What is the meaning of XOR in x86 assembly?

Question

I\'m getting into assembly and I keep running into xor, for example:

xor     ax, ax

Does it just clear the register\'s value?

Answer 1

It determines the logical eXclusive OR

0 XOR 0 = 0
0 XOR 1 = 1
1 XOR 0 = 1
1 XOR 1 = 0

So, TRUE only if one of the expressions is true, not both.

Answer 2

xor reg, reg is often used to clear register. It can be an alternative to mov reg, 0

AFAIR, it was faster (or shorter) in some cases.

And of course, XOR itself is eXclusive OR (a.k.a.: exclusive disjunction) operation (but it's a shame to describe here such basics - use Wikipedia)

Answer 3

xor = exclusive or. See wikipedia's definition for Exclusive or.

If you xor a register with itself, it will zero that register.

0 xor 0 = 0
0 xor 1 = 1
1 xor 0 = 1
1 xor 1 = 0

Let's take the value 41 as example (in binary):

    101001
xor 101001
  = 000000

Answer 4

If I remember correctly xor ax, ax is a one byte assembly instruction, whilst mov ax, 0 would be at least 3 and would probably take slightly longer to execute. It will certainly take longer to decode than the xor instruction.

Answer 5

xor register, register is commonly used to 'zero' a register, because all bits are compared with each other:

0-bits stay zero. 1-bits become zero, because 1 XOR 1 is also 0.