Intel 80286, 80287 manual 10 01, ,1,101, 0101,1, 01 0 10

BASIC INSTRUCTION SET

3.4.2.2 ROTATE INSTRUCTIONS

Rotate instructions allow bits in bytes and words to be rotated. Bits rotated out of an operand are not lost as in a shift, but are "circled" back into the other "end" of the operand.

Rotates affect only the carry and overflow flags. CF may act as an extension of the operand in two of the rotate instructions, allowing a bit to be isolated and then tested by a conditional jump instruction (JC or 1NC). CF always contains the value of the last bit rotated out, even if the instruction does not use this bit as an extension of the rotated operand.

In single-bit rotates, OF is set if the operation changes the high-order (sign) bit of the destination operand. If the sign bit retains its original value, OF is cleared. On multibit rotates, the value of OF is always undefined.

ROL (Rotate Left) rotates the byte or word destination operand left by one or by the number of bits specified in the count operand (an immediate value or the value contained in CL). For each rotation specified, the high-order bit that exists from the left of the operand returns at the right to become the new low-order bit of the operand. See figure 3-9.

Example: ROL AL, 8. Rotates the contents of AL left by 8 bits. This rotate instruction returns AL to its original state but isolates the low-order bit in CF for testing by a 1C or 1NC instruction.

ROR (Rotate Right) rotates the byte or word destination operand right by one or by the number of bits specified in the count operand (an immediate value or the value contained in CL). For each rotation specified, the low-order bit that exits from the right of the operand returns at the left to become the new high-order bit of the operand. See figure 3-10.

1 ,0 10 01, ,1,101, 0101,1, 01 0 10 1

BEFORE ROL

ROL shills the bits In the memory or register operand to the lell by the specified number 01 bit positions. It copies the bit shilled out 01 the lell 01 the operand Into the right 01 the operand. The last bit shilled Into the least slgnilicant bit 01 the operand also appears In CF. This Instruction also operates on byte operands.

G3010B

Figure 3-9. ROL

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