GDT

Global Descriptor Table. More...

Collaboration diagram for GDT:

Data Structures
struct	gdtr_t
	the GDTR register pointing to the GDT More...
union	gdt_selector_t
	A selector in the GDT. More...
struct	gdt_entry_t
	An entry in the GDT. More...

Macros
#define	GDT_ENTRIES   6
	number of entries in the GDT More...
#define	GDT_RING0_CODE_SEG   1
	kernel code segment index
#define	GDT_RING0_DATA_SEG   2
	kernel data segment index
#define	GDT_RING3_CODE_SEG   3
	user code segment index
#define	GDT_RING3_DATA_SEG   4
	user data segment index
#define	GDT_TASK_STATE_SEG   5
	task state segment index

Functions
void	gdt_init_entry (size_t entry, uint32_t base, uint32_t limit)
	Sets basic parameters of a GDT entry. More...
static void	gdt_load ()
	Loads the GDT into the GDTR register.
void	gdt_flush ()
	Loads the kernel code and data segments into the segment registers. More...
void	gdt_init ()
	Initializes the GDT. More...
uint16_t	gdt_get_selector (size_t entry)
	Returns a selector ready to be loaded in a segment register. More...

Variables
static gdt_entry_t	gdt [GDT_ENTRIES]
	The GDT itself. It is located inside the kernel.

Detailed Description

Global Descriptor Table.

The GDT defines memory segments that can be used for memory protection. Here we use a flat memory model, therefore we only need 2 entries for the kernel and user space each and a TSS.

See also

gdt_asm.S

http://www.lowlevel.eu/wiki/Global_Descriptor_Table

http://wiki.osdev.org/Segmentation

http://wiki.osdev.org/GDT

http://wiki.osdev.org/GDT_Tutorial

https://en.wikibooks.org/wiki/X86_Assembly/Global_Descriptor_Table

https://stackoverflow.com/questions/23978486/far-jump-in-gdt-in-bootloader/23979175#23979175

Macro Definition Documentation

#define GDT_ENTRIES   6

number of entries in the GDT

See also

gdt_init

Definition at line 12 of file gdt.h.

Function Documentation

void gdt_flush

(

)

Loads the kernel code and data segments into the segment registers.

See also

gdt_flush in gdt_asm.S

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uint16_t gdt_get_selector

(

size_t

entry

)

Returns a selector ready to be loaded in a segment register.

These selectors are used in segment registers, the IDT and to switch to user space.

Parameters

entry

index into the GDT

Returns

a GDT selector corresponding to the GDT entry

See also

GDT_RING0_CODE_SEG

GDT_RING0_DATA_SEG

GDT_RING3_CODE_SEG

GDT_RING3_DATA_SEG

GDT_TASK_STATE_SEG

Definition at line 103 of file gdt.c.

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void gdt_init

(

)

Initializes the GDT.

Creates the following descriptors for a flat memory model:

• null descriptor
• kernel code segment
• kernel data segment
• user code segment
• user data segment
• task state segment

Then loads the GDT into GDTR and sets the segments registers.

It also loads the TSS into the TR register.

Definition at line 72 of file gdt.c.

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void gdt_init_entry	(	size_t	entry,
		uint32_t	base,
		uint32_t	limit
	)

Sets basic parameters of a GDT entry.

Parameters

entry	index into the GDT
base	where the segment begins
limit	maximum offset allowed

Definition at line 49 of file gdt.c.

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