If you want the top left 2x2 pixels of a 256x256 texture page on a quad I would have thought the UV coordinates would be
0,0, 2.0/256,0, 2.0/256,2.0/256, 0,2.0/256
And the manual should state 0,0 is the top left corner of the top left texel and 1,1 is the bottom right corner of the bottom right texel.
0,0, 2.0/256,0, 2.0/256,2.0/256, 0,2.0/256
And the manual should state 0,0 is the top left corner of the top left texel and 1,1 is the bottom right corner of the bottom right texel.