Global Descriptor Table
||It has been suggested that Local Descriptor Table be merged into this article. (Discuss) Proposed since June 2011.|
The Global Descriptor Table or 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 can hold things other than segment descriptors as well. Every 8-byte entry in the GDT is a descriptor, but these can be Task State Segment (or TSS) descriptors, Local Descriptor Table (LDT) descriptors, or Call Gate descriptors. The last one, Call Gates, are particularly important for transferring control between x86 privilege levels although this mechanism is not used on most modern operating systems.
There is also an LDT or Local Descriptor Table. The LDT is supposed to contain memory segments which are private to a specific program, while the GDT is supposed to contain global segments. The x86 processors contain facilities for automatically switching the current LDT on specific machine events, but no facilities for automatically switching the GDT.
Every memory access which a program can perform always goes through a segment. On the 386 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 in short. 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) 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 automatically reads the GDT or the LDT and stores the properties of the segment inside the processor itself. Subsequent modifications to the GDT or LDT will not be effective unless the segment register is reloaded.
GDT in 64-bit
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.
If the System bit (4th bit of the Access field) is cleared, the size of the descriptor is 16 bytes instead of 8. This because, even though code/data segments are ignored, TSS are not, but the TSS pointer can be 64bit long and thus the descriptor needs more space to insert the higher dword of the TSS pointer.
- "Patching Policy for x64-Based Systems". "If the operating system detects one of these modifications or any other unauthorized patch, it will generate a bug check and shut down the system."
- Intel Architecture Software Developer's Manual Volume 3: System Programming
- GDT Tutorial
- GDT Tutorial by James Molloy, slightly more practically oriented.
- GDT Table at OSDev.org
- GDT Tutorial at OSDev.org
The entire content of this article (accurate as of 16/10/2008) can be verified using section 2.1.1 to 2.1.4 of the "IA-32 Intel(R) Architecture Software Developer's Manual, Volume 3A: System Programming Guide Part 1". This is available from the Intel(R) website as a PDF. It can also be ordered (accurate at the time of writing) for free from the Intel(R) website on CD. See citation links at base of main article.