diff --git a/C/absmi.c b/C/absmi.c
index d8e3a2f17..2bea1c218 100755
--- a/C/absmi.c
+++ b/C/absmi.c
@@ -1086,7 +1086,7 @@ Yap_absmi(int inp)
 	}
 	/* HEY, leave indexing block alone!! */
 	/* check if we are the ones using this code */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	PELOCK(1, ap);
 	PP = ap;
 	DEC_CLREF_COUNT(cl);
@@ -1387,7 +1387,7 @@ Yap_absmi(int inp)
 	}
 	/* HEY, leave indexing block alone!! */
 	/* check if we are the ones using this code */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	PELOCK(2, ap);
 	PP = ap;
 	DEC_CLREF_COUNT(cl);
@@ -1480,7 +1480,7 @@ Yap_absmi(int inp)
       /* only meaningful with THREADS on! */
       /* lock logical updates predicate.  */
       Op(lock_lu, p);
-#if defined(YAPOR) || defined(THREADS)
+#if PARALLEL_YAP
       if (PP) {
 	GONext();
       }
@@ -1508,10 +1508,12 @@ Yap_absmi(int inp)
       check_trail(TR); 
       /* say that an environment is using this clause */
       /* we have our own copy for the clause */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
       {
 	LogUpdClause *cl = PREG->u.L.ClBase;
+#if PARALLEL_YAP
 	PredEntry *ap = cl->ClPred;
+#endif
 
 	/* always add an extra reference */
 	INC_CLREF_COUNT(cl);
@@ -1581,7 +1583,7 @@ Yap_absmi(int inp)
 	}
 	setregs();
 
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	/* always add an extra reference */
 	INC_CLREF_COUNT(cl);
 	TRAIL_CLREF(cl);
@@ -1633,7 +1635,7 @@ Yap_absmi(int inp)
 
 	/* say that an environment is using this clause */
 	/* we have our own copy for the clause */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	/* always add an extra reference */
 	INC_CLREF_COUNT(cl);
 	TRAIL_CLREF(cl);
@@ -1746,7 +1748,7 @@ Yap_absmi(int inp)
 #endif	/* YAPOR */
       SET_BB(B_YREG);
       ENDCACHE_Y();
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
       INC_CLREF_COUNT(ClauseCodeToDynamicClause(PREG));
       UNLOCK(DynamicLock(PREG));
       TRAIL_CLREF(ClauseCodeToDynamicClause(PREG));
@@ -1808,7 +1810,7 @@ Yap_absmi(int inp)
 #endif /* FROZEN_STACKS */
       SET_BB(B_YREG);
       ENDCACHE_Y();
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
       INC_CLREF_COUNT(ClauseCodeToDynamicClause(PREG));
       TRAIL_CLREF(ClauseCodeToDynamicClause(PREG));
       UNLOCK(DynamicLock(PREG));
@@ -2067,7 +2069,7 @@ Yap_absmi(int inp)
 	      if (IN_BETWEEN(H0,pt1,H) && IsAttVar(pt1))
 		goto failloop;		       	    
 	    flags = *pt1;
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	    if (FlagOn(DBClMask, flags)) {
 	      DBRef dbr = DBStructFlagsToDBStruct(pt1);
 	      int erase;
@@ -2086,7 +2088,9 @@ Yap_absmi(int inp)
 		if (flags & IndexMask) {
 		  LogUpdIndex *cl = ClauseFlagsToLogUpdIndex(pt1);
 		  int erase;
+#if PARALLEL_YAP
 		  PredEntry *ap = cl->ClPred;
+#endif
 
 		  PELOCK(8,ap);
 		  DEC_CLREF_COUNT(cl);
@@ -2110,7 +2114,9 @@ Yap_absmi(int inp)
 		} else {
 		  LogUpdClause *cl = ClauseFlagsToLogUpdClause(pt1);
 		  int erase;
+#if PARALLEL_YAP
 		  PredEntry *ap = cl->ClPred;
+#endif
 
