diff --git a/mps/code/protsgix.c b/mps/code/protsgix.c
index e587ac86424..25353aa139d 100644
--- a/mps/code/protsgix.c
+++ b/mps/code/protsgix.c
@@ -17,6 +17,7 @@
  */
 
 #include "mpm.h"
+#include "vm.h"
 
 #if !defined(MPS_OS_XC) && !defined(MPS_OS_FR)
 #error "protsgix.c is Unix-specific, currently for MPS_OS_FR or XC"
@@ -138,6 +139,15 @@ void ProtSetup(void)
 }
 
 
+/* ProtGranularity -- return the granularity of protection */
+
+Size ProtGranularity(void)
+{
+  /* Individual pages can be protected. */
+  return PageSize();
+}
+
+
 /* C. COPYRIGHT AND LICENSE
  *
  * Copyright (C) 2001-2007 Ravenbrook Limited <http://www.ravenbrook.com/>.