[Coco] tetris question

[Arthur Flexser]

Care to explain in more detail why the reset vector (on a read CoCo) would
affect it?  I understand that the value the palette register returns for
bits 6 and 7 depends on some register contents at the time it is read, but
beyond that I'm a bit vague....

Art
On Mon, Jan 18, 2010 at 10:26 AM, Darren A <mechacoco at gmail.com> wrote:

> On 1/18/10, brian palmer wrote:
> > I always assumed that the Tetris Pack automatically determined what coco
> it
> > was plugged into. So that small snippet of code Robert G posted  would be
> > what determines what coco unit it is plugged into. I Find that snippet of
> > code really effective in what it does on real hardware. But I would say
> > emulators would not be able to use it. May need to update that code for
> > emulator users.
>
> ---
>
> That code snippet will work on an emulator so long as the emulator
> returns a value with bits 6 and 7 cleared when you read a palette
> register.  MESS does exactly that, and VCC returns whatever value you
> write to the palette registers. That code snippet should work fine on
> either of those.
>
> What's interesting is that the code works on real hardware only
> because the byte at $FFFF has bits 6 and 7 cleared.  If you were to
> change the CoCo 3's ROM such that the LSB of the Reset vector has
> either bits 6 or 7 set, then that game would not be able to detect
> that it is running on a CoCo 3.  Of course, that would also break any
> program that simply checks the value of the Reset vector to deterimine
> which machine it is running on.
>
> Darren
>
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