Tandy didn't warn us, Motorola did
mike knudsen
compatible with the Coco III, some folks have wished out
loud that Radio Shack had warned us in advance, especially
about the FExx page. Last nite I had to dig into my Motorola
manual for something else, and I noticed that under the SAM
chapter it described FE00-FEFF as "reserved for MPU enhancements"
(MPU is the 6809, not the SAM, weird) and that the portion of
the FFxx I/O page available for add-ons is much more restrictive
than even Tandy's official range, which I believe is FF60-FF7F.
You could interpret Motorola as saying that only FF40-FF5F
is free -- most of that is taken up by Tandy's disk controller
that ghost-addresses all over the place.
I didn't check my old Coco-I tech manual to see how much
of this had been reproduced there -- if it has, then Tandy had
indeed given a clear warning. Of course like everyone else
I figured "enhancements -- sure, when I can ice skate in Hell"
and wrote code that lay all over FExx. Lucky I didn't sell it.
PS: There is plenty of I/O space in FF04-FF1F and FF24-FF3F,
left over from the two PIA chips. To use it, a peripheral
must decode all the bits itself and pull down the Phantom
line (I forget it's official name) to keep the ghosty PIAs
from fighting it. That Phantom lead was a great addition to
the Coco cart slot (it disables all I/O stuff inside from responding
for a cycle) -- too bad no commercial outfits used it.
Don't know how much it works on the III, tho.
--
Mike J Knudsen ...ihnp4!ihwpt!knudsen
Bell Labs (AT&T) (312)-979-4132 (work)
Nobody pays for my opinions, which are mine alone.
"A mind is a terrible thing to waste, but the pay is good."