diff --git a/C/attvar.c b/C/attvar.c
index 7c7993e89..ad9a3e505 100644
--- a/C/attvar.c
+++ b/C/attvar.c
@@ -215,8 +215,6 @@ WakeAttVar(CELL* pt1, CELL reg2)
   Bind_Global(&(attv->Value), reg2);
 }
 
-#ifndef FIXED_STACKS
-
 static void
 mark_attvar(CELL *orig)
 {
@@ -230,8 +228,6 @@ mark_attvar(CELL *orig)
   }
 }
 
-#endif /* FIXED_STACKS */
-
 #if FROZEN_STACKS
 static Term
 CurrentTime(void) {
@@ -635,9 +631,7 @@ void InitAttVarPreds(void)
   attas[attvars_ext].copy_term_op = CopyAttVar;
   attas[attvars_ext].to_term_op = AttVarToTerm;
   attas[attvars_ext].term_to_op = TermToAttVar;
-#ifndef FIXED_STACKS
   attas[attvars_ext].mark_op = mark_attvar;
-#endif
   InitCPred("get_att", 3, p_get_att, SafePredFlag);
   InitCPred("get_all_atts", 2, p_get_all_atts, SafePredFlag);
   InitCPred("free_att", 2, p_free_att, SafePredFlag);
diff --git a/C/corout.c b/C/corout.c
index 37f72221c..353e8e98d 100644
--- a/C/corout.c
+++ b/C/corout.c
@@ -136,10 +136,8 @@ STATIC_PROTO(Int p_non_ground, (void));
 STATIC_PROTO(void Wake, (CELL *, CELL));
 STATIC_PROTO(sus_record *UpdateSVarList, (sus_record *));
 STATIC_PROTO(sus_record *GetSVarList, (void));
-#ifndef FIXED_STACKS
 STATIC_PROTO(void mark_sus_record, (sus_record *));
 STATIC_PROTO(void mark_suspended_goal, (CELL *));
-#endif /* FIXED_STACKS */
 STATIC_PROTO(void AddSuspendedGoals, (sus_record *, sus_record *));
 STATIC_PROTO(void ReleaseGoals, (sus_record *));
 STATIC_PROTO(void wake_if_binding_vars_in_frozen_goal, (Term, sus_record *));
@@ -394,8 +392,6 @@ TermToSuspendedVar(Term gs, Term var)
 }
 
 
-#ifndef FIXED_STACKS
-
 static void
 mark_sus_record(sus_record *sg)
 {
@@ -420,8 +416,6 @@ static void mark_suspended_goal(CELL *orig)
   mark_external_reference(((CELL *)&(sreg->SG)));
 }
 
-#endif /* FIXED_STACKS */
-
 
 /*
 
@@ -1211,9 +1205,7 @@ void InitCoroutPreds(void)
   attas[susp_ext].copy_term_op = CopySuspendedVar;
   attas[susp_ext].to_term_op = SuspendedVarToTerm;
   attas[susp_ext].term_to_op = TermToSuspendedVar;
-#ifndef FIXED_STACKS
   attas[susp_ext].mark_op = mark_suspended_goal;
-#endif /* FIXED_STACKS */
   at = LookupAtom("$wake_up_goal");
   pred = RepPredProp(PredPropByFunc(MkFunctor(at, 2),0));
   WakeUpCode = pred;
diff --git a/C/grow.c b/C/grow.c
index e7f84f547..c49356ef6 100644
--- a/C/grow.c
+++ b/C/grow.c
@@ -635,8 +635,11 @@ growheap(int fix_code)
   int shift_factor = (heap_overflows > 8 ? 8 : heap_overflows);
   unsigned long sz =  size << shift_factor;
 
-#ifdef FIXED_STACKS
-  abort_optyap("noheapleft in function absmi");
+#if defined(YAPOR) || defined(THREADS)
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot grow Heap: more than a worker/thread running");
+    return(FALSE);
+  }
 #endif
   if (SizeOfOverflow > sz)
     sz = AdjustPageSize(SizeOfOverflow);
@@ -674,8 +677,11 @@ growglobal(CELL **ptr)
 {
   unsigned long sz = sizeof(CELL) * 16 * 1024L;
 
-#ifdef FIXED_STACKS
-  abort_optyap("noheapleft in function absmi");
+#if defined(YAPOR) || defined(THREADS)
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot grow Global: more than a worker/thread running");
+    return(FALSE);
+  }
 #endif
   if (!local_growglobal(sz, ptr))
     return(FALSE);
@@ -693,8 +699,11 @@ growstack(long size)
   Int start_growth_time, growth_time;
   int gc_verbose;
 
