[stephencanon]

Anything of the form 0x0000nn00 is representable, as it has at most eight contiguous non-zero bits starting at an even bit position (i.e. these values are 0x000000nn rotated right by 24). Maybe you're wondering how a rotation of 24 can be encoded in four bits? To get the rotation amount, you double the value of the four-bit field. Only even rotation counts are representable.

[userbinator]

Thanks, overlooked that detail. Very interesting encoding scheme.