Detect when the C128 switched to C64 mode

All,

I want to add a way to selectively start the C128 mode or the C64 mode
on a C128 to the EF3 cartridge.

The plan is to assert EXROM until the entry of the C64 mode is detected
and then to release it imediately (or better: leave its control to the
active (emulated) cartridge or KERNAL).

The first idea was to wait for the first read access to $FFFC, which
seems to work so far and is compatible to the C64. But the CPLD is 99%
full, there are not many macrocells nor routing resources left so I
search for an easier solution (in terms of function inputs).

Ideas?

Skoe


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