Intel 80287, 80286 manual Shift and Rotate Instructions, Wordoprnd

BASIC INSTRUCTION SET

Example: AND WORDOPRND, BX. Replaces the contents of WORDOPRND with the logical "and" of the contents of the memory word labeled WORDOPRND and the contents of BX.

NOT (Not) inverts the bits in the specified operand to form a one's complement of the operand. NOT has no effect on the flags.

Example: NOT BYTEOPRND. Replaces the original contents of BYTEOPRND with the one's complement of the contents of the memory word labeled BYTEOPRND.

OR (Or) performs the logical "inclusive or" of the two operands and returns the result to the destina- tion operand. OR clears OF and DF, leaves AF undefined, and updates SF, ZF, and PF.

Example: OR AL,5. Replaces the original contents of AL with the logical "inclusive or" of the contents of AL and the immediate value 5.

XOR (Exclusive OR) performs the logical "exclusive or" of the two operands and returns the result to the destination operand. XOR clears OF and DF, leaves AF undefined, and updates SF, ZF, and PF.

Example: XOR DX, WORDOPRND. Replaces the original contents of DX with the logical "exclu- sive or" or the contents of DX and the contents of the memory word labeled

WORDOPRND.

NEG (Negate) forms a two's complement of a signed byte or word operand. The effect of NEG is to reverse the sign of the operand from positive to negative or from negative to positive. NEG updates OF, SF, ZF, AF, PF, and CF.

Example: NEG AX. Replaces the original contents of AX with the two's complement of the contents of AX.

3.4.2 Shift and Rotate Instructions

The shift and rotate instructions reposition the bits within the specified operand. The shift instructions provide a convenient way to accomplish division or multiplication by binary power. The rotate instruc- tions are useful for bit testing.

3.4.2.1 SHIFT INSTRUCTIONS

The bits in bytes and words may be shifted arithmetically or logically. Depending on the value of a specified count, up to 31 shifts may be performed.

A shift instruction can specify the count in one of three ways. One form of shift instruction implicitly specifies the count as a single shift. The second form specifies the count as an immediate value. The third form specifies the count as the value contained in CL. This last form aiiows the shin ~OullL to t;; a variable that the program supplies during execution. Only the low order 5 bits of CL are used.

Shift instructions affect the flags as follows. AF is always undefined following a shift operation. PF, SF, and ZF are updated normally as in the logical instructions.

CF always contains the value of the last bit shifted out of the destination operand. In a single-bit shift, OF is set if the value of the high-order (sign) bit was changed by the operation. Otherwise, OF is cleared. Following a multibit shift, however, the content of OF is always undefined.

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