diff --git a/C/tracer.c b/C/tracer.c
index c63bcf400..3cbb52861 100644
--- a/C/tracer.c
+++ b/C/tracer.c
@@ -150,7 +150,8 @@ check_area(void)
 }
 */
 
-PredEntry *old_p = NULL;
+PredEntry *old_p[10000];
+Term old_x1[10000], old_x2[10000], old_x3[10000];
 
 void
 low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
@@ -163,14 +164,19 @@ low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
   LOCK(Yap_heap_regs->low_level_trace_lock);
   sc = Yap_heap_regs;
   vsc_count++;
-  old_p = pred;
   if (LCL0[-30]) {
-    UNLOCK(Yap_heap_regs->low_level_trace_lock);
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);    
+    old_p[worker_id] = pred;
+    old_x2[worker_id] = ARG2;
+    old_x1[worker_id] = ARG1;
+    old_x3[worker_id] = ARG3;
     return;
   }
-  if (FALSE && vsc_count == 59855LL) {
-    jmp_deb(1);
-  }
+  jmp_deb(worker_id);
+  old_p[worker_id] = pred;
+  old_x2[worker_id] = ARG2;
+  old_x1[worker_id] = ARG1;
+  old_x3[worker_id] = ARG3;
 #ifdef COMMENTED
   //*(H0+(0xb65f2850-0xb64b2008)/sizeof(CELL))==0xc || 
   //0x4fd4d
diff --git a/H/Yap.h b/H/Yap.h
index fe267bc91..c22bae14c 100644
--- a/H/Yap.h
+++ b/H/Yap.h
@@ -10,7 +10,7 @@
 * File:		Yap.h.m4						 *
 * mods:									 *
 * comments:	main header file for YAP				 *
-* version:      $Id: Yap.h,v 1.23 2007-06-29 01:33:35 vsc Exp $	 *
+* version:      $Id: Yap.h,v 1.24 2008-01-25 00:21:28 vsc Exp $	 *
 *************************************************************************/
 
 #include "config.h"
@@ -237,7 +237,10 @@ extern char Yap_Option[20];
 #define MMAP_ADDR 0x40000000
 #elif mips
 #define MMAP_ADDR 0x02000000
-#elif __APPLE__
+#elif __APPLE__ && __LP64__
+// this address is high enough that it is likely not to confuse Apple's malloc debugger; lowest possible is 0x100200000
+#define MMAP_ADDR 0x200000000
+#elif __APPLE__ && !__LP64__
 #define MMAP_ADDR 0x20000000
 #else
 #define MMAP_ADDR 0x08800000
@@ -635,7 +638,7 @@ typedef enum
 	if you place things in the lower addresses (power to the libc people).
 */
 
-#if (defined(_AIX) || defined(_WIN32) || defined(__APPLE__) || defined(sparc) || defined(__sparc) || defined(mips) || defined(__FreeBSD__) || defined(_POWER) || defined(__POWERPC__) || defined(__linux__) || defined(IN_SECOND_QUADRANT) || defined(__CYGWIN__))
+#if (defined(_AIX) || (defined(__APPLE__) && !defined(__LP64__)) || defined(_WIN32) || defined(sparc) || defined(__sparc) || defined(mips) || defined(__FreeBSD__) || defined(_POWER) || defined(__POWERPC__) || defined(__linux__) || defined(IN_SECOND_QUADRANT) || defined(__CYGWIN__))
 #define USE_LOW32_TAGS 1
 #endif
 
@@ -833,7 +836,7 @@ FunctorOfTerm (Term t)
 }
 
 
-#if USE_SYSTEM_MALLOC || USE_DL_MALLOC
+#if (USE_SYSTEM_MALLOC || USE_DL_MALLOC) && !__LP64__
 #if USE_LOW32_TAGS
 
 inline EXTERN Term MkAtomTerm (Atom);
diff --git a/changes-5.1.html b/changes-5.1.html
index 0e5c27cc3..428ec96f3 100644
--- a/changes-5.1.html
+++ b/changes-5.1.html
@@ -17,6 +17,7 @@
 
 <h2>Yap-5.1.3:</h2>
 <ul>
+<li> FIXED: compilation in OSX/64 (patches from Will Benton).</li>
 <li> FIXED: do not access erased code (obs from Paulo Moura).</li>
 <li> FIXED: clause/3 should not follow fail.</li>
 <li> FIXED: make sure we don't ask for 0-sized blocks, minimal 64K.</li>
diff --git a/configure b/configure
index 197367b68..d9c16e97b 100755
--- a/configure
+++ b/configure
@@ -6964,9 +6964,9 @@ fi
 	SHLIB_CFLAGS="-fno-common"
 	SHLIB_SUFFIX=".dylib"
         DO_SECOND_LD=""
-	SHLIB_LD="gcc -bundle -flat_namespace -undefined suppress"
+	SHLIB_LD="$CC -bundle -flat_namespace -undefined suppress"
 	INSTALL_DLLS=""
-        CC="cc -no-cpp-precomp"
+        CC="$CC -no-cpp-precomp"
 	JAVALIBPATH="-L/System/Library/Frameworks/JavaVM.framework/Libraries"
 	JAVAINCPATH="-I/System/Library/Frameworks/JavaVM.framework/Headers"
 	;;
diff --git a/configure.in b/configure.in
index 88a3afccd..a25a17a87 100644
--- a/configure.in
+++ b/configure.in
@@ -770,9 +770,9 @@ dnl Linux has both elf and a.out, in this case we found elf
 	SHLIB_CFLAGS="-fno-common"
 	SHLIB_SUFFIX=".dylib"
         DO_SECOND_LD=""
-	SHLIB_LD="gcc -bundle -flat_namespace -undefined suppress"
+	SHLIB_LD="$CC -bundle -flat_namespace -undefined suppress"
 	INSTALL_DLLS=""
-        CC="cc -no-cpp-precomp"
+        CC="$CC -no-cpp-precomp"
 	JAVALIBPATH="-L/System/Library/Frameworks/JavaVM.framework/Libraries"
 	JAVAINCPATH="-I/System/Library/Frameworks/JavaVM.framework/Headers"
 	;;
diff --git a/pl/threads.yap b/pl/threads.yap
index b88a7a2d2..df863d869 100644
--- a/pl/threads.yap
+++ b/pl/threads.yap
@@ -48,7 +48,7 @@
 	;
 	    recorda('$thread_exit_status', [Id0|Status], _)
 	),
-	format(user_error,'closing thread ~w~n',[v([Id0|Status])]).
+%	format(user_error,'closing thread ~w~n',[v([Id0|Status])]).
 	'$run_at_thread_exit'(Id0).	
 '$close_thread'(Exception,Detached) :-
 	'$thread_self'(Id0),