xadd

Exchange and Add

XADD r/m, r

Exchanges dest and src, then loads sum into dest.

Details

Exchanges the values of dest and src, then stores their sum into dest. Sets OF, SF, ZF, AF, CF, PF based on the addition result. Implicitly uses the accumulator (EAX/RAX) for implicit operand size determination. Available in 386+ with size variants 8/16/32/64-bit; used in multi-threaded code for atomic operations.

Pseudocode Operation

temp ← src
src ← dest
dest ← dest + temp
flags ← set based on (dest + temp) result
ZF ← (dest == 0); CF ← carry_out; OF ← overflow; SF ← (dest & sign_bit) != 0; PF ← parity(dest); AF ← (((dest ^ temp) ^ result) & 0x10) != 0

Example

XADD rbx, rax

Encoding

Binary Layout

Format Legacy

Opcode 0F C1

Extension Base

Operands

dest
Register or memory operand
src
General-purpose register

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	Description
0F C0 /r	XADD r/m8, r8	MR	Valid Valid	Exchange r8 and r/m8; load sum into r/m8.
0F C1 /r	XADD r/m16, r16	MR	Valid Valid	Exchange r16 and r/m16; load sum into r/m16.
0F C1 /r	XADD r/m32, r32	MR	Valid Valid	Exchange r32 and r/m32; load sum into r/m32.
REX.W + 0F C1 /r	XADD r/m64, r64	MR	Valid N.E.	Exchange r64 and r/m64; load sum into r/m64.

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. In 64-bit mode, the instruction’s default operation size is 32 bits. Using a REX prefix in the form of REX.R permits access to additional registers (R8-R15). Using a REX prefix in the form of REX.W promotes operation to 64 bits. See the summary chart at the beginning of this section for encoding data and limits. This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically. 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.

Operation

TEMP := SRC + DEST;
SRC := DEST;
DEST := TEMP;

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.

Exceptions

Protected Mode Exceptions

#GP(0) If the destination is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. XADD—Exchange and Add Vol. 2D 6-26 #UD If the LOCK prefix is used but the destination is not a memory operand.

Real-Address Mode Exceptions

#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS If a memory operand effective address is outside the SS segment limit. #UD If the LOCK prefix is used but the destination is not a memory operand.

Virtual-8086 Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made. #UD If the LOCK prefix is used but the destination is not a memory operand.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0) If a memory address referencing the SS segment is in a non-canonical form. #GP(0) If the memory address is in a non-canonical form. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used but the destination is not a memory operand. XADD—Exchange and Add Vol. 2D 6-27