Added GDT code in C

src/gdt.c

+#include "gdt.h"
+
+struct gdtr{
+  uint16_t limit;
+  uint32_t base;
+}
+
+static struct gdt_desc _gdt [MAX_GDT_DESC];
+static struct gdtr _gdtr;
+
+static void gdt_install () {
+  //TODO: VOODOO MAGIC
+}
+
+void gdt_set_desc(uint32_t i, uint64_t base, uint64_t limit, uint8_t access, uint8_t gran)
+{
+  if (i > MAX_GDT_DESC)
+    return;
+  memset ((void*)&_gdt[i], 0, sizeof (gdt_descriptor));
+  
+  _gdt[i].low_base = uint16_t(base & 0xffff);
+  _gdt[i].mid_base = uint8_t((base >> 16) & 0xff);
+  _gdt[i].high_base= uint8_t((base >> 24) & 0xff);
+  _gdt[i].lim	   = uint16_t(limit & 0xffff);
+  
+  //! set flags and grandularity bytes
+  _gdt[i].flags = access;
+  _gdt[i].gran = uint8_t((limit >> 16) & 0x0f);
+  _gdt[i].gran |= gran & 0xf0;
+}
+gdt_descriptor* i86_gdt_get_descriptor (int i) {
+
+  if (i > MAX_GDT_DESC)
+    return 0;
+  return &_gdt[i];
+}
+int i86_gdt_initialize () {
+  _gdtr.limit = (sizeof (struct gdt_desc) * MAX_GDT_DESC)-1;
+  _gdtr.base = (uint32_t)&_gdt[0];
+  //null
+  gdt_set_descriptor(0, 0, 0, 0, 0);
+  //code
+  gdt_set_descriptor (1,0,0xffffffff, GDT_RW|GDT_EXE|GDT_CDT|GDT_MEM, GDT_GR_PG|GDT_GR_32|GDT_GR_HI);
+  //data
+  gdt_set_descriptor (2,0,0xffffffff, GDT_RW|GDT_CDT|GDT_MEM, GDT_GR_PG|GDT_GR_32|GDT_GR_HI);
+  gdt_install();  
+  return 0;
+}

src/gdt.h

+#ifndef _GDT_INC
+#define _GDT_INC
+
+#include <stdint.h>
+
+#define MAX_GDT_DESC 3
+#define GDT_ACC 0x0001
+#define GDT_RW  0x0002
+#define GDT_DIR 0x0004
+#define GDT_EXE	0x0008
+#define GDT_CDT 0x0010
+#define GDT_DPL	0x0060
+#define GDT_MEM	0x0080
+
+#define GDT_GR_HI 0x0f
+#define GDT_GR_OS 0x10
+#define GDT_GR_32 0x40
+#define GDT_GR_PG 0x80
+
+struct gdt_desc {
+uint16_t lim;
+uint16_t low_base;
+uint8_t  mid_base;
+uint8_t  flags;
+uint8_t  gran;
+uint8_t  high_base;
+};
+
+extern void gdt_set_desc(uint32_t i, uint64_t base, uint64_t limit, uint8_t access, uint8_t gran);
+extern gdt_desc* gdt_get_desc(int i);
+extern int gdt_init();
+
+#endif