Added the Global Descriptor Table.

kernel/gdt.c

@@ -0,0 +1,50 @@
+#include "../includes/gdt.h"
+/* Code taken from http://www.osdever.net/bkerndev/Docs/gdt.htm with some minor tweaks. */
+
+/* Should be called by main. This will setup the special GDT
+*  pointer, set up the first 3 entries in our GDT, and then
+*  finally call load_gdt() in our assembler file in order
+*  to tell the processor where the new GDT is and update the
+*  new segment registers */
+void gdt_install()
+{
+	struct gdt_entry gdt_entries[3];
+	struct gdt gdt_ptr;
+    /* Setup the GDT pointer and the size of the table */
+    gdt_ptr.size = sizeof(struct gdt_entry) * 3 - 1;
+    gdt_ptr.address = (unsigned int) &gdt_entries;
+
+    /* Our NULL descriptor */
+    gdt_set_gate(0, 0, 0, 0, &gdt_entries[0]);
+
+    /* The second entry is our Code Segment. The base address
+    *  is 0, the limit is 4GBytes, it uses 4KByte granularity,
+    *  uses 32-bit opcodes, and is a Code Segment descriptor.
+    *  Please check the table above in the tutorial in order
+    *  to see exactly what each value means */
+    gdt_set_gate(0, 0xFFFFFFFF, 0x9A, 0xCF, &gdt_entries[1]);
+
+    /* The third entry is our Data Segment. It's EXACTLY the
+    *  same as our code segment, but the descriptor type in
+    *  this entry's access byte says it's a Data Segment */
+    gdt_set_gate(0, 0xFFFFFFFF, 0x92, 0xCF, &gdt_entries[2]);
+
+    /* Flush out the old GDT and install the new changes! */
+    load_gdt(&gdt_ptr);
+}
+
+void gdt_set_gate(unsigned long base, unsigned long limit, unsigned char access, unsigned char gran, struct gdt_entry* gdt_entry)
+{
+    /* Setup the descriptor base address */
+    gdt_entry->base_low = (base & 0xFFFF);
+    gdt_entry->base_middle = (base >> 16) & 0xFF;
+    gdt_entry->base_high = (base >> 24) & 0xFF;
+
+    /* Setup the descriptor limits */
+    gdt_entry->limit_low = (limit & 0xFFFF);
+    gdt_entry->granularity = ((limit >> 16) & 0x0F);
+
+    /* Finally, set up the granularity and access flags */
+    gdt_entry->granularity |= (gran & 0xF0);
+    gdt_entry->access = access;
+}

kernel/io.s

@@ -16,6 +16,25 @@ global inb
 ; stack: [esp + 4] The address of the I/O port
 ;		 [esp    ] The return address
 inb:
-	mov dx, [esp + 4] 	; Move the address of the I/O port to the dx register.
+	mov dx, [esp + 4] 	; Move the address of the I/O port to the dx register
 	in al, dx 			; Read a byte from the I/O port and store it in the al register
 	ret
+	
+global load_gdt
+
+; load_gdt - Installs a new GDT
+; stack: [esp + 4] The address of the GDT
+;		 [esp    ] The return addess
+load_gdt:
+	mov eax, [esp + 4]
+	lgdt [eax]
+	mov ax, 0x10
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	mov ss, ax
+	jmp 0x08:return		; Perform a "far jump" to load 0x08 in the CS register
+	
+return:
+	ret					; Now we've changed CS to 0x08

kernel/kmain.c

@@ -1,4 +1,5 @@
 #include "../includes/io.h"
+#include "../includes/gdt.h"
 #include "../includes/serial.h"
 #include "../includes/screen.h"
 
@@ -26,7 +27,7 @@ void kmain()
 	move_cursor(5, 20);
 	draw_string("Cursor moved over 5 cells and down 4 cells.", 0xF, 0x0);
 	draw_string("And the cursor moves to the end of the string", 0x1, 0xF);
-	draw_string("Now this string should have a newline now:\nAnd now a new line is here.", 0xF, 0x0);
+	draw_string("Now this string should have a newline now:\nAnd now a new line is here.\n", 0xF, 0x0);
 	//draw_rect(20, 5, 50, 7, 0x4, 0xF);
 	//draw_rect(20, 5, 50, 7, 0x4, 0xF);
 	//draw_rect(20, 5, 50, 7, 0x4, 0xF);
@@ -34,5 +35,11 @@ void kmain()
 	//row = 2;
 	///column = 80;
 	//draw_s("And 2nd Row. Now the 3rd row.", 2, 9);
+	move_cursor(0, 3);
+	draw_string("Installing GDT...\n", 0xF, 0x0);
+	gdt_install();
+	move_cursor(0, 4);
+	draw_string("Installed GDT\n", 0xf, 0x0);
+	
 	asm("hlt");
 }