 		  PELOCK(9,ap);
 		  DEC_CLREF_COUNT(cl);
@@ -7926,7 +7932,7 @@ Yap_absmi(int inp)
 	/* fprintf(stderr,"> %p/%p %d %d\n",cl,ap,ap->TimeStampOfPred,PREG->u.Ills.s);*/
 	PREG = PREG->u.Ills.l1;
 	/* indicate the indexing code is being used */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	/* just store a reference */
 	INC_CLREF_COUNT(cl);
 	TRAIL_CLREF(cl);
@@ -8032,7 +8038,7 @@ Yap_absmi(int inp)
 	}
 	/* HEY, leave indexing block alone!! */
 	/* check if we are the ones using this code */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	DEC_CLREF_COUNT(cl);
 	/* clear the entry from the trail */
 	B->cp_tr--;
diff --git a/C/amasm.c b/C/amasm.c
index b0cc48c9b..ef9415126 100755
--- a/C/amasm.c
+++ b/C/amasm.c
@@ -3052,7 +3052,7 @@ do_pass(int pass_no, yamop **entry_codep, int assembling, int *clause_has_blobsp
 	}
 	cl_u->luc.ClExt = NULL;
 	cl_u->luc.ClPrev = cl_u->luc.ClNext = NULL;
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	//INIT_LOCK(cl_u->luc.ClLock);
 	INIT_CLREF_COUNT(&(cl_u->luc));
 #endif
@@ -3071,6 +3071,8 @@ do_pass(int pass_no, yamop **entry_codep, int assembling, int *clause_has_blobsp
 	cl_u->ic.ClRefCount = 0;
 #if defined(YAPOR) || defined(THREADS)
 	INIT_LOCK(cl_u->ic.ClLock);
+#endif
+#ifdef MULTIPLE_STACKS
 	INIT_CLREF_COUNT(&(cl_u->ic));
 #endif
       }
@@ -3122,7 +3124,7 @@ do_pass(int pass_no, yamop **entry_codep, int assembling, int *clause_has_blobsp
 	cl_u->lui.ClSize = size;
 	cl_u->lui.ClRefCount =  0;
 	//	INIT_LOCK(cl_u->lui.ClLock);
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	INIT_CLREF_COUNT(&(cl_u->lui));
 #endif
       }
diff --git a/C/arrays.c b/C/arrays.c
index 2fca5031e..2114aa77d 100644
--- a/C/arrays.c
+++ b/C/arrays.c
@@ -392,7 +392,7 @@ AccessNamedArray(Atom a, Int indx USES_REGS)
 	  if (TRef != 0L) {
 	    DBRef ref = DBRefOfTerm(TRef);
 
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	    LOCK(ref->lock);
 	    INC_DBREF_COUNT(ref);
 	    TRAIL_REF(ref);	/* So that fail will erase it */
@@ -2305,17 +2305,17 @@ p_static_array_to_term( USES_REGS1 )
 
