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

Classes
struct	SegmentDescriptor.__unnamed32__
	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 = 1 }
	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

Class Documentation

◆ GDT::SegmentDescriptor.unnamed32

struct GDT::SegmentDescriptor.__unnamed32__

Fields specific for Data Segment (for debugging purposes)

See also: ISDMv3, 3.4.5.1; Code- and Data-Segment Descriptor Types

Class Members
uint64_t	__pad0__: 40	Ignored (set via `limit_low` and `base_low`)
bool	data_accessed: 1	If set, the data segment was used since the last reset of this value.
bool	writeable: 1	If set, data is writable (otherwise read only)
bool	expand_down: 1	Growing direction for dynamically growing segments.
bool	notData: 1	Has to be cleared (`false`)
uint64_t	__pad1__: 9	Ignored (set via `privilege_level` ... `available`)
uint64_t	reserved: 1	Reserved, always set to `0`!
bool	big: 1	Size of the stack pointer (`false` = 16 bit, `true` = 32 bit) Warning Has a different meaning in case expand_down is set to `1`.
uint64_t	__pad2__: 0	Remainder ignored.

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	64-bit (D/B = 0, L = 1)

Classes

Enumerations

Functions