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?

As a sweetener, this is now the *preferred* way to zero a register on modern x86-64 micro-architectures. It doesn't require any execution units (essentially handled in the decoder), effectively eliminates stalls (waiting) on the dst=src register, and breaks partial flags register stalls. — Brett Hale, Feb 08 '14 at 18:02
possible duplicate of [Any reason to do a "xor eax, eax"?](http://stackoverflow.com/questions/1396527/any-reason-to-do-a-xor-eax-eax) — Ciro Santilli OurBigBook.com, Jul 08 '15 at 15:07

score 70 · Accepted Answer · answered Jan 20 '11 at 16:17

70

A XOR B in english would be translated as "are A and B not equal". So xor ax, ax will set ax to zero since ax is always equal to itself.

A B | A XOR B
0 0 | 0
1 0 | 1
0 1 | 1
1 1 | 0

answered Jan 20 '11 at 16:17

orlp

112,504
36
218
315

1

This is really helpful. This answer helped me in RE, so just in case: It would translate to something like `if(A != B)` in a high level language. – TheRookierLearner Oct 10 '13 at 05:13
10

@TheRookierLearner: `A XOR B` is a primitive building block for higher-level constructs. It *can be used* to implement `(A != B)`, but it is a distinctly different operation in its own right. Also, nightcracker's statement that `A XOR B` in english would be translated as "are A and B not equal" is only correct when you're looking at the result from a Boolean zero/nonzero perspective. A popular application of XOR is to toggle one or more bits of some input bitfield: the result of `A XOR 1` is the inversion of only the low bit of A. – phonetagger Mar 21 '14 at 16:43

Lukasz · Answer 2 · 2011-01-20T16:22:34.707

26

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)

edited Jan 20 '11 at 16:22

answered Jan 20 '11 at 16:16

Lukasz

7,572
4
41
50

if I run `xor a, b` then will both register's values would be changed based on the condition? I mean if `a == b` then after running the code will they become `a = b = 0`? – Sunlight Jan 07 '23 at 13:05
@Sunlight no it doesn't work this way. Only one register would be affected (the first one) – Lukasz Jan 08 '23 at 14:09

Salman A · Answer 3 · 2016-02-01T06:25:45.047

xor ax, ax is the fastest possible way to set the ax register to 0. Fastest in terms of the size of instruction and number of instructions. For detail about how it works you need a little knowledge of bit arithmetic.

XOR operation between two bits returns 1 if one and only one of the two bits is 1; 0 otherwise. Another way to explain is that that it returns 1 if the two bits are different; 0 otherwise.

XOR operation between two binary numbers of same length works likewise on a bit-by-bit basis. XOR two numbers you get a number with bits set to 1 where corresponding bits of the two operands differ, 0 when corresponding bits are same.

From this knowledge its fairly easy to see that if the two operands are the same (ax and ax for example) the result will be 0.

In 32bit or 64bit mode, it's faster to zero ax with `xor eax, eax`. Shorter encoding, and no false dependency on the previous value of the upper 16b. In 16b mode on a modern CPU, probably `xor eax,eax` is still better, because of the [many special benefits of using a recognized zeroing idiom](http://stackoverflow.com/questions/33666617/which-is-best-way-to-set-a-register-to-zero-in-x86-assembly-xor-mov-or-and/33668295#33668295) — Peter Cordes, Jan 31 '16 at 12:34

score 7 · Answer 4 · answered Jan 20 '11 at 16:16

7

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.

answered Jan 20 '11 at 16:16

Daniel Stelter

466
3
6

jweyrich · Answer 5 · 2011-01-20T16:22:34.907

5

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

edited Jan 20 '11 at 16:22

answered Jan 20 '11 at 16:16

jweyrich

31,198
5
66
97

score 3 · Answer 6 · answered Jan 20 '11 at 16:15

3

A B | XOR
0 0 | 0
1 0 | 1
0 1 | 1
1 1 | 0

The XOR instruction does the above operation on every pair of bits in the two operands. So 0xFF xor 0xFF would be 0x00 and 0x55 xor 0xAA would be 0xFF. And yes, xor ax ax clears ax.

answered Jan 20 '11 at 16:15

nmichaels

49,466
12
107
135

score 2 · Answer 7 · answered Jan 20 '11 at 16:16

2

In this case it will clear the register... XOR is an "exclusive or"... so if ax contains 1010 and you exclusive or that with 1010 you'll get 0000 (cleared)

answered Jan 20 '11 at 16:16

John K.

5,426
1
21
20

score 2 · Answer 8 · answered Jan 20 '11 at 16:26

2

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.

answered Jan 20 '11 at 16:26

Sean

60,939
11
97
136

Depends on bittage, but xor (e)ax,(e)ax is two bytes. – Jens Björnhager Jan 20 '11 at 16:50
@Jens: Sure,but the question is about ax, not eax! – Sean Jan 20 '11 at 18:30
If you're in 16-bit mode, the instruction is two bytes. If 32-bit, 3 bytes. – Jens Björnhager Jan 20 '11 at 21:33

score 2 · Answer 9 · answered Jan 27 '11 at 20:23

When I started programming a long time ago there was no exclusive or on either the processor or in the compiler. When I got around to it I stuck to the descriptions:

or: true if a=1 or b=1 or both=1
xor: true if a=1 or b=1 but not both=1

so:

0 or 0 = 0
0 or 1 = 1
1 or 0 = 1
1 or 1 = 1

and

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

score 1 · Answer 10 · answered Feb 01 '15 at 00:06

1

xor ax,ax is used to set ax to 0.

Reason: typically xor instruction on any processor takes less bytes in assembling, than using movl 0,%ax

answered Feb 01 '15 at 00:06

Vedhas Deshpande

66
8

6

This doesn't seem to add anything to previous answers that mentioned the same thing in more detail. – Nathan Tuggy Feb 01 '15 at 00:13

score 1 · Answer 11 · answered Jan 20 '11 at 16:16

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.

answered Jan 20 '11 at 16:16

Dirk

2,167
2
20
29

What is the meaning of XOR in x86 assembly?

11 Answers11

Linked