logical instructions

LOGICAL INSTRUCTIONS

 NOT Instruction - Invert each bit of operand

NOT perform the bitwise complement of operand and stores the result back into operand itself.

Syntax–NOT destination

Example :

NOT BX                            ;Complement contents of BX register. - DX =F038h
NOT DX                           ;after the instruction DX = 0FC7h

• AND Instruction - AND corresponding bits of two operands

This Performs a bitwise Logical AND of two operands. The result of the operation is stored in the op1 and used to set the flags. AND op1, op2 To perform a bitwise AND of the two operands, each bit of the result is set to 1 if and only if the corresponding bit in both of the operands is 1, otherwise the bit in the result I cleared to 0 .

Example :

AND BH, CL                 ;AND byte in CL with byte in BH ;result in BH
AND BX,00FFh             ;AND word in BX with immediate 00FFH. Mask upper byte,                         leave lower unchanged

AND CX,[SI]                ; AND word at offset [SI] in data segment with word in CX register . Result in CX register and BX = 10110011 01011110

AND BX,00FFh              ;Mask out upper 8 bits of BX. ;Result BX = 00000000 01011110 and CF =0 , OF = 0, PF = 0, SF = 0 ,ZF = 0.

• OR Instruction - Logically OR corresponding of two operands

OR Instruction - OR instruction perform the bit wise logical OR of two operands .Each bit of the result is cleared to 0 if and only if both corresponding bits in each operand are 0, other wise the bit in the result is set to 1.

Syntax–- OR destination, source.

Examples :

OR AH, CL                         ;CL is OR’ed with AH, result in AH.
                                       ;CX = 00111110 10100101
OR CX,FF00h                     ;OR CX with immediate FF00h result in CX = 11111111 10100101
                                       ;Upper byte are all 1’s lower bytes are unchanged.

• XOR Instruction - Exclusive XOR destination, source

XOR Instruction - XOR performs a bit wise logical XOR of the operands specified by op1 and op2. The result of the operand is stored in op1 and is used to set the flag.

Syntax–- XOR destination, source.

Example : ( Numerical )
                                       ; BX = 00111101 01101001 and CX = 00000000 11111111
XOR BX, CX                       ;Exclusive OR CX with BX and Result BX = 00111101 10010110

• TEST Instruction – AND operand to update flags

TEST Instruction - This instruction ANDs the contents of a source byte or word with the contents of specified destination word. Flags are updated but neither operand is changed . TEST instruction is often used to set flags before a condition jump instruction

Examples:

TEST AL, BH                     ;AND BH with AL. no result is stored . Update PF, SF, ZF TEST CX, 0001H ;AND CX with immediate number
                                      ;no result is stored, Update PF,SF

Example :

                                      ;AL = 01010001
TEST Al, 80H                    ;AND immediate 80H with AL to test f MSB of AL is 1 or 0
                                     ;ZF = 1 if MSB of AL = 0 and AL = 01010001 (unchanged),PF = 0 , SF = 0,
                                     ;ZF = 1 because ANDing produced is 00