The 8 bit unsigned immediate offset is shifted left 2 bits and either added to (U=1) or subtracted from (U=0)
the base register (Rn); this calculation may be performed either before (P=1) or after (P=0) the base is used
as the transfer address. The modified base value may be overwritten back into the base register (if W=1), or
the old value of the base may be preserved (W=0). Note that post-indexed addressing modes require
explicit setting of the W bit, unlike LDR and STR which always write-back when post-indexed.