 	  if (TRef != 0L) {
 	    DBRef ref = DBRefOfTerm(TRef);
-#if defined(YAPOR) || defined(THREADS)
 	    LOCK(ref->lock);
+#if MULTIPLE_STACKS
 	    INC_DBREF_COUNT(ref);
 	    TRAIL_REF(ref);	/* So that fail will erase it */
-	    UNLOCK(ref->lock);
 #else
 	    if (!(ref->Flags & InUseMask)) {
 	      ref->Flags |= InUseMask;
 	      TRAIL_REF(ref);	/* So that fail will erase it */
 	    }
 #endif
+	    UNLOCK(ref->lock);
 	  } else {
 	    TRef = TermNil;
 	  }
diff --git a/C/cdmgr.c b/C/cdmgr.c
index ce34e43c6..6620385ae 100644
--- a/C/cdmgr.c
+++ b/C/cdmgr.c
@@ -1451,12 +1451,12 @@ Yap_kill_iblock(ClauseUnion *blk, ClauseUnion *parent_blk, PredEntry *ap)
     LogUpdIndex *c = (LogUpdIndex *)blk;
     if (parent_blk != NULL) {
       LogUpdIndex *cl = (LogUpdIndex *)parent_blk;
-#if defined(THREADS) || defined(YAPOR)
+#if MULTIPLE_STACKS
       /* protect against attempts at erasing */
       cl->ClRefCount++;
 #endif
       kill_first_log_iblock(c, cl, ap);
-#if defined(THREADS) || defined(YAPOR)
+#if MULTIPLE_STACKS
       cl->ClRefCount--;
 #endif
     } else {
@@ -1501,12 +1501,12 @@ Yap_ErLogUpdIndex(LogUpdIndex *clau)
   if (clau->ClFlags & SwitchRootMask) {
     kill_first_log_iblock(clau, NULL, clau->ClPred);
   } else {
-#if defined(THREADS) || defined(YAPOR)
+#if MULTIPLE_STACKS
     /* protect against attempts at erasing */
     clau->ClRefCount++;
 #endif
     kill_first_log_iblock(clau, clau->ParentIndex, clau->ClPred);
-#if defined(THREADS) || defined(YAPOR)
+#if MULTIPLE_STACKS
     /* protect against attempts at erasing */
     clau->ClRefCount--;
 #endif
@@ -2274,7 +2274,7 @@ addclause(Term t, yamop *cp, int mode, Term mod, Term *t4ref)
   if (pflags & LogUpdatePredFlag) {
     LogUpdClause *cl = (LogUpdClause *)ClauseCodeToLogUpdClause(cp);
     tf = MkDBRefTerm((DBRef)cl);
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
     TRAIL_CLREF(cl);		/* So that fail will erase it */
     INC_CLREF_COUNT(cl);
 #else
@@ -4428,7 +4428,7 @@ fetch_next_lu_clause(PredEntry *pe, yamop *i_code, Term th, Term tb, Term tr, ya
     return FALSE;
   }
   rtn = MkDBRefTerm((DBRef)cl);
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   TRAIL_CLREF(cl);		/* So that fail will erase it */
   INC_CLREF_COUNT(cl);
 #else
@@ -4575,7 +4575,7 @@ fetch_next_lu_clause_erase(PredEntry *pe, yamop *i_code, Term th, Term tb, Term
     return FALSE;
   }
   rtn = MkDBRefTerm((DBRef)cl);
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   TRAIL_CLREF(cl);		/* So that fail will erase it */
   INC_CLREF_COUNT(cl);
 #else
@@ -5071,7 +5071,7 @@ p_nth_clause( USES_REGS1 )
     return FALSE;
   }
   if (pe->PredFlags & LogUpdatePredFlag) {
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
     TRAIL_CLREF(cl);		/* So that fail will erase it */
     INC_CLREF_COUNT(cl);
 #else
diff --git a/C/dbase.c b/C/dbase.c
index c7272fc3f..efa76d597 100755
--- a/C/dbase.c
+++ b/C/dbase.c
@@ -113,7 +113,7 @@ typedef struct idb_queue
 {
   Functor id;		/* identify this as being pointed to by a DBRef */
   SMALLUNSGN    Flags;  /* always required */
-#if defined(YAPOR) || defined(THREADS)
+#if PARALLEL_YAP
   rwlock_t    QRWLock;         /* a simple lock to protect this entry */
 #endif
   QueueEntry *FirstInQueue, *LastInQueue;
@@ -1670,7 +1670,7 @@ record(int Flag, Term key, Term t_data, Term t_code USES_REGS)
   else
     x->Flags |= DBNoCode;
   x->Parent = p;
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   x->Flags |= DBClMask;
   x->ref_count = 1;
 #else
@@ -1742,7 +1742,7 @@ record_at(int Flag, DBRef r0, Term t_data, Term t_code USES_REGS)
   else
     x->Flags |= DBNoCode;
   x->Parent = p;
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   x->Flags |= DBClMask;
   x->ref_count = 1;
 #else
@@ -1806,7 +1806,7 @@ new_lu_db_entry(Term t, PredEntry *pe)
   struct db_globs dbg;
   int d_flag = 0;
 
