Right. I'm back. So quick update:
Turns out I'm a fool and apparently COMBEL does need the FC[2:0] lines driven to decode addresses properly. I didn't wire them up with this board as I, without any justifiable reason in hindsight, thought they served no purpose on the motherboard. Damn.
In retrospect, the FC lines would be needed for the onboard FPU chip select decode, so it was perhaps obvious I needed to pass them through.
Anyway three bodge wires later the Falcon is now decoding addresses and reading from ROM.
I found and fixed a bus error logic fault in my verilog which now allows the Atari logo to come up and the memory test to pass *but* both IDE and floppy fail to work (the IDE at all and the floppy properly). When the desktop comes up, it's devoid of icons.
Four bombs on inserting a floppy disc at boot.
When the desktop comes up, the screen doesn't redraw unless a blitter event (bus master change) occurs. So moving the mouse has no response unless you hit a menu or if, for example, you click and drag so the rubber band is refreshed. I can see interrupt events being requested and serviced, however.
Diagnostic cartridge fails to boot.
Now, I'm sure I've seen all this behaviour before. But I can't remember what the issue was. My immediate suspicion is the UDS/LDS decode logic is perhaps wrong, but that's taken from a working project so it could mean something upstream of that is faulty (perhaps shorted SIZE[1:0] lines?).
Anyway, progress bringing it up, but we're not there yet.
Anyone with any obvious insight into the symptoms above, in the mean time?
Ta,
BW