If I recall correctly, at least on 32-bit mode the segment registers were a 13-bit offset into either the GDT or the LDT (so a maximum of 8192 entries on each),

If I recall correctly, at least on 32-bit mode the segment registers were a 13-bit offset into either the GDT or the LDT (so a maximum of 8192 entries on each), 1 bit to choose between the GDT and the LDT, and 2 bits for the protection ring. The base and limit (and other details) were loaded from the GDT/LDT when the segment register was loaded; the undocumented SAVEALL/LOADALL instructions could save and load the "hidden" base/limit/etc directly, leading to tricks like the "unreal mode".