68300

I am porting some CPU16 code in which the addressing was performed using
ZK:ZX = 0 and the SCIM control registers were accessed at negative 16-bit
displacements from the index register.

I like this addressing mode, which would be (d16,An) in CPU32.  I am trying
to determine if address calculations are full 32 bit in the 683xx or if they
are still 24 bit addresses with roll-over.

IE, if the scim registers are at $FFF800-$FFFFFF, are they only at $00FFF800
or at $xxFFF800?

I was off doing microwaves and RF the entire period of time between learning
6800 assembly in 1983 and self-teaching the extension to CPU16 a couple
years ago.  I missed the entire 68000 family until a month or so ago and I
am slowly learning it...  I have P&E Micro assembler, so using C and letting
the compiler deal with it isn't an option...

Does the scim map to $FFFF F800?
Is (d16,An) really going to save as many clock cycles and bytes over direct
32 bit addressing as it appears?