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(nasm.info)Section B.4.335

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B.4.335. `XOR': Bitwise Exclusive OR
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     XOR r/m8,reg8                 ; 30 /r                [8086]
     XOR r/m16,reg16               ; o16 31 /r            [8086]
     XOR r/m32,reg32               ; o32 31 /r            [386]

     XOR reg8,r/m8                 ; 32 /r                [8086]
     XOR reg16,r/m16               ; o16 33 /r            [8086]
     XOR reg32,r/m32               ; o32 33 /r            [386]

     XOR r/m8,imm8                 ; 80 /6 ib             [8086]
     XOR r/m16,imm16               ; o16 81 /6 iw         [8086]
     XOR r/m32,imm32               ; o32 81 /6 id         [386]

     XOR r/m16,imm8                ; o16 83 /6 ib         [8086]
     XOR r/m32,imm8                ; o32 83 /6 ib         [386]

     XOR AL,imm8                   ; 34 ib                [8086]
     XOR AX,imm16                  ; o16 35 iw            [8086]
     XOR EAX,imm32                 ; o32 35 id            [386]

   `XOR' performs a bitwise XOR operation between its two operands (i.e.
each bit of the result is 1 if and only if exactly one of the
corresponding bits of the two inputs was 1), and stores the result in
the destination (first) operand.

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.

   The `MMX' instruction `PXOR' (see *Note Section B.4.266::) performs
the same operation on the 64-bit `MMX' registers.