Each rotate instruction shifts the bits of the register or memory operand given. The left rotate instructions shift all the bits upward, except for the top bit, which is returned to the bottom. The right rotate instructions do the reverse: the bits shift downward until the bottom bit arrives at the top.
For the RCL and RCR instructions, the CF flag is part of the rotated quantity. The RCL instruction shifts the CF flag into the bottom bit and shifts the top bit into the CF flag; the RCR instruction shifts the CF flag into the top bit and shifts the bottom bit into the CF flag. For the ROL and ROR instructions, the original value of the CF flag is not a part of the result, but the CF flag receives a copy of the bit that was shifted from one end to the other.
The rotate is repeated the number of times indicated by the second operand, which is either an immediate number or the contents of the CL register. To reduce the maximum instruction processing time, the Pentium processor does not allow rotation counts greater than 31. If a rotation count greater than 31 is attempted, only the bottom five bits of the rotation are used. The 8086 does not mask rotation counts. The Pentium processor in Virtual 8086 Mode does mask rotation counts.
The OF flags is defined only for the single-rotate forms of the instructions (second operand is a 1). It is undefined in all other cases. For left shifts/rotates, the CF bit after the shift is XORed with the high-order result bit. For right shifts/rotates, the high-order two bits of the result are XORed to get the OF flag.