[Coco] Solution to switch between 6809 and 6309

[CoCo Demus]

100% compatible does not mean equivalent. The 6309 is 100% compatible for
all documented 6809 features.

Jumping in the middle of a instruction byte is a non-defined behaviour. As
executing an invalid instruction.

On Mon, Jun 15, 2015 at 2:24 AM, Arthur Flexser <flexser at fiu.edu> wrote:

> Another example is where the undocumented 6809 opcode $3E is used, a handy
> way to get a single byte equivalent to pressing the reset button.  But it
> gets vectored to the illegal-opcode address, probably crashing the CoCo,
> when used with a 6309.
>
> Art
>
> On Sun, Jun 14, 2015 at 5:04 PM, Darren A <mechacoco at gmail.com> wrote:
>
> > On Sun, Jun 14, 2015 at 2:30 PM, Arthur Flexser wrote:
> >
> > > Bill, I agree with you about there being no need to switch back and
> forth
> > > between 6809 and 6309, but I'd put compatibility at more like 99.9%.  I
> > > once did receive a Basic program that turned out not to work right on
> > > account of my having a 6309 installed.  The author, who was not an
> > assembly
> > > language programmer, used an EXEC he found listed in some handy guide
> > > somewhere.  It jumped into the middle of a ROM instruction, and worked
> > okay
> > > with a 6809 because the first byte executed was an illegal opcode and
> was
> > > therefore ignored by the 6809 under which the program was developed.
> But
> > > when I tried the program with a 6309, the 6309 vectored the illegal
> > opcode
> > > to its error handler address, which hadn't been initialized, and the
> > > program crashed.  So, no 101% compatibility, or even 100%.   But close.
> > >
> >
> >
> > Another situation I have seen is where one of the invalid register
> > encodings has been intentionally used with the TFR instruction.  This
> > undocumented "feature" can be used on a 6809 to load the value $FF into
> the
> > DP resgister without having to use A or B as an intermediate.  Running
> the
> > same code on a 6309 will have different behavior since all of the
> encodings
> > are valid on that CPU.
> >
> > - Darren
> >
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> >
>
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