Add GDT initialization

cpu/i386/cpu_init.c

@@ -1,2 +1,5 @@
+#include "gdt.h"
+
 void cpu_init() {
+  gdt_init();
 }

cpu/i386/gdt.c

@@ -0,0 +1,39 @@
+#include <stdint.h>
+#define NUM_ENTRIES 4
+
+typedef struct {
+  uint16_t limit_low16;
+  uint16_t base_low16;
+  uint8_t base_mid8;
+  uint8_t access;
+  uint8_t limit_flags;
+  uint8_t base_high8;
+} gdt_entry;
+
+typedef struct {
+  uint16_t size;
+  uint32_t address;
+} gdt_description;
+
+gdt_entry gdt[NUM_ENTRIES-1];
+gdt_description gdt_desc;
+
+
+void gdt_init() {
+  set_entry(0,0,0,0);
+  set_entry(1,0,0xFFFFF,0x9E);
+  set_entry(2,0,0xFFFFF,0x92);
+  gdt_desc.size=(sizeof(gdt_entry)*NUM_ENTRIES)-1;
+  gdt_desc.address=&gdt[0];
+  asm volatile("lgdt gdt_desc");
+}
+
+void set_entry(int i,uint32_t base,uint32_t limit,uint8_t access) {
+  gdt[i].limit_low16=limit&0xFFFF;
+  gdt[i].base_low16=base&0xFFFFF;
+  gdt[i].base_mid8=(base&0xFF0000)>>16;
+  gdt[i].access=access;
+  uint8_t limit_high4=(limit&0xF0000)>>16;
+  gdt[i].limit_flags=0xC0&limit_high4;
+  gdt[i].base_high8=(base&0xFF000000)>>24;
+}

cpu/i386/gdt.h

@@ -0,0 +1,6 @@
+#ifndef GDT_H
+#define GDT_H
+
+void gdt_init();
+
+#endif