[Coco] Strange results reading the palette registers on Coco3

[Arthur Flexser]

Are you using a 6809?  I expect a 6309 might possibly behave
differently in some respects from a 6809 with regard to this weird
stuff.

Art

On Mon, Dec 5, 2011 at 4:54 PM, Robert Gault <robert.gault at att.net> wrote:
> Art, that's a very interesting explanation but testing indicates additional
> results with different code. One critical section of code, is that which
> runs during LOADM. I've a CER-COMP program which sets the palette registers
> during the LOADM process and part of the Disk ROM checks to see if the
> process was successful.
>
> Disk 1.1
> $D004   sta     ,x
>        cmpa    ,x+
>        beq     $D00D
>        jmp     I/O error on bad store
>
> When regX=$FFBn, the result becomes contentX&$3F. That is bits 6&7 are
> ignored and effective values range from 0-$3F.
> This is somewhat different from the result quoted in your message.
>
> Anyway, the main point is that recent changes to the MESS source code had an
> effect on how the palette registers were read. Programs like my CER-COMP
> program stopped loading with an I/O error because the comparison above
> failed.
>
> Looks like there is no practical way to fully emulate the Coco3 hardware in
> emulators regards un-mapped bits or bytes.
>
>
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