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   /* we cannot allow sharing between threads (for now) */ 
   if (!(pe->PredFlags & ThreadLocalPredFlag))
     d_flag |= InQueue;
@@ -1838,7 +1838,7 @@ new_lu_db_entry(Term t, PredEntry *pe)
     cl->ClTimeStart = 0L;
   }
   cl->ClTimeEnd = TIMESTAMP_EOT; 
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   //  INIT_LOCK(cl->ClLock);
   INIT_CLREF_COUNT(cl);
   ipc->opc = Yap_opcode(_copy_idb_term);
@@ -1960,7 +1960,7 @@ p_rcda( USES_REGS1 )
     cl = record_lu(pe, Deref(ARG2), MkFirst);
     if (cl != NULL) {
       TRAIL_CLREF(cl);
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
       INC_CLREF_COUNT(cl);
 #else
       cl->ClFlags |= InUseMask;
@@ -2070,7 +2070,7 @@ p_rcdz( USES_REGS1 )
     cl = record_lu(pe, t2, MkLast);
     if (cl != NULL) {
       TRAIL_CLREF(cl);
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
       INC_CLREF_COUNT(cl);
 #else
       cl->ClFlags |= InUseMask;
@@ -2917,7 +2917,7 @@ lu_nth_recorded(PredEntry *pe, Int Count USES_REGS)
   cl = Yap_NthClause(pe, Count);
   if (cl == NULL)
     return FALSE;
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   PELOCK(65,pe);
   TRAIL_CLREF(cl);		/* So that fail will erase it */
   INC_CLREF_COUNT(cl);
@@ -2959,7 +2959,7 @@ nth_recorded(DBProp AtProp, Int Count USES_REGS)
       return FALSE;
     }
   }
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   LOCK(ref->lock);
   READ_UNLOCK(AtProp->DBRWLock);
   TRAIL_REF(ref);		/* So that fail will erase it */
@@ -3276,7 +3276,7 @@ i_recorded(DBProp AtProp, Term t3 USES_REGS)
   /* This should be after any non-tagged terms, because the routines in grow.c
      go from upper to lower addresses */
   TRef = MkDBRefTerm(ref);
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   LOCK(ref->lock);
   TRAIL_REF(ref);		/* So that fail will erase it */
   INC_DBREF_COUNT(ref);
@@ -3421,7 +3421,7 @@ c_recorded(int flags USES_REGS)
   READ_UNLOCK(ref0->Parent->DBRWLock);
   TRef = MkDBRefTerm(ref);
   EXTRA_CBACK_ARG(3,1) = (CELL)ref;
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   LOCK(ref->lock);
   TRAIL_REF(ref);	/* So that fail will erase it */
   INC_DBREF_COUNT(ref);
@@ -3664,17 +3664,17 @@ p_first_instance( USES_REGS1 )
   }
   TRef = MkDBRefTerm(ref);
   /* we have a pointer to the term available */
-#if defined(YAPOR) || defined(THREADS)
   LOCK(ref->lock);
+#if MULTIPLE_STACKS
   TRAIL_REF(ref);	/* So that fail will erase it */
   INC_DBREF_COUNT(ref);
-  UNLOCK(ref->lock);
 #else 
   if (!(ref->Flags & InUseMask)) {
     ref->Flags |= InUseMask;
     TRAIL_REF(ref);	/* So that fail will erase it */
   }
 #endif
+  UNLOCK(ref->lock);
   while ((TermDB = GetDBTermFromDBEntry(ref PASS_REGS)) == (CELL)0) {
     /* oops, we are in trouble, not enough stack space */
     if (Yap_Error_TYPE == OUT_OF_ATTVARS_ERROR) {
@@ -3942,7 +3942,7 @@ RemoveDBEntry(DBRef entryref USES_REGS)
        || B->cp_ap == RETRY_C_RECORDEDP_CODE) &&
       EXTRA_CBACK_ARG(3,1) == (CELL)entryref) {
     /* make it clear the entry has been released */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
     DEC_DBREF_COUNT(entryref);
 #else 
     entryref->Flags &= ~InUseMask;
@@ -3993,7 +3993,7 @@ find_next_clause(DBRef ref0 USES_REGS)
     newp = ref->Code;
     /* and next let's tell the world this clause is being used, just
        like if we were executing a standard retry_and_mark */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
     {
       DynamicClause *cl = ClauseCodeToDynamicClause(newp);
 
@@ -4805,7 +4805,7 @@ Yap_LUInstance(LogUpdClause *cl, UInt arity)
       }
     }
   }
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
   cl->ClRefCount++;
   TRAIL_CLREF(cl);	/* So that fail will erase it */
 #else
@@ -5427,17 +5427,17 @@ p_fetch_reference_from_index( USES_REGS1 )
     return FALSE;
   pos = IntOfTerm(t2);
   el = (DBRef)(table->DBT.Contents[pos]);
-#if defined(YAPOR) || defined(THREADS)
   LOCK(el->lock);
+#if MULTIPLE_STACKS
   TRAIL_REF(el);	/* So that fail will erase it */
   INC_DBREF_COUNT(el);
-  UNLOCK(el->lock);
 #else
   if (!(el->Flags & InUseMask)) {
     el->Flags |= InUseMask;
     TRAIL_REF(el);
   }
 #endif
+  UNLOCK(el->lock);
   return Yap_unify(ARG3, MkDBRefTerm(el));
 }
 
