reserve space for mbr partition table

boot.s

@@ -34,13 +34,23 @@ bits 16
   ; TODO: grab all the info we can from bios interrupts and deliver it to ths OS nicely
   ; e.g. in a fixed memory location
 
-  ; Make sure we fit in a single 512-byte sector
-  %if ($ - $$) > 510
-  %error "exceeded boot sector size"
+  ; MBR bootstrap field is 440 bytes long
+  %if ($ - $$) > 440
+  %error "exceeded mbr bootstrap field size"
   %endif
 
-  ; Pad to the boot signature offset
+  ; Pad to the end of the MBR bootstrap field
+  times 440 - ($ - $$) db 0
+
+  ; Unique disk ID
+  dd 0x00000000
+  ; Reserved
+  dw 0x0000
+
+  ; Zero the partition fields
+  ; TODO: fill in the field pointing to the GPT table
   times 510 - ($ - $$) db 0
+
   ; Boot signature
   db 0x55
   db 0xaa