Texas Instruments MSP430x1xx manual Rra.W, Rra.B

Instruction Set

RRA[.W]	Rotate right arithmetically
RRA.B	Rotate right arithmetically
Syntax	RRA	dst	or	RRA.W dst
	RRA.B	dst
Operation	MSB −> MSB, MSB −> MSB−1, ... LSB+1 −> LSB, LSB −> C
Description	The destination operand is shifted right one position as shown in Figure 3−16.
	The MSB is shifted into the MSB, the MSB is shifted into the MSB−1, and the
	LSB+1 is shifted into the LSB.

Figure 3−16. Destination Operand—Arithmetic Right Shift

Word	15	0
C
Byte
	15	0

Status Bits	N: Set if result is negative, reset if positive
	Z:	Set if result is zero, reset otherwise
	C:	Loaded from the LSB
	V:	Reset
Mode Bits	OSCOFF, CPUOFF, and GIE are not affected.
Example	R5 is shifted right one position. The MSB retains the old value. It operates
	equal to an arithmetic division by 2.
	RRA		R5	; R5/2 −> R5
;	The value in R5 is multiplied by 0.75 (0.5 + 0.25).
;
	PUSH		R5	; Hold R5 temporarily using stack
	RRA		R5	; R5 ⋅ 0.5 −> R5
	ADD		@SP+,R5	; R5 ⋅ 0.5 + R5 = 1.5 ⋅ R5 −> R5
	RRA		R5	; (1.5 ⋅ R5) ⋅ 0.5 = 0.75 ⋅ R5 −> R5
	......
Example	The low byte of R5 is shifted right one position. The MSB retains the old value.
	It operates equal to an arithmetic division by 2.
	RRA.B		R5	; R5/2 −> R5: operation is on low byte only
				; High byte of R5 is reset
	PUSH.B		R5	; R5 ⋅ 0.5 −> TOS
	RRA.B		@SP	; TOS ⋅ 0.5 = 0.5 ⋅ R5 ⋅ 0.5 = 0.25 ⋅ R5 −> TOS
	ADD.B		@SP+,R5	; R5 ⋅ 0.5 + R5 ⋅ 0.25 = 0.75 ⋅ R5 −> R5

......

3-60 RISC 16−Bit CPU