Compare and Exchange

  1. This instruction is not supported on processors earlier than the 80486.

Encoding

EncodingOperand 1Operand 2
mrModRM.r/m[rw]ModRM.reg[r]

Description

The CMPXCHG instruction compares the value in the accumulator with the first ("destination") operand. If the two values are equal, the second ("source") operand is written into the first, and EFLAGS.ZF set. Otherwise, the second operand is loaded into the accumulator and EFLAGS.ZF cleared.

For 64 bit operation in legacy modes, the CMPXCHG8B instruction can be used.

This instruction can be used with the LOCK prefix to allow atomic exectution. To simplify bus operation with locked operation, the destination operand is always written to, even if it is not updated (a locked bus cycle cannot end without a write).

Operation

// if the instruction is LOCKed and `dest` and the accumulator are different, `dest` is reloaded with itself

public void CMPXCHG(ref U8 dest, U8 src)
{
    if (dest == AL)
        dest = src;
    else
        AL = dest;
}

public void CMPXCHG(ref U16 dest, U16 src)
{
    if (dest == AX)
        dest = src;
    else
        AX = dest;
}

public void CMPXCHG(ref U32 dest, U32 src)
{
    if (dest == EAX)
        dest = src;
    else
        EAX = dest;
}

public void CMPXCHG(ref U64 dest, U64 src)
{
    if (dest == RAX)
        dest = src;
    else
        RAX = dest;
}

Flags Affected

CF (carry flag)
Set according to the temporary result.
PF (parity flag)
Set according to the temporary result.
AF (auxiliary flag)
Set according to the temporary result.
ZF (zero flag)
Set if an exchange occurs. Cleared otherwise.
SF (sign flag)
Set according to the temporary result.
OF (overflow flag)
Set according to the temporary result.

Intrinsics

None. Auto-generated by compiler.

Exceptions

Real-Address Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.

Virtual-8086 Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.
#PF(fc)
  • If a page fault occurs.
#AC(0)
  • If alignment checking is enabled while the current privilege level is 3 and an unaligned memory access is made.

Protected Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If the destination is located in a non-writable segment.
  • If a memory operand uses a segment containing a NULL selector.
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.
#PF(fc)
  • If a page fault occurs.
#AC(0)
  • If alignment checking is enabled while the current privilege level is 3 and an unaligned memory access is made.

Compatibility Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If the destination is located in a non-writable segment.
  • If a memory operand uses a segment containing a NULL selector.
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.
#PF(fc)
  • If a page fault occurs.
#AC(0)
  • If alignment checking is enabled while the current privilege level is 3 and an unaligned memory access is made.

Long Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment is in non-canonical form.
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If a memory operand (using a segment other than SS) is in non-canonical form.
  • If the destination is located in a non-writable segment.
  • If a memory operand uses a segment containing a NULL selector.
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.
#PF(fc)
  • If a page fault occurs.
#AC(0)
  • If alignment checking is enabled while the current privilege level is 3 and an unaligned memory access is made.