The GDT is still present in 64-bit mode; a GDT must be defined, but is generally never changed or used for segmentation. The size of the register has been extended from 48 to 80 bits, and 64-bit selectors are always "flat" (thus, from 0x0000000000000000 to 0xFFFFFFFFFFFFFFFF). However, the base of FS and GS are not constrained to 0, and they continue to be used as pointers to the offset of items such as the process environment block and the thread information block.