FC0-FC2 are used for systems that separate user data, user instruction, supervisor data and supervisor instruction into separate memory banks. In theory this expands your capacity and could prevent buffer overflow attacks somewhat like DEP on newer x86.
It's not user accessible though, it's fully controlled by the CPU involuntarily. When it fetches an instruction, those pins would bank select the user instruction bank, then when it fetches the data for that instruction, they toggle to the user data bank. In supervisor mode, it selects the supervisor banks instead.
So how do you load a program from disk then ? Because during load the CPU considers it as data but during execution as instructions.
So I think you external DMA hardware that is then also aware of the user data/instruction data split.
greets,
Staf.