-#ifdef FIXED_STACKS
-  abort_optyap("nostackleft in function absmi");
+#if defined(YAPOR) || defined(THREADS)
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot grow Local: more than a worker/thread running");
+    return(FALSE);
+  }
 #endif
   /* adjust to a multiple of 256) */
   size = AdjustPageSize(size);
@@ -812,8 +821,11 @@ growstack_in_parser(tr_fr_ptr *old_trp, TokEntry **tksp, VarEntry **vep)
   int gc_verbose;
   long size  = sizeof(CELL)*(LCL0-(CELL *)GlobalBase);
 
-#ifdef FIXED_STACKS
-  abort_optyap("nostackleft in parser");
+#if defined(YAPOR) || defined(THREADS)
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot grow Parser Stack: more than a worker/thread running");
+    return(FALSE);
+  }
 #endif
   /* adjust to a multiple of 256) */
   size = AdjustPageSize(size);
@@ -867,8 +879,11 @@ growtrail(long size)
   Int start_growth_time = cputime(), growth_time;
   int gc_verbose = is_gc_verbose();
 
-#ifdef FIXED_STACKS
-  abort_optyap("notrailleft in function absmi");
+#if defined(YAPOR) || defined(THREADS)
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot grow trail: more than a worker/thread running");
+    return(FALSE);
+  }
 #endif
   /* adjust to a multiple of 256) */
   size = AdjustPageSize(size);
diff --git a/C/heapgc.c b/C/heapgc.c
index 6051be316..94fc60e37 100644
--- a/C/heapgc.c
+++ b/C/heapgc.c
@@ -48,8 +48,6 @@ struct gc_ma_h_entry *live_list;
 STATIC_PROTO(Int  p_inform_gc, (void));
 STATIC_PROTO(Int  p_gc, (void));
 
-#ifndef FIXED_STACKS
-
 #ifdef EASY_SHUNTING
 static choiceptr current_B;
 
@@ -3100,8 +3098,6 @@ do_gc(Int predarity, CELL *current_env, yamop *nextop)
   return(effectiveness);
 }
 
-#endif /* FIXED_STACKS */
-
 int
 is_gc_verbose(void)
 {
@@ -3140,14 +3136,17 @@ p_inform_gc(void)
 int 
 gc(Int predarity, CELL *current_env, yamop *nextop)
 {
-#ifdef FIXED_STACKS
-  abort_optyap("garbage collection");
-#else /* FIXED_STACKS */
   Int           gc_margin = 128;
   Term          Tgc_margin;
   Int           effectiveness = 0;
   int           gc_on = FALSE;
 
+#if defined(YAPOR) || defined(THREADS)
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot perform garbage collection: more than a worker/thread running");
+    return(FALSE);
+  }
+#endif
   if (GetValue(AtomGc) != TermNil)
     gc_on = TRUE;
   if (IsIntTerm(Tgc_margin = GetValue(AtomGcMargin)))
@@ -3186,7 +3185,6 @@ gc(Int predarity, CELL *current_env, yamop *nextop)
    * debug for(save_total=1; save_total<=N; ++save_total)
    * plwrite(XREGS[save_total],DebugPutc,0); 
    */
-#endif /* FIXED_STACKS */
   return ( TRUE );
 }
 
@@ -3194,9 +3192,7 @@ gc(Int predarity, CELL *current_env, yamop *nextop)
 static Int
 p_gc(void)
 {
-#ifndef FIXED_STACKS
   do_gc(0, ENV, P);
-#endif  /* FIXED_STACKS */
   return(TRUE);
 }
 
diff --git a/C/init.c b/C/init.c
index ef92ad852..9e9234f5e 100644
--- a/C/init.c
+++ b/C/init.c
@@ -834,6 +834,7 @@ InitCodes(void)
   INIT_LOCK(heap_regs->heap_used_lock);
   INIT_LOCK(heap_regs->heap_top_lock);
   INIT_LOCK(heap_regs->dead_clauses_lock);
+  heap_regs->n_of_threads = 1;
   heap_regs->heap_top_owner = -1;
 #endif /* YAPOR */
   heap_regs->clausecode.arity = 0;
diff --git a/C/save.c b/C/save.c
index 477d5b637..7f6db8cdb 100644
--- a/C/save.c
+++ b/C/save.c
@@ -553,6 +553,10 @@ do_save(int mode) {
 static Int 
 p_save(void)
 {
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot perform save: more than a worker/thread running");
+    return(FALSE);
+  }
   which_save = 1;
   return(do_save(DO_EVERYTHING));
 }
@@ -561,6 +565,10 @@ p_save(void)
 static Int 
 p_save2(void)
 {
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot perform save: more than a worker/thread running");
+    return(FALSE);
+  }
   which_save = 2;
   return(do_save(DO_EVERYTHING) && unify(ARG2,MkIntTerm(1)));
 }
@@ -1499,6 +1507,10 @@ p_restore(void)
   int mode;
 
