求高人解释下面的Code， 为什么效果上是在Output上显示 0到7 RRA R4 4次有什么用？
#include"msp430x20x3.h"
;-------------------------------------------------------------------------------
ORG
0F800h
; Program Reset
;-------------------------------------------------------------------------------
RESET
mov.w #0280h,SP
; Initialize stackpointer
StopWDT
mov.w #WDTPW+WDTHOLD,&WDTCTL; Stop WDT
SetupP1
mov.b #00Fh,&P1DIR
; P1.0 output, else input
mov.b #001h,&P1OUT
; P1.4 set, else reset
Mainloop
nop;
mov.b &P1IN,r4;
rra.b
r4;
rra.b
r4;
rra.b
r4;
rra.b
r4;
mov.b r4,&P1OUT;
br
#Mainloop; Required only for debugger
;
;
ORG
0FFFEh
; MSP430 RESET Vector
DW
RESET
;
END
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