At 08:31 PM 9/19/2007, you wrote: How does the address decoding work for a 32K cartridge??? IIUC CTS# decodes $C000-$FFEF, so decoding $8000-$BFFF would contend with the mainboard ROMS?!? When you tell port $FF90 on a CoCo 3 that ROM mapping is 32k external (except for vectors), the CTS decodes for $8000-$FFEF. That's all there is to it. Steve