diff --git a/asminc/zeropage.inc b/asminc/zeropage.inc
index 872105fb5..6e6b81bcb 100644
--- a/asminc/zeropage.inc
+++ b/asminc/zeropage.inc
@@ -1,7 +1,7 @@
 ;
 ; zeropage.inc
 ;
-; (C) Copyright 2002 Ullrich von Bassewitz (uz@cc65.org)
+; (C) Copyright 2002-2012, Ullrich von Bassewitz (uz@cc65.org)
 ;
 
 ; Assembler include file that imports the runtime zero page locations used
@@ -13,10 +13,14 @@
 	.importzp   	tmp1, tmp2, tmp3, tmp4
 	.importzp   	regbank
 
+; The size of the register bank
+regbanksize     = 6
+
 ; The total amount of zero page space used
+zpspace         = 26
 
-zpspace = 26
-
-
-
+; The amount of space that needs to be saved by an interrupt handler that
+; calls C code (does not include the register bank, which is saved by the
+; generated C code if required).
+zpsavespace     = zpspace - regbanksize