diff --git a/C/globals.c b/C/globals.c
index 61afec71d..1e0b51d17 100644
--- a/C/globals.c
+++ b/C/globals.c
@@ -1024,7 +1024,7 @@ p_nb_linkval( USES_REGS1 )
 
 
 static Int
-p_nb_create_accumulator(void)
+p_nb_create_accumulator( USES_REGS1 )
 {
   Term t = Deref(ARG1), acct, to;
   CELL *destp;
@@ -1041,7 +1041,7 @@ p_nb_create_accumulator(void)
   if (!Yap_unify(ARG2, acct)) {
     return FALSE;
   }
-  to = CopyTermToArena(t, GlobalArena, TRUE, TRUE, 2, &GlobalArena, garena_overflow_size(ArenaPt(GlobalArena)));
+  to = CopyTermToArena(t, GlobalArena, TRUE, TRUE, 2, &GlobalArena, garena_overflow_size(ArenaPt(GlobalArena) PASS_REGS) PASS_REGS);
   if (to == 0L)
     return FALSE;
   destp = RepAppl(Deref(ARG2));
@@ -1050,7 +1050,7 @@ p_nb_create_accumulator(void)
 }
 
 static Int
-p_nb_add_to_accumulator(void)
+p_nb_add_to_accumulator( USES_REGS1 )
 {
   Term t = Deref(ARG1), t0, tadd;
   Functor f;
@@ -1091,7 +1091,7 @@ p_nb_add_to_accumulator(void)
 	target[1] = source[1];
       } else {
 	/* we need to create a new long int */
-	new = CopyTermToArena(new, GlobalArena, TRUE, TRUE, 2, &GlobalArena, garena_overflow_size(ArenaPt(GlobalArena)));
+	new = CopyTermToArena(new, GlobalArena, TRUE, TRUE, 2, &GlobalArena, garena_overflow_size(ArenaPt(GlobalArena) PASS_REGS) PASS_REGS);
 	destp = RepAppl(Deref(ARG1));
 	destp[1] = new;
       }
@@ -1118,7 +1118,7 @@ p_nb_add_to_accumulator(void)
     new = Yap_MkApplTerm(FunctorPlus, 2, t2);
 
     new = Yap_Eval(new);
-    new = CopyTermToArena(new, GlobalArena, TRUE, TRUE, 2, &GlobalArena, garena_overflow_size(ArenaPt(GlobalArena)));
+    new = CopyTermToArena(new, GlobalArena, TRUE, TRUE, 2, &GlobalArena, garena_overflow_size(ArenaPt(GlobalArena) PASS_REGS)  PASS_REGS);
     destp = RepAppl(Deref(ARG1));
     destp[1] = new;    
 
@@ -1129,7 +1129,7 @@ p_nb_add_to_accumulator(void)
 
 
 static Int
-p_nb_accumulator_value(void)
+p_nb_accumulator_value( USES_REGS1 )
 {
   Term t = Deref(ARG1), to;
   Functor f;
diff --git a/C/grow.c b/C/grow.c
index c4166c176..07e752eb9 100755
--- a/C/grow.c
+++ b/C/grow.c
@@ -214,7 +214,7 @@ MoveLocalAndTrail( USES_REGS1 )
 #endif
 }
 
-#ifdef THREADS
+#ifdef YAPOR_THREADS
 
 static void
 CopyLocalAndTrail( USES_REGS1 )
@@ -401,14 +401,14 @@ AdjustTrail(int adjusting_heap, int thread_copying USES_REGS)
 {
   volatile tr_fr_ptr ptt, tr_base = (tr_fr_ptr)Yap_TrailBase;
 
-#if defined(YAPOR) && defined(THREADS)
+#if defined(YAPOR_THREADS)
   if (thread_copying == STACK_INCREMENTAL_COPYING) {
     ptt =  (tr_fr_ptr)(LOCAL_end_trail_copy);
     tr_base =  (tr_fr_ptr)(LOCAL_start_trail_copy);
   } else {
 #endif
     ptt = TR;
-#if defined(YAPOR) && defined(THREADS)
+#if defined(YAPOR_THREADS)
   }
 #endif
   /* moving the trail is simple */
@@ -462,7 +462,7 @@ AdjustLocal(int thread_copying USES_REGS)
   register CELL   reg, *pt, *pt_bot;
 
