Posted By
Csabo
on 2021-05-20
15:01:31
|  Re: Chip Selects for Plus 4 an C16?
Hey Superglob,
Sorry about the confusion around your question, obviously we're not all clear on what you're trying to do. This info should be more readily available on our site :-/ Anyway, let me try to clear some of this up.
This may be unrelated to your question, but...
- after writing to $FF3E, the memory will be mapped to $0000-$7FFF: RAM, $8000-$FCFF: ROM.
- after writing to $FF3F, the memory will be mapped to $0000-$FCFF: RAM
The chips can only be mapped to $8000-$BFFF / $C000-$FCFF (so, you cannot map ROM below $8000, no matter what). You can only control this from software, by writing to $FDDX. (On C64, apparently there are chip select signals on the expansion port, which control the memory layout, but there's no such thing on the C16/Plus4.)
Hope this helps! 
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