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

struct GDT_REGISTER GDTR = { 0, 0 };

void gdt_memcpy(){
  GDTR.limit=8*4; // Each entry is 8 bytes and 4 entries
  GDTR.base=0x800;
  
  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((GDT_ENTRY){0,0,0,0},GDTR.base); // First one must be null
  gdt_write_entry(cs_desc, GDTR.base+8); // Each entry is 64 bits (8 bytes)
  gdt_write_entry(ds_desc, GDTR.base+8*2);
  gdt_write_entry(ss_desc, GDTR.base+8*3);
}

void gdt_write_entry(GDT_ENTRY entry, u32 addr){
  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*)addr,8);
}