enter protected mode

boot/boot.asm

@@ -1,8 +1,47 @@
-bits 16
+[org 0x7c00]
-org 0x7c00
+[bits 16]
+
+CODE_SEG equ gdt_code - gdt_start
+
+cli
+lgdt [gdt_descriptor]
+mov eax, cr0
+or eax, 1
+mov cr0, eax
+jmp CODE_SEG:start_protected_mode
+jmp $
+
+gdt_start:
+    gdt_null:
+        dd 0x0
+        dd 0x0
+
+    gdt_code:
+        dw 0xffff
+        dw 0x0
+        db 0x0
+        db 0b10011010
+        db 0b11001111
+        db 0x0
+
+    gdt_data:
+        dw 0xffff
+        dw 0x0
+        db 0x0
+        db 0b10010010
+        db 0b11001111
+        db 0x0
+
+gdt_end:
+
+gdt_descriptor:
+    dw gdt_end - gdt_start - 1
+    dd gdt_start
+
+[bits 32]
+start_protected_mode:
+    jmp $
 
-halt:
-    jmp halt
 
 times 510-($-$$) db 0
-db 0x55, 0xaa
+dw 0xaa55