x86 Instruction Set Reference

XADD - Exchange and Add

Opcode Mnemonic Description
0F C0 /r XADD r/m8, r8 Exchange r8 and r/m8; load sum into r/m8.
0F C1 /r XADD r/m16, r16 Exchange r16 and r/m16; load sum into r/m16.
0F C1 /r XADD r/m32, r32 Exchange r32 and r/m32; load sum into r/m32.
Description

Exchanges the first operand (destination operand) with the second operand (source operand), then loads the sum of the two values into the destination operand. The destination operand can be a register or a memory location; the source operand is a register.

This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically.
Operation 
Temporary = Source + Destination;
Source = Destination;
Destination = Temporary;
Flags affected

The CF, PF, AF, SF, ZF, and OF flags are set according to the result of the addition, which is stored in the destination operand.

IA-32 Architecture Compatibility

IA-32 processors earlier than the Intel486 processor do not recognize this instruction. If this instruction is used, you should provide an equivalent code sequence that runs on earlier processors.