[Coco] Some notes on ROM Paks

[Boisy G. Pitre]

As Art posted a few days ago, we've been having a private thread about 
getting to the last 512 bytes of a ROM Pak (either 16K or 32K), or even 
the 32K of CoCo 3 ROM.

Here are some conclusions that I've drawn from the discussion.  Feel 
free to prove or disprove them.

(1) On a CoCo 2, it is not possible to access the last 256 bytes of a 
ROM Pak ($FFXX).  Therefore, 16K ROM Paks do not have valid code in the 
last 256 bytes.

(2) On a CoCo 3, it IS possible to access the last 512 bytes of a ROM 
Pak ($FE00-$FFFF) through use of the MMU register trick as Art 
outlined.

(3) Even so, it is unlikely that CoCo 3 ROM Paks use the last 512 bytes 
of the ROM Pak, unless the game uses the MMU register trick to get at 
those last 512 bytes.

Assuming these hold true, then:

(1) It is safe to copy all but the last 256 bytes of a 16K ROM Pak into 
a blank EPROM, and that game should run fine on a CoCo 1/2.

(2) It *may*, and in all likelihood *IS* safe to copy all but the last 
512 bytes of a 16K or 32K ROM Pak from a CoCo 3 into a blank EPROM, and 
that game should run fine on a CoCo 3.

(3) Since 16K ROM Paks designed for the CoCo 2 could possibly use the 
second to last 256 bytes ($FEXX), such a PAK would NOT work properly on 
a CoCo 3 since the CoCo 1/2 era Pak wouldn't know about the MMU or the 
CoCo 3 (does anyone have an example of such a Pak?)

Boisy