// by R bits, and repeated with period of 64, 32, 16, 8, 4, or 2.
// it is encoded in logical instructions with 3 bitfields
// N (1 bit) : R (6 bits) : S (6 bits), where
// N=1           -- period=64
// N=0, S=0xxxxx -- period=32
// N=0, S=10xxxx -- period=16
// N=0, S=110xxx -- period=8
// N=0, S=1110xx -- period=4
// N=0, S=11110x -- period=2
// R is the shift amount, low bits of S = n-1