   Term t1 = Deref(ARG1);
+  if (NOfThreads != 1) {
+    Error(SYSTEM_ERROR,TermNil,"cannot perform save: more than a worker/thread running");
+    return(FALSE);
+  }
   if (!GetName(FileNameBuf, YAP_FILENAME_MAX, t1)) {
     Error(TYPE_ERROR_LIST,t1,"restore/1");
     return(FALSE);
diff --git a/C/sysbits.c b/C/sysbits.c
index ebf387787..38cfdde26 100644
--- a/C/sysbits.c
+++ b/C/sysbits.c
@@ -1010,7 +1010,7 @@ SearchForTrailFault(void)
 #ifdef DEBUG
   /*  fprintf(stderr,"Catching a sigsegv at %p with %p\n", TR, TrailTop); */
 #endif
-#ifndef FIXED_STACKS
+#if  !OS_HANDLES_TR_OVERFLOW
   if ((TR > (tr_fr_ptr)TrailTop-1024  && 
        TR < (tr_fr_ptr)TrailTop+(64*1024))|| DBTrailOverflow()) {
     if (!growtrail(64 * 1024L)) {
@@ -1019,7 +1019,7 @@ SearchForTrailFault(void)
     /* just in case, make sure the OS keeps the signal handler. */
     /*    my_signal_info(SIGSEGV, HandleSIGSEGV); */
   } else
-#endif /* FIXED_STACKS */
+#endif /* OS_HANDLES_TR_OVERFLOW */
     Error(FATAL_ERROR, TermNil,
 	  "likely bug in YAP, segmentation violation");
 }
diff --git a/H/Heap.h b/H/Heap.h
index 031dfef18..f234de037 100644
--- a/H/Heap.h
+++ b/H/Heap.h
@@ -10,7 +10,7 @@
 * File:		Heap.h         						 *
 * mods:									 *
 * comments:	Heap Init Structure					 *
-* version:      $Id: Heap.h,v 1.32 2002-10-14 16:25:33 vsc Exp $	 *
+* version:      $Id: Heap.h,v 1.33 2002-10-21 22:52:36 vsc Exp $	 *
 *************************************************************************/
 
 /* information that can be stored in Code Space */
@@ -128,9 +128,6 @@ typedef struct various_codes {
   Int maxdepth, maxlist;
   int update_mode;
   Atom atprompt;
-#if defined(YAPOR) || defined(THREADS)
-  lockvar  heap_used_lock;        /* protect HeapUsed */
-#endif
   char prompt[MAX_PROMPT];
   OPCODE undef_op; 
   OPCODE index_op; 
@@ -149,8 +146,11 @@ typedef struct various_codes {
   char *char_conversion_table;
   char *char_conversion_table2;
 #if defined(YAPOR) || defined(THREADS)
+  lockvar  heap_used_lock;        /* protect HeapUsed */
   lockvar  heap_top_lock;        /* protect HeapTop */
+  lockvar  dead_clauses_lock;        /* protect DeadClauses */
   int      heap_top_owner;
+  unsigned int n_of_threads;      /* number of threads and processes in system */
 #endif
   unsigned int size_of_overflow;
   UInt  number_of_cpreds;
@@ -159,9 +159,6 @@ typedef struct various_codes {
   struct pred_entry *module_pred[MaxModules];
   SMALLUNSGN   no_of_modules;
   struct clause_struct *dead_clauses;
-#if defined(YAPOR) || defined(THREADS)
-  lockvar  dead_clauses_lock;        /* protect DeadClauses */
-#endif
   int   primitives_module;
   int   user_module;
   struct idb_queue *db_queues, *db_queues_cache;
@@ -520,6 +517,7 @@ typedef struct various_codes {
 #define  FreeBlocksLock           heap_regs->free_blocks_lock
 #define  HeapTopLock              heap_regs->heap_top_lock
 #define  HeapTopOwner             heap_regs->heap_top_owner
+#define  NOfThreads               heap_regs->n_of_threads
 #define  HeapUsedLock             heap_regs->heap_used_lock
 #define  DeadClausesLock          heap_regs->dead_clauses_lock
 #endif
diff --git a/H/absmi.h b/H/absmi.h
index eabcefcf7..4537f850b 100644
--- a/H/absmi.h
+++ b/H/absmi.h
@@ -674,7 +674,7 @@ Macros to check the limits of stacks
 /* for the moment I don't know how to handle trail overflows
    in a pure Windows environment 
 */
-#if !_MSC_VER && !defined(__MINGW32__)
+#if !_MSC_VER && !defined(__MINGW32__) && !defined(THREADS) && !defined(YAPOR)
 #define OS_HANDLES_TR_OVERFLOW 1
 #endif
 #endif
diff --git a/OPTYap/opt.init.c b/OPTYap/opt.init.c
index 3482b8114..52057d1ac 100644
--- a/OPTYap/opt.init.c
+++ b/OPTYap/opt.init.c
@@ -229,6 +229,7 @@ void make_root_frames(void) {
 #ifdef YAPOR
 void init_workers(void) {
   int proc;
+  NOfThreads = number_workers;
 #ifdef ACOW
   if (number_workers > 1) {
     int son;