   /* Adjusting the local */
-#if defined(YAPOR) && defined(THREADS)
+#if defined(YAPOR_THREADS)
   if (thread_copying == STACK_INCREMENTAL_COPYING) {
     pt =  (CELL *) (LOCAL_end_local_copy);
     pt_bot =  (CELL *) (LOCAL_start_local_copy);
@@ -470,7 +470,7 @@ AdjustLocal(int thread_copying USES_REGS)
 #endif
     pt = LCL0;
     pt_bot = ASP;
-#if defined(YAPOR) && defined(THREADS)
+#if defined(YAPOR_THREADS)
   }
 #endif
   while (pt > pt_bot) {
@@ -551,7 +551,7 @@ AdjustGlobal(long sz, int thread_copying USES_REGS)
    * to clean the global now that functors are just variables pointing to
    * the code 
    */
-#if defined(YAPOR) && defined(THREADS)
+#if defined(YAPOR_THREADS)
   if (thread_copying == STACK_INCREMENTAL_COPYING) {
     pt =  (CELL *) (LOCAL_start_global_copy);
     pt_max =  (CELL *) (LOCAL_end_global_copy);
@@ -559,7 +559,7 @@ AdjustGlobal(long sz, int thread_copying USES_REGS)
 #endif
     pt = H0;
     pt_max = (H-sz/CellSize);
-#if defined(YAPOR) && defined(THREADS)
+#if defined(YAPOR_THREADS)
   }
 #endif
   pt = H0;
@@ -790,7 +790,7 @@ static_growheap(long size, int fix_code, struct intermediates *cip, tr_fr_ptr *o
   gc_verbose = Yap_is_gc_verbose();
   heap_overflows++;
   if (gc_verbose) {
-#if  defined(YAPOR) || defined(THREADS)
+#if  defined(YAPOR_THREADS)
     fprintf(Yap_stderr, "%% Worker Id %d:\n", worker_id);
 #endif
     fprintf(Yap_stderr, "%% Database Overflow %d\n", heap_overflows);
@@ -936,7 +936,7 @@ static_growglobal(long request, CELL **ptr, CELL *hsplit USES_REGS)
       vb_msg1 = 'D';
       vb_msg2 = "Delay";
     }
-#if  defined(YAPOR) || defined(THREADS)
+#if  defined(YAPOR_THREADS)
     fprintf(Yap_stderr, "%% Worker Id %d:\n", worker_id);
 #endif
     fprintf(Yap_stderr, "%% %cO %s Overflow %d\n", vb_msg1, vb_msg2, delay_overflows);
@@ -1357,7 +1357,7 @@ growatomtable( USES_REGS1 )
   }
   atom_table_overflows++;
   if (gc_verbose) {
-#if  defined(YAPOR) || defined(THREADS)
+#if  defined(YAPOR_THREADS)
     fprintf(Yap_stderr, "%% Worker Id %d:\n", worker_id);
 #endif
     fprintf(Yap_stderr, "%% Atom Table Overflow %d\n", atom_table_overflows);
@@ -1441,7 +1441,7 @@ Yap_growglobal(CELL **ptr)
   CACHE_REGS
   unsigned long sz = sizeof(CELL) * K16;
 
-#if defined(YAPOR) && !defined(THREADS)
+#if defined(YAPOR_THREADS)
   if (Yap_number_workers != 1) {
     Yap_Error(OUT_OF_STACK_ERROR,TermNil,"cannot grow Global: more than a worker/thread running");
     return(FALSE);
@@ -1837,7 +1837,7 @@ p_inform_heap_overflows( USES_REGS1 )
   return(Yap_unify(tn, ARG1) && Yap_unify(tt, ARG2));
 }
 
