#include "gdt.h"
#include "mem.h"

struct GDT_REGISTER GDTR = { 0, 0 };
GDT_TSS TSS;

void gdt_memcpy(){
  GDTR.limit=8*8; // Each entry is 8 bytes and 8 entries
  GDTR.base=0x800;
  gdt_write_entry((GDT_ENTRY){0,0,0,0},0); // First one must be null

  // ----- Kernel Segments
  GDT_ENTRY cs_desc;
  cs_desc.base=0;
  cs_desc.limit=0xFFFFF;
  cs_desc.flags=GDT_SZ|GDT_GR;
  cs_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_EXEC|GDT_RW;
  
  GDT_ENTRY ds_desc;
  ds_desc.base=0;
  ds_desc.limit=0xFFFFF;
  ds_desc.flags=GDT_SZ|GDT_GR;
  ds_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_RW;
  
  GDT_ENTRY ss_desc;
  ss_desc.base=0;
  ss_desc.limit=0;
  ss_desc.flags=GDT_SZ|GDT_GR;
  ss_desc.access=GDT_PR|GDT_PRVL_0|GDT_S|GDT_RW|GDT_DC;
  
  // Write GDT descriptors into memory
  gdt_write_entry(cs_desc, 1);
  gdt_write_entry(ds_desc, 2);
  gdt_write_entry(ss_desc, 3);

  // ----- User Segments
  GDT_ENTRY ucs_desc;
  ucs_desc.base=0x300000;
  ucs_desc.limit=0xFFFFF;
  ucs_desc.flags=GDT_SZ|GDT_GR;
  ucs_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_EXEC|GDT_RW|GDT_DC;
  
  GDT_ENTRY uds_desc;
  uds_desc.base=0x300000;
  uds_desc.limit=0xFFFFF;
  uds_desc.flags=GDT_SZ|GDT_GR;
  uds_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_RW;
  
  GDT_ENTRY uss_desc;
  uss_desc.base=0; // Not used in stack descriptor
  uss_desc.limit=0x64;  // Define how much entry it can contains
  uss_desc.flags=GDT_SZ|GDT_GR;
  uss_desc.access=GDT_PR|GDT_PRVL_3|GDT_S|GDT_RW|GDT_DC;
  
  // Write GDT descriptors into memory
  gdt_write_entry(ucs_desc, 4); // Each entry is 64 bits (8 bytes)
  gdt_write_entry(uds_desc, 5);
  gdt_write_entry(uss_desc, 6);

  // Init TSS segment
  TSS.t_reserved=0;
  TSS.io_map=0;
  TSS.ss0=0x18;
  TSS.esp0=0x50000;

  GDT_ENTRY tss_desc;
  tss_desc.base=(u32)&TSS; // Not used in stack descriptor
  tss_desc.limit=0x68;  // Define how much entry it can contains
  tss_desc.flags=0;
  tss_desc.access=0x89 | GDT_PRVL_3; // Note that 0x89 is specific to TSS!
  gdt_write_entry(tss_desc, 7);

}

void gdt_write_entry(GDT_ENTRY entry, u32 id){
  int descriptor[2];

  // First row of the descriptor
  descriptor[0]=(entry.limit & 0xFFFF)|(entry.base << 16);

  // Format second row of the descriptor
  u16 base=(entry.base >> 16) & 0xFF;
  u16 access=entry.access & 0xFF;
  u16 limit=(entry.limit >> 16) & 0xF; // Remember: limits it is on 20 bits so 4 last bits
  u8 flags=entry.flags & 0xF;
  u8 base2=entry.base >> 24; // Take the last 8 bits
  descriptor[1]=base|access<<8|limit<<16|flags<<20|base2<<24;

  // Copy descriptor into memory
  memcpy(descriptor,(void*)GDTR.base+8*id,8); // Each entry is 64 bits (8 bytes)
}