[Coco] Interrupt Vectors Physical Location ???

[RETRO Innovations]

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