GDT

Global Descriptor Table. More...

Collaboration diagram for GDT:

Detailed Description

Global Descriptor Table.

The global descriptor table is a legacy feature from the days of 32-bit x86 and older. Most of its features are unused, but is still required in 64-bit mode to specify the current privilage level and to load the TSS.

See also

OSDev Wiki GDT

Data Structures
struct	gdt_desc_t
	GDT descriptor structure. More...
struct	gdt_segment_t
	A single GDT segment descriptor. More...
struct	gdt_long_system_segment_t
	A long mode system segment descriptor, used for TSS and LDT. More...
struct	gdt_t
	The actual GDT structure. More...

Enumerations
enum	gdt_ring_t { GDT_RING0 = 0 , GDT_RING1 = 1 , GDT_RING2 = 2 , GDT_RING3 = 3 }
enum	gdt_segment_selector_t {   GDT_NULL = 0 , GDT_KERNEL_CODE = 0x08 , GDT_KERNEL_DATA = 0x10 , GDT_USER_DATA = 0x18 ,   GDT_USER_CODE = 0x20 , GDT_TSS = 0x28 , GDT_CS_RING0 = GDT_KERNEL_CODE | GDT_RING0 , GDT_SS_RING0 = GDT_KERNEL_DATA | GDT_RING0 ,   GDT_CS_RING3 = GDT_USER_CODE | GDT_RING3 , GDT_SS_RING3 = GDT_USER_DATA | GDT_RING3 }
	Segment selectors. More...
enum	gdt_access_t {   GDT_ACCESS_ACCESSED = 1 << 0 , GDT_ACCESS_READ_WRITE = 1 << 1 , GDT_ACCESS_DIRECTION_CONFORMING = 1 << 2 , GDT_ACCESS_EXEC = 1 << 3 ,   GDT_ACCESS_SYSTEM = 0 << 4 , GDT_ACCESS_DATA_CODE = 1 << 4 , GDT_ACCESS_TYPE_LDT = 0x2 , GDT_ACCESS_TYPE_TSS_AVAILABLE = 0x9 ,   GDT_ACCESS_TYPE_TSS_BUSY = 0xB , GDT_ACCESS_RING0 = 0 << 5 , GDT_ACCESS_RING1 = 1 << 5 , GDT_ACCESS_RING2 = 2 << 5 ,   GDT_ACCESS_RING3 = 3 << 5 , GDT_ACCESS_PRESENT = 1 << 7 }
	Access byte. More...
enum	gdt_flags_t {   GDT_FLAGS_NONE = 0 , GDT_FLAGS_RESERVED = 1 << 0 , GDT_FLAGS_LONG_MODE = 1 << 1 , GDT_FLAGS_SIZE = 1 << 2 ,   GDT_FLAGS_4KB = 1 << 3 }
	Flags byte. More...

Functions
void	gdt_load_descriptor (gdt_desc_t *descriptor)
	Loads a GDT descriptor.
void	gdt_init (void)
	Initialize the GDT.
void	gdt_cpu_load (void)
	Load the GDT on the current CPU.
void	gdt_cpu_tss_load (tss_t *tss)
	Load a TSS into the GDT and load it using the ltr instruction on the current CPU.

Enumeration Type Documentation

gdt_ring_t

enum gdt_ring_t

Enumerator
GDT_RING0
GDT_RING1
GDT_RING2
GDT_RING3

Definition at line 21 of file gdt.h.

gdt_segment_selector_t

enum gdt_segment_selector_t

Segment selectors.

These values are stored in the segment registers (CS, DS, ES, FS, GS, SS) to tell the CPU which segment of the GDT to use.

Enumerator
GDT_NULL	Null segment selector, unused but the gdt must start with it.
GDT_KERNEL_CODE	Kernel code segment selector.
GDT_KERNEL_DATA	Kernel data segment selector.
GDT_USER_DATA	User data segment selector.
GDT_USER_CODE	User code segment selector.
GDT_TSS	TSS segment selector.
GDT_CS_RING0	Value to load into the CS register for kernel code.
GDT_SS_RING0	Value to load into the SS register for kernel data.
GDT_CS_RING3	Value to load into the CS register for user code.
GDT_SS_RING3	Value to load into the SS register for user data.

Definition at line 36 of file gdt.h.

gdt_access_t

enum gdt_access_t

Access byte.

Enumerator
GDT_ACCESS_ACCESSED	Will be set to 1 when accessed, but its best to set it to 1 manually.
GDT_ACCESS_READ_WRITE	If set on a code segment, its readable. If set on a data segment, its writable.
GDT_ACCESS_DIRECTION_CONFORMING	If set on a data segment, the segment grows downwards. If set on a code segment, conforming.
GDT_ACCESS_EXEC	If set, the segment is a code segment. If clear, its a data segment.
GDT_ACCESS_SYSTEM	Is a system segment.
GDT_ACCESS_DATA_CODE	Is a data or code segment.
GDT_ACCESS_TYPE_LDT	Local Descriptor Table. Only used if the SYSTEM bit is 0.
GDT_ACCESS_TYPE_TSS_AVAILABLE	Available 64-bit Task State Segment. Only used if the SYSTEM bit is 0.
GDT_ACCESS_TYPE_TSS_BUSY	Busy 64-bit Task State Segment. Only used if the SYSTEM bit is 0.
GDT_ACCESS_RING0	Descriptor Privilege Level 0 (kernel).
GDT_ACCESS_RING1	Descriptor Privilege Level 1, unused.
GDT_ACCESS_RING2	Descriptor Privilege Level 2, unused.
GDT_ACCESS_RING3	Descriptor Privilege Level 3 (user).
GDT_ACCESS_PRESENT	Must be 1 for all valid segments.

Definition at line 56 of file gdt.h.

gdt_flags_t

enum gdt_flags_t

Flags byte.

Enumerator
GDT_FLAGS_NONE	No flags set.
GDT_FLAGS_RESERVED	Must be 0.
GDT_FLAGS_LONG_MODE	If set, its a 64-bit code segment.
GDT_FLAGS_SIZE	If set, its a 32-bit segment. If clear, its a 16-bit segment. Ignored in 64-bit mode.
GDT_FLAGS_4KB	If set, the limit is in 4KiB blocks. If clear, the limit is in bytes.

Definition at line 82 of file gdt.h.

Function Documentation

gdt_load_descriptor()

void gdt_load_descriptor ( gdt_desc_t *  descriptor )

extern

Loads a GDT descriptor.

Dont use this directly use gdt_cpu_load() instead.

Parameters

descriptor

The GDT descriptor to load.

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gdt_init()

void gdt_init

(

void

)

Initialize the GDT.

This will setup the GDT structure in memory, but will not load it. Loading is done in gdt_cpu_load().

Definition at line 35 of file gdt.c.

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gdt_cpu_load()

void gdt_cpu_load

(

void

)

Load the GDT on the current CPU.

This will load the GDT using the lgdt instruction and then reload all segment registers to use the new GDT.

Must be called after gdt_init().

Definition at line 55 of file gdt.c.

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gdt_cpu_tss_load()

void gdt_cpu_tss_load

(

tss_t *

tss

)

Load a TSS into the GDT and load it using the ltr instruction on the current CPU.

Note that the actual TTS descriptor in the GDT can be shared between CPUs, as its only used while loading the TSS, after that its just useless.

Parameters

tss	The TSS to load.

Definition at line 63 of file gdt.c.

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