Global Descriptor Table

The Global Descriptor Table (GDT) is a data structure used by Intel x86-family processors starting with the 80286 in order to define the characteristics of the various memory areas used during program execution, including the base address, the size, and access privileges like executability and writability. These memory areas are called segments in Intel terminology. The GDT is a table of 8-byte entries. Each entry may refer to a segment descriptor, Task State Segment (TSS), Local Descriptor Table (LDT), or call gate. Call gates were designed for transferring control between x86 privilege levels, although this mechanism is not used on most modern operating systems. There is also a Local Descriptor Table (LDT). Multiple LDTs can be defined in the GDT, but only one is current at any one time: usually associated with the current Task. While the LDT contains memory segments which are private to a specific process, the GDT contains global segments. The x86 processors have facilities for automatically switching the current LDT on specific machine events, but no facilities for automatically switching the GDT. Every memory access performed by a process always goes through a segment. On the 80386 processor and later, because of 32-bit segment offsets and limits, it is possible to make segments cover the entire addressable memory, which makes segment-relative addressing transparent to the user. 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 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. Loading a selector into a segment register reads the GDT or LDT entry at the time it is loaded, and caches the properties of the segment in a hidden register.