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

[Ciaran Anscomb]

"Harry Hurst" wrote:
> 
> 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.

FWIW, I also have

1RRI1010 ($8A, etc.) -> ea = REG_PC | 0xff

(ie, PC with the lower 8 bits set to 1).

But I really need to re-verify that, and check whether bit 4 being set
indirects as expected.

Strangely though, I have $87 etc. down as being zero-offset (ie a
duplicate of $84, etc.), so it looks like I have to re-verify everything!

I don't have anything noted for 1RRI1110 ($8E, etc.).

..ciaran