-#if defined(THREADS) && defined(YAPOR)
+#if defined(YAPOR_THREADS)
 void
 Yap_CopyThreadStacks(int worker_q, int worker_p, int incremental)
 {
diff --git a/C/index.c b/C/index.c
index 10ebf5263..eedfc0d02 100644
--- a/C/index.c
+++ b/C/index.c
@@ -2190,6 +2190,7 @@ emit_type_switch(compiler_vm_op op, struct intermediates *cint)
 static yamop *
 emit_switch_space(UInt n, UInt item_size, struct intermediates *cint, CELL func_mask)
 {
+  CACHE_REGS
   PredEntry *ap = cint->CurrentPred;
 
   if (ap->PredFlags & LogUpdatePredFlag) {
@@ -5218,7 +5219,7 @@ kill_clause(yamop *ipc, yamop *bg, yamop *lt, path_stack_entry *sp0, PredEntry *
     return sp;
   } else {
     if (
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	blk->ClRefCount == 0
 #else
 	!(blk->ClFlags & InUseMask)
@@ -6821,7 +6822,7 @@ Yap_FollowIndexingCode(PredEntry *ap, yamop *ipc, Term Terms[3], yamop *ap_pc, y
 	}
 	*--ASP = MkIntegerTerm(ap->TimeStampOfPred);
 	/* indicate the indexing code is being used */
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	/* just store a reference */
 	INC_CLREF_COUNT(cl);
 	TRAIL_CLREF(cl);
@@ -6883,7 +6884,7 @@ Yap_FollowIndexingCode(PredEntry *ap, yamop *ipc, Term Terms[3], yamop *ap_pc, y
 	} else {
 	  newpc = ipc->u.OtILl.d;
 	}
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	B->cp_tr--;
 	TR--;
 	DEC_CLREF_COUNT(cl);
diff --git a/C/tracer.c b/C/tracer.c
index bab8c7c37..9874ca225 100755
--- a/C/tracer.c
+++ b/C/tracer.c
@@ -163,6 +163,10 @@ 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++;
+  if (vsc_count < 45000LL)
+    return;
+  if (vsc_count == 47456LL)
+    jmp_deb(1);
 #ifdef THREADS
   MY_ThreadHandle.thread_inst_count++;
 #endif  
diff --git a/C/utilpreds.c b/C/utilpreds.c
index 543db8b2e..8684cc32c 100644
--- a/C/utilpreds.c
+++ b/C/utilpreds.c
@@ -146,7 +146,7 @@ copy_complex_term(CELL *pt0, CELL *pt0_end, int share, int newattvs, CELL *ptf,
 	f = (Functor)(*ap2);
 
 	if (IsExtensionFunctor(f)) {
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 	  if (f == FunctorDBRef) {
 	    DBRef  entryref = DBRefOfTerm(d0);
 	    if (entryref->Flags & LogUpdMask) {
diff --git a/H/Yap.h b/H/Yap.h
index 9d320f73f..bd06fd5e7 100755
--- a/H/Yap.h
+++ b/H/Yap.h
@@ -43,7 +43,7 @@
 #error Do not define multiple or-parallel models
 #endif /* (YAPOR_COPY && (YAPOR_COW || YAPOR_SBA)) || (YAPOR_COW && YAPOR_SBA) */
 
-#if defined(YAPOR_COPY) || defined(YAPOR_COW) || defined(YAPOR_SBA) || defined(THREADS)
+#if defined(YAPOR_COPY) || defined(YAPOR_COW) || defined(YAPOR_SBA) || defined(YAPOR_THREADS)
 #define YAPOR 1
 #endif /* YAPOR_COPY || YAPOR_COW || YAPOR_SBA */
 
@@ -60,6 +60,14 @@
 #endif
 #endif /* YAPOR */
 
+#if defined(YAPOR) || defined(THREADS) || defined(TABLING)
+#define MULTIPLE_STACKS 1
+#endif
+
+#if defined(YAPOR) || defined(THREADS)
+#define PARALLEL_YAP 1
+#endif
+
 #if defined(YAPOR) || defined(TABLING)
 #undef TRAILING_REQUIRES_BRANCH
 #endif /* YAPOR || TABLING */
@@ -76,7 +84,7 @@
 #endif
 #endif
 
