the LDA ,X are different 
>6809 instructions with different value for the addressing mode.
>But this does not account to different values for bits 6 and 7.
>The ,X has a binary pattern of %10000100 and 0,X the binary pattern 
>is %0000000. Only bit that changes is bit 7 in the addressing mode 
>but its bit 6 that that changes in the two reading of $7x and $3x.
>
>I think there is something else (like the micro-code of the 6809) 
>creating the noise on the bits.
>
>Roger, this is a mute point anyway because they are undefined bits 
>of data and should be ignored.
>
>Steve


I can't ignore them, though.  ;)

There's something here that (if predictable) is very powerful if used 
like I plan to.  I've toyed with this before and made it work, but 
I'm looking to get a better grasp on it all.

Again, thanks to the people who responded with the simple answer I 
was looking for, the results of ?PEEK(65441) on *your* real CoCo 3 system(s).