That isn't X86, probably MIPS64. The .D means double precision floating point.
ADD.D F4,F0,F2
Means add the contents of floating registers F0 and F2 as double precision floating point numbers and store the result into register F4.
http://www.tik.ee.ethz.ch/education/lectures/TI1/materials/assemblylanguageprogdoc.pdf