Rotates all bits of the register one place right through the C (carry)
        bit. This is a 9-bit rotation.

        moved the carry flag into bit 8
        moved bit 7 into carry flag

        source code forms: ROR Q; RORA; RORB

        CC bits "HNZVC": -aa-s

def ROR(self, a):
        """
        Rotates all bits of the register one place right through the C (carry)
        bit. This is a 9-bit rotation.

        moved the carry flag into bit 8
        moved bit 7 into carry flag

        source code forms: ROR Q; RORA; RORB

        CC bits "HNZVC": -aa-s
        """
        r = (a >> 1) | (self.C << 7)
        self.clear_NZ()
        self.update_NZ_8(r)
        self.C = get_bit(a, bit=0) # same as: self.C = (a & 1)
        return r