This instruction can be used with a LOCK prefix. In order to simplify interface to the processor's bus, the destination operand receives a write cycle without regard to the result of the comparison. DEST is written back if the comparison fails, and SRC is written into the destination otherwise. (The processor never produces a locked read without also producing a locked write.)
The "r/m64" syntax had previously been used only in the context of floating point operations. It indicates a 64-bit value, in memory at an address determined by the modr/m byte. This instruction is not supported on Intel486 processors.