diff --git a/mps/code/ll.gmk b/mps/code/ll.gmk
index 787380fb3ed..1bdbef3afcb 100644
--- a/mps/code/ll.gmk
+++ b/mps/code/ll.gmk
@@ -32,7 +32,7 @@ CFLAGSCOMPILER := \
 	-Wstrict-prototypes \
 	-Wunreachable-code \
 	-Wwrite-strings
-CFLAGSCOMPILERSTRICT :=
+CFLAGSCOMPILERSTRICT := -ansi
 
 # A different set of compiler flags for less strict compilation, for
 # instance when we need to #include a third-party header file that
diff --git a/mps/code/shield.c b/mps/code/shield.c
index f7cc4aef672..7eabf5f8986 100644
--- a/mps/code/shield.c
+++ b/mps/code/shield.c
@@ -234,7 +234,7 @@ static Seg shieldDequeue(Shield shield, Index i)
   seg = shield->queue[i];
   AVERT(Seg, seg);
   AVER(seg->queued);
-  shield->queue[i] = NULL;
+  shield->queue[i] = NULL; /* to ensure it can't get re-used */
   seg->queued = FALSE;
   return seg;
 }
diff --git a/mps/code/xci3ll.gmk b/mps/code/xci3ll.gmk
new file mode 100644
index 00000000000..a0af3b7079d
--- /dev/null
+++ b/mps/code/xci3ll.gmk
@@ -0,0 +1,72 @@
+# -*- makefile -*-
+#
+# xci3ll.gmk: BUILD FOR MAC OS X/i386/Clang PLATFORM
+#
+# $Id$
+# Copyright (c) 2001-2014 Ravenbrook Limited.  See end of file for license.
+#
+# .prefer.xcode: The documented and preferred way to develop the MPS
+# for this platform is to use the Xcode project (mps.xcodeproj). This
+# makefile provides a way to compile the MPS one source file at a
+# time, rather than all at once via mps.c (which can hide errors due
+# to missing headers).
+
+PFM = xci3ll
+
+MPMPF = \
+    lockix.c \
+    prmci3xc.c \
+    proti3.c \
+    protix.c \
+    protxc.c \
+    span.c \
+    ssixi3.c \
+    thxc.c \
+    vmix.c
+
+include ll.gmk
+
+CC = clang -arch i386
+
+include comm.gmk
+
+
+# C. COPYRIGHT AND LICENSE
+#
+# Copyright (C) 2001-2014 Ravenbrook Limited <http://www.ravenbrook.com/>.
+# All rights reserved.  This is an open source license.  Contact
+# Ravenbrook for commercial licensing options.
+# 
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+# 
+# 1. Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+# 
+# 2. Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+# 
+# 3. Redistributions in any form must be accompanied by information on how
+# to obtain complete source code for this software and any accompanying
+# software that uses this software.  The source code must either be
+# included in the distribution or be available for no more than the cost
+# of distribution plus a nominal fee, and must be freely redistributable
+# under reasonable conditions.  For an executable file, complete source
+# code means the source code for all modules it contains. It does not
+# include source code for modules or files that typically accompany the
+# major components of the operating system on which the executable file
+# runs.
+# 
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
+# IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+# TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+# PURPOSE, OR NON-INFRINGEMENT, ARE DISCLAIMED. IN NO EVENT SHALL THE
+# COPYRIGHT HOLDERS AND CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+# ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+# THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.