42_KFS/headers/gdt.h

#pragma once

#include <stdint.h>

#define GDT_ADDRESS 0xC0105040
#define GDT_SIZE    7

// https://wiki.osdev.org/Global_Descriptor_Table#GDTR
struct gdt_descriptor {
	uint16_t size;
	uint32_t base;
} __attribute__((packed));

void init_gdt();

#define GDT_FLAG_64BIT_MODE 0b0010
#define GDT_FLAG_32BIT_MODE 0b0100
#define GDT_FLAG_PAGE_MODE  0b1000
#define GDT_FLAG_BYTE_MODE  0b1000

#define GDT_ACCESS_P_VALID                                0b10000000
#define GDT_ACCESS_DPL_KERNEL_MODE                        0b00000000
#define GDT_ACCESS_DPL_USER_MODE                          0b01100000
#define GDT_ACCESS_S_CODE_OR_DATA                         0b00010000
#define GDT_ACCESS_S_SYSTEM_SEGMENT                       0b00010000
#define GDT_ACCESS_E_EXECUTABLE                           0b00001000
#define GDT_ACCESS_E_NOT_EXECUTABLE                       0b00000000
#define GDT_ACCESS_DC_CONFORM                             0b00000100 // TODO UNDERSTAND THIS BIT
#define GDT_ACCESS_DC_NOT_CONFORM                         0b00000000
#define GDT_ACCESS_RW_READABLE_FOR_CODE_WRITABLE_FOR_DATA 0b00000010
#define GDT_ACCESS_A_ACCESSED                             0b00000001
#define GDT_ACCESS_A_NOT_ACCESSED                         0b00000000

extern uint8_t gdt_entries[GDT_SIZE * 8];

#define GDT_OFFSET_KERNEL_CODE  0x08
#define GDT_OFFSET_KERNEL_DATA  0x10
#define GDT_OFFSET_KERNEL_STACK 0x18
#define GDT_OFFSET_USER_CODE    0x20
#define GDT_OFFSET_USER_DATA    0x28
#define GDT_OFFSET_USER_STACK   0x30