x86 Instruction Set Reference

Moves a scalar double-precision floating-point value from the source operand (second operand) to the destination operand (first operand). The source and destination operands can be XMM registers or 64-bit memory locations. This instruction can be used to move a double-precision floating-point value to and from the low quadword of an XMM register and a 64-bit memory location, or to move a double-precision floating-point value between the low quadwords of two XMM registers. The instruction cannot be used to transfer data between memory locations.

When the source and destination operands are XMM registers, the high quadword of the destination operand remains unchanged. When the source operand is a memory location and destination operand is an XMM registers, the high quadword of the destination operand is cleared to all 0s.

//MOVSD instruction when source and destination operands are XMM registers:
if(IsXMM(Source) && IsXMM(Destination)) Destination[0..63] = Source[0..63];
//Destination[64..127] remains unchanged
//MOVSD instruction when source operand is XMM register and destination operand is memory location:
else if(IsXMM(Source) && IsMemory(Destination)) Destination = Source[0..63];
//MOVSD instruction when source operand is memory location and destination operand is XMM register:
else {
	Destination[0..63] = Source;
	Destination[64..127] = 0;
}