[Coco] Interrupt Vectors Physical Location ???

[Arthur Flexser]

If I'm understanding you correctly, you're suggesting that it would have
made more sense to put the vectors at the top of Extended Basic rather than
the Color Basic ROM.  The clear problem with that is that many CoCo's,
especially in the early days, were sold with ONLY the Color Basic ROM, no
Extended Basic.

Art

On Sat, Dec 28, 2019 at 5:33 PM RETRO Innovations <go4retro at go4retro.com>
wrote:

> On 12/28/2019 4:09 PM, Arthur Flexser wrote:
> > That would be $BFF2-BFFF in the Color Basic ROM.  No idea how the magic
> is
> > accomplished that causes them to appear at $FFF2-FFFF.
>
> The Magic is called "SAM" :-)
>
> Grab a datasheet for the sn74ls783 (74ls naming for the SAM IC).
>
> Look at page 17.
>
> Notice that the vectors are listed at top middle, in the "fine"
> section.  It's an expansion of $ff00-$ffff.  On the left side of the
> fine section, you see "S=" things.  The S means the output of the SAM
> pins S0,S1,S2, which the designer then needs to feed to a 74ls138, which
> is the reference Motorola design that begat the CoCo.
>
> Well, the vectors are listed besides S=2.  Now, if you look at the left
> side ("course", which is the wrong spelling, it should be "coarse"), You
> see that ROM1 is also listed besides S=2.  So, this means that the
> vectors should be placed in the top of the ROM1 ROM.  Why the SAM didn't
> put them at the top of the S=3 (ROM2) ROM, I have no idea, because it
> just wastes $bff2-$bfff, whereas $fff2-$ffff is always unused in ROM2.
>
> Jim
>
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