ROR moves every bit one place to the right, count times. The bit leaving bit 0 wraps around into the top bit and is also copied into CF. No bits are lost; rotating a byte by 8, or a word by 16, restores it. ROR by n is the same as ROL by the operand width minus n.

     +---------------------------+
     |                           |
     +-> b7 b6 b5 b4 b3 b2 b1 b0 -+
                                  |
                            CF <--+
 
      1000 0001   81h
ROR 1 1100 0000   C0h, CF = 1

Forms and flags

Register or memory operand; count 1 or CL on the 8086. Only CF and OF change. CF is the bit that wrapped around, which is now the top bit. With a count of 1, OF is set when the two highest bits of the result differ, meaning the sign bit changed. For other counts OF is undefined. SF, ZF, PF and AF are not touched.

Idioms

Walk a single 1 bit through a byte, for example to select one of eight things in turn: start with 80h and ROR by 1 each step; after bit 0 it comes back to bit 7 by itself. Look at bit 0 without losing it: ROR AL, 1 then JC, and ROL AL, 1 to undo. Bring a field down to the bottom of a register: a rotate followed by AND with a mask extracts it, and the rotate in the other direction puts it back.

Prints 10000001 11000000 CF=1 00011000
MOV     AL, 81h       ; 1000 0001
CALL    SHOWBIN
ROR     AL, 1         ; bit 0 comes in at the top
CALL    SHOWBIN       ; 1100 0000
CALL    SHOWCF
MOV     CL, 3
ROR     AL, CL
CALL    SHOWBIN       ; 0001 1000
 
SHOWBIN PROC    NEAR  ; print AL as 8 binary digits and a space
PUSHF                 ; keep the caller's flags
PUSH    AX
PUSH    BX
PUSH    CX
PUSH    DX
MOV     BL, AL
MOV     CX, 8
NEXTBIT:
SHL     BL, 1         ; the top bit falls into CF
MOV     DL, "0"
ADC     DL, 0         ; "0" + CF = "0" or "1"
MOV     AH, 02h
INT     21h
LOOP    NEXTBIT
MOV     DL, " "
MOV     AH, 02h
INT     21h
POP     DX
POP     CX
POP     BX
POP     AX
POPF
RET
SHOWBIN ENDP
SHOWCF  PROC    NEAR  ; print "CF=0" or "CF=1" and a space
PUSHF
PUSH    AX
PUSH    DX
MOV     DL, "0"
ADC     DL, 0         ; "0" + CF, before anything changes CF
PUSH    DX
MOV     AH, 09h
MOV     DX, OFFSET CFTXT
INT     21h
POP     DX
MOV     AH, 02h
INT     21h
MOV     DL, " "
MOV     AH, 02h
INT     21h
POP     DX
POP     AX
POPF
RET
SHOWCF  ENDP
CFTXT   DB      "CF=$"

Later CPUs

On the 8086 the count in CL is used in full. The 80186 and later mask it to 5 bits and add an immediate count form, such as ROR AL, 3.