quick question about BANK()

[win-ini]

so BANK(x) returns the bank of x, correct? what if MBC5 is used and x is in bank $100-$1ff, and the code used is ld a,BANK(x) or loading to another bank? I expect a will contain the lowest 8 bits of the bank no., but where will the 9th bit be stored? (in carry?)

edit: tried it; Value 0x1fe is not 8-bit...

[aaaaaa123456789]

BANK(xxx) is returning the value, which is $1FE in your case. There is nothing special about that.
The rest is the error you'd normally get for trying to write that instruction: ld a, $1fe directly will give you the same error. If you actually do this, the value will be truncated (it's equivalent to ld a, $fe) and it won't do what you want.