XOR Swap Algorithm - The Algorithm

The Algorithm

Conventional swapping requires the use of a temporary storage variable. Using the XOR swap algorithm, however, no temporary storage is needed. The algorithm is as follows:

The algorithm typically corresponds to three machine code instructions. Since XOR is a commutative operation, X XOR Y can be replaced with Y XOR X in any of the lines. When coded in assembly language, this commutativity is often exercised in the second line:

In the above System/370 assembly code sample, R1 and R2 are distinct registers, and each XR operation leaves its result in the register named in the first argument. Using x86 assembly, values X and Y are in registers eax and ebx (respectively), and xor places the result of the operation in the first register.

However, the algorithm fails if x and y use the same storage location, since the value stored in that location will be zeroed out by the first XOR instruction, and then remain zero; it will not be "swapped with itself". (Note that this is not the same as if x and y have the same values. The trouble only comes when x and y use the same storage location, in which case their values must already be equal.)