1 files changed, 50 insertions, 0 deletions
diff --git a/kernel/global_descriptor_table.c b/kernel/global_descriptor_table.c
new file mode 100644
index 0000000..16b99dc
--- /dev/null
+++ b/kernel/global_descriptor_table.c
@@ -0,0 +1,50 @@
+extern void flush_gdt();
+
+struct gdt_entry
+{
+  unsigned short limit_low;
+  unsigned short base_low;
+  unsigned char base_middle;
+  unsigned char access;
+  unsigned char granularity;
+  unsigned char base_high;
+} __attribute__((packed));
+
+struct gdt_ptr
+{
+  unsigned short limit;
+  unsigned int base;
+} __attribute__((packed));
+
+struct gdt_entry gdt[3];
+struct gdt_ptr gdt_pointer;
+
+void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)
+{
+
+  gdt[num].base_low = (base & 0xFFFF);
+  gdt[num].base_middle = (base >> 16) & 0xFF;
+  gdt[num].base_high = (base >> 24) & 0xFF;
+
+  gdt[num].limit_low = (limit & 0xFFFF);
+  gdt[num].granularity = ((limit >> 16) & 0x0F);
+
+  gdt[num].granularity |= (gran & 0xF0);
+  gdt[num].access = access;
+}
+
+void init_gdt()
+{
+  gdt_pointer.limit = (sizeof(struct gdt_entry) * 3) - 1;
+  gdt_pointer.base = (unsigned int) &gdt;
+
+  // Values taken from bkerndev
+  // First GDT segment must be null
+  gdt_set_gate(0, 0, 0, 0, 0);
+  // Code segment
+  gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
+  // Data segment
+  gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
+
+  flush_gdt();
+}
+\ No newline at end of file