1 files changed, 98 insertions, 0 deletions
diff --git a/src/core/gdt.c b/src/core/gdt.c
new file mode 100644
index 0000000..8c1c70d
--- /dev/null
+++ b/src/core/gdt.c
@@ -0,0 +1,98 @@
+#include "gdt.h"
+#include "mem.h"
+
+struct GDT_REGISTER GDTR = { 0, 0 };
+GDT_TSS TSS;
+
+void gdt_memcpy(){
+  GDTR.limit=8*GDT_MAX_ENTRIES; // 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=0; // With paging will take care of allocating the right address for each task
+  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=0;
+  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;
+
+  GDT_ENTRY tss_desc;
+  tss_desc.base=(u32)&TSS; // Not used in stack descriptor
+  tss_desc.limit=0x68;  // Define how much bytes it occupies
+  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)
+}
+
+int gdt_user_ds_base(){
+  char *addr=(char*)GDTR.base+48;
+  int *base0_15=(int*)addr+2;
+  int *base16_31=(int*)addr+7;
+  int base0_15_content=*base0_15 & 0xFFFF;
+  int base16_21_content=*base16_31 & 0xFFFF;
+  return(base16_21_content<<16 & base0_15_content);
+}
+\ No newline at end of file