In order to reference a segment, a program must use its index inside the GDT or the LDT. Such an index is called a segment selector (or selector). The selector must generally be loaded into a segment register to be used. Apart from the machine instructions which allow one to set/get the position of the GDT, and of the Interrupt Descriptor Table (IDT), in memory, every machine instruction referencing memory has an implicit Segment Register, occasionally two. Most of the time this Segment Register can be overridden by adding a Segment Prefix before the instruction.