[Coco] 6309/6809 opcodes with mixed 8/16 bit registers

[Harry Hurst]

> Go to this web page:
> <https://sites.google.com/a/aaronwolfe.com/cococoding/home/docs>
>
> Download the document titled "6x09_Instruction_Sets.pdf".  This has
> answers to all of the above questions.
>
> Darren

Thanks for the rapid response, Darren. That's a big help. Very thorough.
The only thing I can see that's missing is some details on the
index/indirect addressing modes, and maybe that's in there, too. After
all, I haven't read the whole thing yet. :)

One thing I've found, that may not be in your book, Darren, is that the
supposedly "not allowed" modes [,R+] and [,-R] are actually functional on
the 68B09E. They are "not allowed" in every reference I could find, and
yet they work. I think most, if not all, assemblers disallow them, for
good reason, but if the CPU encounters an instance of this, it will
execute it just fine. The 6309 uses the [,R+] opcodes for some limited
indexing with the W register. Here's a short list of these and other
invalid-on-the-6809 indexing mode differences:

                                CPU
Postbyte              6809               6309
$90                  [,X+]("NA")           [,W]
$B0                  [,Y+]("NA")           [n16,W]
$D0                  [,U+]("NA")           [,W++]
$F0                  [,S+]("NA")           [,--W]
$87                   A,X (dup/$86)         E,X
$A7                   A,Y (dup/$A6)         E,Y
$C7                   A,U (dup/$C6)         E,U
$E7                   A,S (dup/$E6)         E,S
$97                  [A,X] (dup/$96)       [E,X]
Presumably $B7, $D7, and $F7 follow suit for the Y, U, and S registers and
are duplicates of $B6, $D6, and $F6.

Thanks again,
HH