Is used to define the characteristics of the various memory areas used during program execution, including the base address, the size and access privileges like executability and writability. More...

Classes
struct	gdt_descriptor_t
	Data structure representing a GDT descriptor. More...

struct	gdt_pointer_t
	Data structure used to load the GDT into the GDTR. More...

Macros
#define	IDT_PADDING 14U
	Used in IDT for padding.

Typedefs
typedef struct gdt_descriptor_t	_syscall0 = _syscall0( (packed) )= _syscall1( (packed) )= _syscall2( (packed) )= _syscall3( (packed) )= gdt_descriptor_t
	Data structure representing a GDT descriptor. More...

Enumerations
enum	gdt_bits_t { GDT_PRESENT = 128U , GDT_KERNEL = 0U , GDT_USER = 96U , GDT_S = 16U , GDT_EX = 8U , GDT_DC = 4U , GDT_RW = 2U , GDT_AC = 1U , GDT_CODE = (GDT_S \| GDT_EX) , GDT_DATA = (GDT_S \| GDT_RW) , GDT_GRANULARITY = 128U , GDT_OPERAND_SIZE = 64U }
	Bitmasks used to access to specific bits of the GDT. More...

Functions
void	init_gdt (void)
	Initialise the GDT. More...

void	gdt_set_gate (uint8_t index, uint32_t base, uint32_t limit, uint8_t access, uint8_t granul)
	Sets the value of one GDT entry. More...

Detailed Description

Is used to define the characteristics of the various memory areas used during program execution, including the base address, the size and access privileges like executability and writability.

Typedef Documentation

◆ _syscall0

typedef struct page_directory_t _syscall0 = _syscall0( (packed) )= _syscall1( (packed) )= _syscall2( (packed) )= _syscall3( (packed) )= gdt_descriptor_t

Data structure representing a GDT descriptor.

A page directory.

A pointer structure used for informing the CPU about our IDT.

Data structure used to load the GDT into the GDTR.

In the two-level paging, this is the first level.

Enumeration Type Documentation

◆ gdt_bits_t

enum gdt_bits_t

Bitmasks used to access to specific bits of the GDT.

PRIV (Privilege bits)

Contains the ring level, specifically:

00 (0) = highest (kernel),
11 (3) = lowest (user applications).

S (Descriptor type)

This bit should be set for code or data segments and should be cleared for system segments (eg. a Task State Segment).

EX (Executable bit)

If 1 code in this segment can be executed, ie. a code selector.
If 0 it is a data selector.

DC (Direction bit/Conforming bit)

Direction bit for data selectors: Tells the direction. 0 the segment grows up. 1 the segment grows down, ie., the offset has to be greater than the limit.

Conforming bit for code selectors:

If 1 code in this segment can be executed from an equal or lower privilege level. For example, code in ring 3 can far-jump to conforming code in a ring 2 segment. The privl-bits represent the highest privilege level that is allowed to execute the segment. For example, code in ring 0 cannot far-jump to a conforming code segment with privl==0x2, while code in ring 2 and 3 can. Note that the privilege level remains the same, ie. a far-jump form ring 3 to a privl==2-segment remains in ring 3 after the jump.
If 0 code in this segment can only be executed from the ring set in privl.

RW (Readable bit/Writable bit)

Readable bit for code selectors:
- Whether read access for this segment is allowed.
- Write access is never allowed for code segments.
Writable bit for data selectors:
- Whether write access for this segment is allowed.
- Read access is always allowed for data segments.

AC (Accessed bit)

Just set to 0. The CPU sets this to 1 when the segment is accessed.

GR (Granularity bit)

If 0 the limit is in 1 B blocks (byte granularity);
if 1 the limit is in 4 KiB blocks (page granularity).

SZ (Size bit)

If 0 the selector defines 16 bit protected mode;
if 1 it defines 32 bit protected mode.

You can have both 16 bit and 32 bit selectors at once.

Enumerator
GDT_PRESENT	`0b10000000U` (Present): This must be 1 for all valid selectors.
GDT_KERNEL	`0b00000000U` (Privilege): Sets the 2 privilege bits (ring level) to 0 = highest (kernel).
GDT_USER	`0b01100000U` (Privilege): Sets the 2 privilege bits (ring level) to 3 = lowest (user applications).
GDT_S	`0b00010000U` (Descriptor): This bit should be set for code or data segments and should be cleared for system segments (eg. a Task State Segment)
GDT_EX	`0b00001000U` (Executable): If 1 code in this segment can be executed, ie. a code selector. If 0 it is a data selector.
GDT_DC	`0b00000100U` (Direction/Conforming)
GDT_RW	`0b00000010U` (Readable/Writable)
GDT_AC	`0b00000001U` (Accessed): Just set to 0. The CPU sets this to 1 when the segment is accessed.
GDT_CODE	`0b00001100U` (Executable Code): Identifies an executable code segment.
GDT_DATA	`0b00001001U` (Writable Data): Identifies a writable data segment.
GDT_GRANULARITY	`0b10000000U` (Granularity): If 0 the limit is in 1 B blocks (byte granularity), if 1 the limit is in 4 KiB blocks (page granularity).
GDT_OPERAND_SIZE	`0b01000000U` (Size): If 0 the selector defines 16 bit protected mode. If 1 it defines 32 bit protected mode. You can have both 16 bit and 32 bit selectors at once.

Function Documentation

◆ gdt_set_gate()

void gdt_set_gate	(	uint8_t	index,
		uint32_t	base,
		uint32_t	limit,
		uint8_t	access,
		uint8_t	granul
	)

Sets the value of one GDT entry.

Parameters

index	The index inside the GDT.
base	Memory address where the segment we are defining starts.
limit	The memory address which determines the end of the segnment.
access	Type (4bit) - S (1) bit -DPL (2 bit) - P(1 bit).
granul	SegLimit_hi(4 bit) AVL(1 bit) L(1 bit) D/B(1 bit) G(1bit).

◆ init_gdt()

void init_gdt ( void )

Initialise the GDT.

This will setup the special GDT pointer, set up the first 3 entries in our GDT, and then finally call gdt_flush() in our assembler file in order to tell the processor where the new GDT is and update the new segment registers.

Classes

Macros

Typedefs

Enumerations

Functions