diff --git a/src/filo-fs-vfs.ads b/src/filo-fs-vfs.ads
new file mode 100644
index 0000000..500c77b
--- /dev/null
+++ b/src/filo-fs-vfs.ads
@@ -0,0 +1,66 @@
+with System;
+with Interfaces.C;
+with Interfaces.C.Strings;
+
+use Interfaces.C;
+
+generic
+
+   type T (<>) is private;
+
+   with function Is_Mounted (State : T) return Boolean is <>;
+   with function Is_Open (State : T) return Boolean is <>
+   with
+      Post => (if Is_Open'Result then Is_Mounted (State));
+
+   Initial : T;
+
+   with procedure Mount
+     (State    : in out T;
+      Part_Len : in     Partition_Length;
+      Success  :    out Boolean)
+   is <>
+   with
+      Pre => not Is_Mounted (State),
+      Post => Success = Is_Mounted (State);
+
+   with procedure Open
+     (State       : in out T;
+      File_Len    :    out File_Length;
+      File_Path   : in     String;
+      Success     : out    Boolean)
+   is <>
+   with
+      Pre => Is_Mounted (State) and not Is_Open (State),
+      Post => Success = Is_Open (State);
+
+   with procedure Close (State : in out T)
+   is <>
+   with
+      Pre => Is_Open (State),
+      Post => Is_Mounted (State);
+
+   with procedure Read
+     (State    : in out T;
+      File_Len : in     File_Length;
+      File_Pos : in out File_Offset;
+      Buf      :    out Buffer_Type;
+      Len      :    out Natural)
+   is <>
+   with
+      Pre => Is_Open (State),
+      Post => Is_Open (State);
+
+package FILO.FS.VFS
+with
+   Convention => C
+is
+
+   function C_Mount return int;
+
+   function C_Open (File_Path : Strings.chars_ptr) return int;
+   procedure C_Close;
+   
+   function C_Read (Buf : System.Address; Len : int) return int;
+
+end FILO.FS.VFS;