https://cdrdv2-public.intel.com/819723/325462-sdm-vol-1-2abc..., Vol. 2B page 4-42, (physical page 1263): "MOV—Move to/from Debug Registers ... At the opcode level, the reg field within the ModR/M byte specifies which of the debug registers is loaded or read. The two bits in the mod field are ignored. The r/m field specifies the general-purpose register loaded or read."