x86 Instruction Set Reference

Moves two packed single-precision floating-point values from the source operand (second operand) and the destination operand (first operand). The source and destination operands can be an XMM register or a 64-bit memory location. This instruction allows two single-precision floating-point values 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.

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