Coco FDC hacks (was: Ebay reaches new low

[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.

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.