ROR          ROR Rotate one bit right (memory or accumulator)         ROR

               +------------------------------+
               |                              |
               |   +-+    +-+-+-+-+-+-+-+-+   |
  Operation:   +-> |C| -> |7|6|5|4|3|2|1|0| >-+         N V - B D I Z C
                   +-+    +-+-+-+-+-+-+-+-+             / . . . . . / /

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Accumulator   |   ROR A               |   $6A   |    1    |    2     |
  |  ZeroPage      |   ROR $FF             |   $66   |    2    |    5     |
  |  ZeroPage,X    |   ROR $FF,X           |   $76   |    2    |    6     |
  |  Absolute      |   ROR $FFFF           |   $6E   |    3    |    6     |
  |  Absolute,X    |   ROR $FFFF,X         |   $7E   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+
  For penalty cycles on the 65816, check the desired addressing mode.

    Note: ROR instruction is available on MCS650X microprocessors after
          June, 1976.

Major uses: To divide a byte by 2. ROR can be used with LSR to divide multiple-byte numbers since ROR puts any carry into bit 7. If an LSR resulted in a carry, it would be thus taken into account in the next lower byte in a multiple-byte number.

Notice how the act of moving columns of binary numbers to the right has the effect of dividing by 2:

1000    (the number 8 in binary)
0100    (the number 4)

1000    (the number 1000 in decimal)
0100    (the number 100)