GDT Namespace Reference

Abstracts the GDT that, primarily, contains descriptors to memory segments. More...

Classes
union	SegmentDescriptor
	Describes the structure of segment descriptors. More...

Structs
struct	Pointer
	Structure that describes a GDT Pointer (aka GDT Descriptor). More...
struct	SegmentDescriptor.__unnamed0__
struct	SegmentDescriptor.__unnamed1__
	Fields specific for Code Segment (for debugging purposes). More...
struct	SegmentDescriptor.__unnamed2__
	Fields specific for Data Segment (for debugging purposes). More...

Enumerations
enum	Segments {   SEGMENT_NULL = 0 ,   SEGMENT_KERNEL_CODE ,   SEGMENT_KERNEL_DATA }
enum	Granularity {   GRANULARITY_BYTES = 0 ,   GRANULARITY_4KBLOCK = 1 }
	Unit of the segment limit. More...
enum	DescriptorType {   DESCRIPTOR_SYSTEM = 0 ,   DESCRIPTOR_CODEDATA = 1 }
	Descriptor type. More...
enum	Size {   SIZE_16BIT = 0 ,   SIZE_32BIT = 2 ,   SIZE_64BIT_CODE = 1 ,   SIZE_64BIT_DATA = 0 }
	Address width. More...

Functions
constexpr Pointer	gdt_protected_mode_pointer (protected_mode)
constexpr Pointer	gdt_long_mode_pointer (long_mode)

Detailed Description

Abstracts the GDT that, primarily, contains descriptors to memory segments.

The GDT is a table that primarily contains segment descriptors. Segment descriptors has a size of 8 Bytes and contains the size, position, access rights, and purpose of such a segment. Unlike the LDT, the GDT is shared between all processes and may contain TSS and LDT descriptors. For the kernel, the first entry is required to be a null descriptor and the code and data segments. To support user-mode processes, additional TSS, code, and data segments for ring 3 must be added.

The base address and size of the GDT are written to the GDTR register during boot (via. lgdt).

See also

ISDMv3, 2.4.1; Global Descriptor Table Register (GDTR)

ISDMv3, 3.5.1; Segment Descriptor Tables

Enumeration Type Documentation

Granularity

enum GDT::Granularity

Unit of the segment limit.

Enumerator
GRANULARITY_BYTES	Segment limit in Bytes.
GRANULARITY_4KBLOCK	Segment limit in blocks of 4 Kilobytes.

DescriptorType

enum GDT::DescriptorType

Descriptor type.

Enumerator
DESCRIPTOR_SYSTEM	entry is a system segment
DESCRIPTOR_CODEDATA	entry is a code/data segment

Size

enum GDT::Size

Address width.

Enumerator
SIZE_16BIT	16-bit (D/B = 0, L = 0)
SIZE_32BIT	32-bit (D/B = 1, L = 0)
SIZE_64BIT_CODE	64-bit (D/B = 0, L = 1)
SIZE_64BIT_DATA	64-bit (D/B = 0, L = 0)