#include <stdio.h>
#include <stdint.h>
 
struct gdt_entry_struct {
  uint16_t limit_low;
  uint16_t base_low;
  uint8_t base_middle;
  uint8_t access;
  uint8_t granularity;
  uint8_t base_high;
}__attribute__((packed));
typedef struct gdt_entry_struct gdt_entry_t;

struct gdt_ptr_struct {
  uint16_t limit;
  uint32_t base;
}__attribute__((packed));
typedef struct gdt_ptr_struct gdt_ptr_t;

gdt_entry_t gdt_entries[5];
gdt_ptr_t gdt_ptr;

extern void gdt_flush(struct gdt_ptr* gdt_ptr_addr);

static void gdt_set_gate(int32_t num, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran)
{
  gdt_entries[num].base_low = (base & 0xFFFF);
  gdt_entries[num].base_middle = (base >> 16) & 0xFF;
  gdt_entries[num].base_high = (base >> 24) & 0xFF;

  gdt_entries[num].limit_low = (limit & 0xFFFF);
  gdt_entries[num].granularity = (limit >> 16) & 0x0F;
 
  gdt_entries[num].granularity |= gran & 0xF0;
  gdt_entries[num].access = access;
}

void gdt_init() {
  gdt_ptr.limit = (sizeof(gdt_entry_t) * 5) - 1;
  gdt_ptr.base = (uint32_t)&gdt_entries;
 
  gdt_set_gate(0, 0, 0, 0, 0);
  gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
  gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
  
  
  gdt_flush((uint32_t)&gdt_ptr);
}