x86 Instruction Set Reference

Moves a double-precision floating-point value from the source operand (second operand) to the destination operand (first operand). The source and destination operands can be an XMM register or a 64-bit memory location. This instruction allows a double-precision floating-point value to be moved to and from the low quadword of an XMM register and memory. It cannot be used for register to register or memory to memory moves. When the destination operand is an XMM register, the high quadword of the register remains unchanged.

//MOVLPD instruction for memory to XMM move:
if(IsXMM(Destination)) Destination[0..63] = Source;
//Destination[64..127] unchanged
//MOVLPD instruction for XMM to memory move:
else Destination = Source[0..63];