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

[Harry Hurst]

> On Tue, Nov 20, 2012 at 2:14 PM, Harry Hurst <hhos at st-tel.net> 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.
>
> Whoa, so do these increment and decrement the index register (to use
> the next pointer next time) or the memory pointer? Hummm, since it's a
> one byte increment it must be an increment to the memory pointer, it
> wouldn't be useful otherwise. Am I right?
>
> But the question is valid for [,W++].
>
> I'm having some 6502 feelings here. Such feelings are never good.
>
>> 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.
>
Let's say the X register contains $EC00, the PC is at $EB00, and memory
word at $EC00 is $8000. The instruction at $EB00 is $33 $90 which would
mean LEAU [,X+] to the 6809. After executing this instruction:

PC: $EB02
X:  $EC01
U:  $8000

This is not useful. LEAU [,X++], on the other hand, which is a valid
index/indirect form, can be quite useful. I just want to make my 6x09
simulator as accurate as I possibly can, so I'm investigating as many
non-valid, unlikely byte sequences as I can imagine. These are just a few
that I have tested so far.

HH