1 files changed, 52 insertions, 0 deletions
diff --git a/src/utils/gdt.c b/src/utils/gdt.c
new file mode 100644
index 0000000..85c4a09
--- /dev/null
+++ b/src/utils/gdt.c
@@ -0,0 +1,52 @@
+#include "gdt.h"
+#include "print.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);
+}