diff --git a/mps/code/protxc.c b/mps/code/protxc.c
index a3377d28449..6dbe0d91c13 100644
--- a/mps/code/protxc.c
+++ b/mps/code/protxc.c
@@ -24,16 +24,22 @@
  * thread to progress.
  *
  *
- * SOURCES
- *
- * .source.man: <http://felinemenace.org/~nemo/mach/manpages/>
- *              <http://web.mit.edu/darwin/src/modules/xnu/osfmk/man/>
- *
- *
  * REFERENCES
  *
  * [Fuller_2013] "Mach Exception Handlers"; Landon Fuller;
- *               <http://www.mikeash.com/pyblog/friday-qa-2013-01-11-mach-exception-handlers.html>
+ *               <http://www.mikeash.com/pyblog/friday-qa-2013-01-11-mach-exception-handlers.html>.
+ *
+ * [XNU] "xnu-2050.22.13" source code;
+ *       Apple Computer;
+ *       <http://www.opensource.apple.com/source/xnu/xnu-2050.22.13/>.
+ *
+ * [Mach_man]  Mach man pages within XNU;
+ *             Apple Computer;
+ *             <http://www.opensource.apple.com/source/xnu/xnu-2050.22.13/osfmk/man/>.
+ *
+ * [Libc] "Libc-825.26" source code;
+ *        Apple Computer;
+ *        <http://www.opensource.apple.com/source/Libc/Libc-825.26/>.
  *
  *
  * TRANSGRESSIONS
@@ -172,7 +178,8 @@ static void protMustSend(mach_msg_header_t *head)
  *
  * Mac OS X provides a function exc_server (in
  * /usr/lib/system/libsystem_kernel.dylib) that's documented in the XNU
- * sources and generated by the Mach Interface Generator (mig).  It unpacks
+ * sources <http://www.opensource.apple.com/source/xnu/xnu-2050.22.13/osfmk/man/exc_server.html>
+ * and generated by the Mach Interface Generator (mig).  It unpacks
  * an exception message structure and calls one of several handler functions.
  * We can't use it because:
  *
@@ -251,7 +258,7 @@ static void protCatchOne(void)
      task-wide exception handler, but the code is pretty hairy and not
      necessary as long as the MPS is registering threads individually.
      If we ever need to reinstate that code, look at
-     //info.ravenbrook.com/project/mps/branch/2013-06-18/macosx-threads/code/protxc.c#3 */
+     https://info.ravenbrook.com/project/mps/prototype/2013-06-24/machtest */
   
   protBuildReply(&reply, &request, KERN_FAILURE);
   protMustSend(&reply.Head);
@@ -352,8 +359,8 @@ static void protSetup(void)
 
   /* Launch the exception handling thread.  We use pthread_create because
      it's much simpler than setting up a thread from scratch using Mach,
-     and that's basically what it does (by inspection of the XNU source
-     code). */
+     and that's basically what it does.  See [Libc]
+     <http://www.opensource.apple.com/source/Libc/Libc-825.26/pthreads/pthread.c> */
   pr = pthread_create(&excThread, NULL, protCatchThread, NULL);
   AVER(pr == 0);
   if (pr != 0)
diff --git a/mps/code/thxc.c b/mps/code/thxc.c
index e36e4cd9886..659ad5dd7e1 100644
--- a/mps/code/thxc.c
+++ b/mps/code/thxc.c
@@ -4,10 +4,10 @@
  * Copyright (c) 2013 Ravenbrook Limited.  See end of file for license.
  *
  * REFERENCES
- * "Abusing Mach on Mac OS X" <http://www.uninformed.org/?v=4&a=3&t=pdf>
- * Mach man pages <http://felinemenace.org/~nemo/mach/manpages/>
  *
- * TODO: Consider making Register a no-op and using task_threads().
+ * [Mach_man]  Mach man pages within XNU;
+ *             Apple Computer;
+ *             <http://www.opensource.apple.com/source/xnu/xnu-2050.22.13/osfmk/man/>.
  */
 
 #include "mpm.h"