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

[Harry Hurst]

> The only possible opcodes on page one that could be the HCF are
>
> $14
> $15
> $18
> $1B
> $87
> $8F
> $C7
> $CD
> $CF

As I said before, opcodes $14, $15, and $CD all halt any useful output
from the CPU. There is no apparent difference between them, although, as
someone else pointed out, an 0-scope might tell a different story. You can
see the interference with the display from the address strobing the CPU
does. It is apparently indiscriminate in its timing of the strobe, as
normally it would not interfere with the GIME's display time. Reset is the
only way out of this (these?) mode(s). I've checked these and their
associated $10 & $11 ext. They all act the same. I haven't checked these:

 $18  DarrenA's "6x09 Instruction Set" covers it. You should read the
footnote.
 $1B  6x09 Instruction Set (6IS) indicates it is another NOP on the 6809
 $87 & $C7 6IS says it skips the next byte, set some bits, clear other bits
 $8F & $CF 6IS covers it. You should read the footnote.

HH