diff --git a/sys/arch/i386/pci/pci_machdep.h b/sys/arch/i386/pci/pci_machdep.h
index ade41223288c..593004ca92bd 100644
--- a/sys/arch/i386/pci/pci_machdep.h
+++ b/sys/arch/i386/pci/pci_machdep.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: pci_machdep.h,v 1.7 1997/06/06 23:29:18 thorpej Exp $	*/
+/*	$NetBSD: pci_machdep.h,v 1.8 1997/08/26 03:14:06 thorpej Exp $	*/
 
 /*
  * Copyright (c) 1996 Christopher G. Demetriou.  All rights reserved.
@@ -34,6 +34,14 @@
  * Machine-specific definitions for PCI autoconfiguration.
  */
 
+/*
+ * Many i386 PCI systems only work properly with I/O mapped space, in
+ * particular, buses behind PCI-PCI bridges may not have memory
+ * space mapped at all.  For this reason, tell drivers that have
+ * a choice that we "prefer" I/O space.
+ */
+#define	PCI_PREFER_IOSPACE
+
 /*
  * i386-specific PCI structure and type definitions.
  * NOT TO BE USED DIRECTLY BY MACHINE INDEPENDENT CODE.