ROL and ROR perform a bitwise rotation operation on the given
source/destination (first) operand. Thus, for example, in the
operation ROL AL,1, an 8-bit rotation is performed in which
AL is shifted left by 1 and the original top bit of AL moves
round into the low bit.
The number of bits to rotate by is given by the second operand. Only the bottom five bits of the rotation count are considered by processors above the 8086.
You can force the longer (286 and upwards, beginning with a C1
byte) form of ROL foo,1 by using a BYTE prefix: \c{ROL
foo,BYTE 1}. Similarly with ROR.