-#if defined(SUPPORT_THREADS) || defined(SUPPORT_CONDOR)
+#if defined(THREADS) || defined(SUPPORT_CONDOR)
 #define USE_SYSTEM_MALLOC 1
 #endif
 
diff --git a/H/Yatom.h b/H/Yatom.h
index 751ab0ecb..95f7ca01f 100755
--- a/H/Yatom.h
+++ b/H/Yatom.h
@@ -863,6 +863,8 @@ typedef struct DB_STRUCT
   CELL Flags;			/* Term Flags                           */
 #if defined(YAPOR) || defined(THREADS)
   lockvar lock;			/* a simple lock to protect this entry */
+#endif
+#if MULTIPLE_STACKS
   Int ref_count;		/* how many branches are using this entry */
 #endif
   CELL NOfRefsTo;		/* Number of references pointing here   */
@@ -880,7 +882,7 @@ typedef struct DB_STRUCT
 
 #define DBStructFlagsToDBStruct(X) ((DBRef)((char *)(X) - (CELL) &(((DBRef) NULL)->Flags)))
 
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STAACKS
 #define INIT_DBREF_COUNT(X) (X)->ref_count = 0
 #define  INC_DBREF_COUNT(X) (X)->ref_count++
 #define  DEC_DBREF_COUNT(X) (X)->ref_count--
diff --git a/H/amiops.h b/H/amiops.h
index ec4818797..d58cfb0f1 100644
--- a/H/amiops.h
+++ b/H/amiops.h
@@ -267,9 +267,15 @@ Binding Macros for Multiple Assignment Variables.
 #define REF_TO_TRENTRY(REF)    AbsPair(((CELL *)&((REF)->Flags)))
 #define CLREF_TO_TRENTRY(REF)  AbsPair(((CELL *)&((REF)->ClFlags)))
 
+#if FROZEN_STACKS
 #define TRAIL_REF(REF)         RESET_VARIABLE(&TrailVal(TR)), TrailTerm(TR++) = REF_TO_TRENTRY(REF)
 #define TRAIL_CLREF(REF)       RESET_VARIABLE(&TrailVal(TR)), TrailTerm(TR++) = CLREF_TO_TRENTRY(REF)
 #define TRAIL_LINK(REF)        RESET_VARIABLE(&TrailVal(TR)), TrailTerm(TR++) = AbsPair((CELL *)(REF))
+#else
+#define TRAIL_REF(REF)         TrailTerm(TR++) = REF_TO_TRENTRY(REF)
+#define TRAIL_CLREF(REF)       TrailTerm(TR++) = CLREF_TO_TRENTRY(REF)
+#define TRAIL_LINK(REF)        TrailTerm(TR++) = AbsPair((CELL *)(REF))
+#endif
 #define TRAIL_FRAME(FR)        DO_TRAIL(AbsPair((CELL *)(Yap_TrailBase)), FR)
 
 extern void	Yap_WakeUp(CELL *v);
diff --git a/H/clause.h b/H/clause.h
index 09a7aa5aa..a4a5212e6 100644
--- a/H/clause.h
+++ b/H/clause.h
@@ -182,7 +182,7 @@ typedef struct dbterm_list {
 
 #define DynamicLock(X)		(ClauseCodeToDynamicClause(X)->ClLock)
 
-#if defined(YAPOR) || defined(THREADS)
+#if MULTIPLE_STACKS
 #define INIT_CLREF_COUNT(X) (X)->ClRefCount = 0
 #define  INC_CLREF_COUNT(X) (X)->ClRefCount++
 #define  DEC_CLREF_COUNT(X) (X)->ClRefCount--
diff --git a/config.h.in b/config.h.in
index cd2a85c5d..b6c216c59 100755
--- a/config.h.in
+++ b/config.h.in
@@ -48,7 +48,6 @@
 #undef HOST_ALIAS
 
 #undef SUPPORT_CONDOR
-#undef SUPPORT_THREADS
 #undef USE_PTHREAD_LOCKING
 
 #undef HAVE_SYS_WAIT_H
diff --git a/misc/LOCALS b/misc/LOCALS
index b16b4c798..1f1c205a6 100644
--- a/misc/LOCALS
+++ b/misc/LOCALS
@@ -149,7 +149,7 @@ struct PL_local_data	       *Yap_ld_			PL_local_data_p		=Yap_InitThreadIO(wid)
 struct open_query_struct*	_execution		execution		=NULL
 
 // Ricardo's stuff
-#if (defined(YAPOR) || defined(TABLING)) && defined(THREADS)
+#if MULTIPLE_STACKS
 struct worker			worker			WORKER		void
 #endif