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. More...

Classes
struct	Pointer
	Structure that describes a GDT Pointer (aka GDT Descriptor) More...

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

struct	SegmentDescriptor.__unnamed__

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	Type { TYPE_SYSTEM16 = 0, TYPE_SYSTEM32 = 1, TYPE_DATA = 2, TYPE_CODE = 3 }
	Segment type. More...

enum	Size { SIZE_16BIT = 0, SIZE_32BIT = 1 }
	Address width in code segment. More...

enum	Gate { GATE_TSS = 0x1, GATE_LDT = 0x2, GATE_TSS_BUSY = 0x3, GATE_CALL = 0x4, GATE_TASK = 0x5, GATE_INTERRUPT = 0x6, GATE_TRAP = 0x7 }
	gate type More...

enum	Mode { REAL_MODE, PROTECTED_MODE, LONG_MODE }
	Modus.

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.unnamed

struct GDT::SegmentDescriptor.__unnamed__

Class Members
uint64_t	limit_low: 16	Least-significant bits of segment size (influenced by granularity!)
uint64_t	base_low: 24	Least-significant bits of base address.
uint64_t	type: 3	Meaning of those 3 bits depends on segment_type below.
enum Type	segment_type: 2	Segment type (influences the meaning of the 3 bits above)
uint64_t	privilege_level: 2	Ring for this segment.
bool	present: 1	Entry is valid iff set to 1.
uint64_t	limit_high: 4	Most-significant bits of segment size.
bool	available: 1	One bit to use for other purposes.
uint64_t	custom: 2	Meaning of those 2 bits depends on segment_type below.
Granularity	granularity: 1	Unit used as granularity for the segment limit.
uint64_t	base_high: 8	most-significant bits of base address

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.

◆ Type

enum GDT::Type

Segment type.

Enumerator
TYPE_SYSTEM16	entry is a 16 bit system segment
TYPE_SYSTEM32	entry is a 32 bit system segment (e.g., TSS)
TYPE_DATA	entry is a data segment
TYPE_CODE	entry is a code segment

◆ Size

enum GDT::Size

Address width in code segment.

Enumerator
SIZE_16BIT	16-bit addresses by default
SIZE_32BIT	32-bit addresses by default

◆ Gate

enum GDT::Gate

gate type

See also: ISDMv3, Table 3-2; System-Segment and Gate-Descriptor Types

Enumerator
GATE_TSS_BUSY	when the task is currently running, otherwise GATE_TSS
GATE_TASK	only for 16 bit

Classes

Enumerations

Functions