Coco FDC hacks (was: Ebay reaches new low

From: Roger Merchberger <>
Date: Tue Nov 9 20:32:24 1999

Rumor has it that Tony Duell may have mentioned these words:

>[2 FDCs]

>Now this is where it gets interesting...
>All I/O on the CoCo is memory mapped. It has to be -- the CoCo uses a
>6809 CPU which doesn't have special I/O instructions.

Right. That confused me for a while when I started learning (unforch, I
only got as far as starting... :-( ) 80C85 code for my Tandy 200, with the
non-memory-mapped I/O capabilities...

>Now, there are 2 chip select signals decoded on the coco mainboard that
>are brought out to the expansion connector. One enables the expansion
>ROM (in a plug-in cartridge). The other is used to enable I/O devices in
>the cartridge (IIRC, it's active from FF40-FF5F (or is it FF4F?)).
>AFAIK, _all_ ROMs in cartridges are enabled by the first signal. Some
>cartridges use the I/O select signal, others decode the address lines and
>map their I/O devices elsewhere in the memory map. IIRC, the disk
>controller is one of the few devices that uses this I/O select signal.
>The multipak interface switches both the above select signals so that
>they're only active on one slot at a time. It doesn't fiddle with any of
>the address lines, though.
>What this means is that many I/O devices (but not the disk controller)
>are always accessible if plugged into a multipak, no matter which slot is
>'selected'. But the disk controller is only accessible if the slot it's
>plugged into is the active one. So in theory you can just plug in 2 disk
>controllers and switch between then using the slot select register in the
>multipack (I forget where this is -- FF7F????). Just don't ask me to
>write the software.

I had forgotten about that tidbit... time to get out my spec's on the
Multipak and disk drivers again. Don't worry - I'm better with software
than hardware as that's where I started, so I can prolly write/modify
device drivers to handle that stuff... it's the hardware where I'm rusty.

Thanks... this weekend I might have to rustle up some books again, and
start plinking again... (or, get the Commie 128 I rescued up and running; I
have a Zork trilogy I want to play. ;-)

Glad to help improve the S/N ratio... ;-)
See ya,
Roger "Merch" Merchberger
Roger "Merch" Merchberger   ---   sysadmin, Iceberg Computers
Recycling is good, right???  Ok, so I'll recycle an old .sig.
If at first you don't succeed, nuclear warhead
disarmament should *not* be your first career choice.
Received on Tue Nov 09 1999 - 20:32:24 GMT

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