diff --git a/src/filo.ads b/src/filo.ads
index b85fa19..c0a7e3d 100644
--- a/src/filo.ads
+++ b/src/filo.ads
@@ -4,6 +4,12 @@
 
 package FILO is
 
+   type Partition_Length is range 0 .. Interfaces.Unsigned_64'Last;
+   subtype Partition_Offset is Partition_Length range 0 .. Partition_Length'Last - 1;
+
+   BLOCK_SIZE : constant := 512;
+   type Block_Offset is range 0 .. Partition_Offset'Last / BLOCK_SIZE;
+
    subtype Index_Type is Natural range 0 .. Natural'Last - 1;
    subtype Index_Type16 is Index_Type range 0 .. Index_Type'Last - 1;
    subtype Index_Type32 is Index_Type16 range 0 .. Index_Type16'Last - 2;