The offset from the base may be either a 12 bit unsigned binary immediate value in the instruction, or a
second register (possibly shifted in some way). The offset may be added to (U=1) or subtracted from (U=0)
the base register Rn. The offset modification may be performed either before (pre-indexed, P=1) or after
(post-indexed, P=0) the base is used as the transfer address.