What is "movddup"

In the x86 assembly programming language, MOVDDUP is the name for a specific action performable by modern x86 processors with 3rd-generation Streaming SIMD Extensions (SSE3). This action involves copying a number to temporary space in the processor for use in other computations.

Specifically, MOVDDUP causes one, double-precision, floating-point source to be copied to both the lower half and upper half of an XMM register.