Global Descriptor Table

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The Global Descriptor Table (GDT) is a binary data structure specific to the IA-32 and x86-64 architectures. It contains entries telling the CPU about memory segments. A similar Interrupt Descriptor Table exists containing task and interrupt descriptors.

It is recommended to read the GDT Tutorial.

GDTR

The GDT is pointed to by the value in the GDTR register. This is loaded using the LGDT assembly instruction, whose argument is a pointer to a GDT Descriptor structure:

GDT Descriptor (GDTR)
79 (64-bit mode)
48 (32-bit mode)   16 		15   0
Offset
63 (64-bit mode)
31 (32-bit mode)   0 		Size

15   0
  • Size: The size of the table in bytes subtracted by 1. This subtraction occurs because the maximum value of Size is 65535, while the GDT can be up to 65536 bytes in length (8192 entries). Further, no GDT can have a size of 0 bytes.
  • Offset: The linear address of the GDT (not the physical address, paging applies).

Note that the amount of data loaded by LGDT differs in 32-bit and 64-bit modes, the offset is 4 bytes long in 32-bit mode and 8 bytes long in 64-bit mode.

For more information, see Section 2.4.1: Global Descriptor Table Register (GDTR) and Figure 2-6: Memory Management Registers of the Intel Software Developer Manual, Volume 3-A.

Table

The entries in the GDT are 8 bytes long and form a table like this:

Global Descriptor Table
Address Content
GDTR Offset + 0 Null
GDTR Offset + 8 Entry 1
GDTR Offset + 16 Entry 2
GDTR Offset + 24 Entry 3
... ...

The first entry in the GDT (Entry 0) should always be null and subsequent entries should be used instead.

Entries in the table are accessed by Segment Selectors, which are loaded into Segmentation registers either by assembly instructions or by hardware functions such as Interrupts.

Segment Descriptor

Each entry in the table has a complex structure:

Segment Descriptor
63   56 55   52 51   48 47   40 39   32
Base
31   24 		Flags
3   0 		Limit
19   16 		Access Byte
7   0 		Base
23   16
31   16 15   0
Base
15   0 		Limit
15   0
  • Base: A 32-bit value containing the linear address where the segment begins.
  • Limit: A 20-bit value, tells the maximum addressable unit, either in 1 byte units, or in 4KiB pages. Hence, if you choose page granularity and set the Limit value to 0xFFFFF the segment will span the full 4 GiB address space in 32-bit mode.

In 64-bit mode, the Base and Limit values are ignored, each descriptor covers the entire linear address space regardless of what they are set to.

For more information, see Section 3.4.5: Segment Descriptors and Figure 3-8: Segment Descriptor of the Intel Software Developer Manual, Volume 3-A.

Access Byte
7 6 5 4 3 2 1 0
P DPL S E DC RW A
  • P: Present bit. Allows an entry to refer to a valid segment. Must be set (1) for any valid segment.
  • DPL: Descriptor privilege level field. Contains the CPU Privilege level of the segment. 0 = highest privilege (kernel), 3 = lowest privilege (user applications).
  • S: Descriptor type bit. If clear (0) the descriptor defines a system segment (eg. a Task State Segment). If set (1) it defines a code or data segment.
  • E: Executable bit. If clear (0) the descriptor defines a data segment. If set (1) it defines a code segment which can be executed from.
  • DC: Direction bit/Conforming bit.
    • For data selectors: Direction bit. If clear (0) the segment grows up. If set (1) the segment grows down, ie. the Offset has to be greater than the Limit.
    • For code selectors: Conforming bit.
      • If clear (0) code in this segment can only be executed from the ring set in DPL.
      • If set (1) code in this segment can be executed from an equal or lower privilege level. For example, code in ring 3 can far-jump to conforming code in a ring 2 segment. The DPL field represent the highest privilege level that is allowed to execute the segment. For example, code in ring 0 cannot far-jump to a conforming code segment where DPL is 2, while code in ring 2 and 3 can. Note that the privilege level remains the same, ie. a far-jump from ring 3 to a segment with a DPL of 2 remains in ring 3 after the jump.
  • RW: Readable bit/Writable bit.
    • For code segments: Readable bit. If clear (0), read access for this segment is not allowed. If set (1) read access is allowed. Write access is never allowed for code segments.
    • For data segments: Writeable bit. If clear (0), write access for this segment is not allowed. If set (1) write access is allowed. Read access is always allowed for data segments.
  • A: Accessed bit. The CPU will set it when the segment is accessed unless set to 1 in advance. This means that in case the GDT descriptor is stored in read only pages and this bit is set to 0, the CPU trying to set this bit will trigger a page fault. Best left set to 1 unless otherwise needed.
Flags
3 2 1 0
G DB L Reserved
  • G: Granularity flag, indicates the size the Limit value is scaled by. If clear (0), the Limit is in 1 Byte blocks (byte granularity). If set (1), the Limit is in 4 KiB blocks (page granularity).
  • DB: Size flag. If clear (0), the descriptor defines a 16-bit protected mode segment. If set (1) it defines a 32-bit protected mode segment. A GDT can have both 16-bit and 32-bit selectors at once.
  • L: Long-mode code flag. If set (1), the descriptor defines a 64-bit code segment. When set, DB should always be clear. For any other type of segment (other code types or any data segment), it should be clear (0).

System Segment Descriptor

For system segments, such as those defining a Task State Segment or Local Descriptor Table, the format of the Access Byte differs slightly, in order to define different types of system segments rather than code and data segments.

For more information, see Section 3.5: System Descriptor Types and Figure 3-2: System-Segment and Gate-Descriptor Types of the Intel Software Developer Manual, Volume 3-A.

Access Byte
7 6 5 4 3 2 1 0
P    DPL S Type
  • Type: Type of system segment.

Types available in 32-bit protected mode:

  • 0x1: 16-bit TSS (Available)
  • 0x2: LDT
  • 0x3: 16-bit TSS (Busy)
  • 0x9: 32-bit TSS (Available)
  • 0xB: 32-bit TSS (Busy)

Types available in Long Mode:

  • 0x2: LDT
  • 0x9: 64-bit TSS (Available)
  • 0xB: 64-bit TSS (Busy)

Long Mode System Segment Descriptor

For a Task State Segment or Local Descriptor Table in Long Mode, the format of a Segment Descriptor differs to ensure that the Base value can contain a 64-bit Linear Address. It takes up the space in the table of two usual entries, in a little endian format, such that the lower half of this entry precedes the higher half in the table.

For more information, see Section 8.2.3: TSS Descriptor in 64-bit Mode and Figure 8-4: Format of TSS and LDT Descriptors in 64-bit Mode of the Intel Software Developer Manual, Volume 3-A.

64-bit System Segment Descriptor
127   96
Reserved
95   64
Base
63   32
63   56 55   52 51   48 47   40 39   32
Base
31   24 		Flags
3   0 		Limit
19   16 		Access Byte
7   0 		Base
23   16
31   16 15   0
Base
15   0 		Limit
15   0

See Also

Articles

External references

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