#ifndef GDT_H
#define GDT_H
#include <string.h>
#include <typedefs.h>

#define GDT_ENTRY_SIZE 0x8
#define GDT_NULL_SEGMENT 0x0
#define GDT_KERNEL_CODE_SEGMENT 0x1
#define GDT_KERNEL_DATA_SEGMENT 0x2
#define GDT_USERMODE_CODE_SEGMENT 0x3
#define GDT_USERMODE_DATA_SEGMENT 0x4
#define GDT_TSS_SEGMENT 0x5

struct GDT_Entry {
  u16 limit_low;
  u32 base_low : 24;
  u32 accessed : 1;
  u32 read_write : 1;             // readable for code, writable for data
  u32 conforming_expand_down : 1; // conforming for code, expand down for data
  u32 code : 1;                   // 1 for code, 0 for data
  u32 code_data_segment : 1;      // should be 1 for everything but TSS and LDT
  u32 DPL : 2;                    // privilege level
  u32 present : 1;
  u32 limit_high : 4;
  u32 available : 1; // only used in software; has no effect on hardware
  u32 long_mode : 1;
  u32 big : 1;  // 32-bit opcodes for code, u32 stack for data
  u32 gran : 1; // 1 to use 4k page addressing, 0 for byte addressing
  u8 base_high;
} __attribute__((packed));

typedef struct GDT_Pointer {
  u16 size;
  u32 offset;
} __attribute__((packed)) GDT_Pointer;

struct tss_entry_struct {
  u32 prev_tss; // The previous TSS - with hardware task switching these form a
                // kind of backward linked list.
  u32 esp0;     // The stack pointer to load when changing to kernel mode.
  u32 ss0;      // The stack segment to load when changing to kernel mode.
  // Everything below here is unused.
  u32 esp1; // esp and ss 1 and 2 would be used when switching to rings 1 or 2.
  u32 ss1;
  u32 esp2;
  u32 ss2;
  u32 cr3;
  u32 eip;
  u32 eflags;
  u32 eax;
  u32 ecx;
  u32 edx;
  u32 ebx;
  u32 esp;
  u32 ebp;
  u32 esi;
  u32 edi;
  u32 es;
  u32 cs;
  u32 ss;
  u32 ds;
  u32 fs;
  u32 gs;
  u32 ldt;
  u16 trap;
  u16 iomap_base;
} __attribute__((packed));

void gdt_init();

u8 gen_access_byte(u8 priv, u8 s, u8 ex, u8 dc, u8 rw);
u64 gen_gdt_entry(u32 base, u32 limit, u8 access_byte, u8 flag);
u8 gen_flag(u8 gr, u8 sz);
#endif