diff --git a/src/mzscheme/gc2/README b/src/mzscheme/gc2/README
index 54e6e9daeb..4250dc0d41 100644
--- a/src/mzscheme/gc2/README
+++ b/src/mzscheme/gc2/README
@@ -83,7 +83,8 @@ MzScheme allocates the following kinds of memory objects:
 
    Note that a two-space copying collector might use only the fixup
    operation, while a non-moving collector might use only the mark
-   operation.
+   operation. However, MzScheme currently relies on at least fixup
+   calls for scheme_runstack_rt-tagged objects.
 
  * Atomic - The allocated object contains no pointers to other
    allocated objects.
diff --git a/src/mzscheme/gc2/compact.c b/src/mzscheme/gc2/compact.c
index 8003f29410..c8ecaa9011 100644
--- a/src/mzscheme/gc2/compact.c
+++ b/src/mzscheme/gc2/compact.c
@@ -440,7 +440,7 @@ static int just_checking, the_size;
 
 #if defined(OS_X)
 # if GENERATIONS
-static void designate_modified(void *p);
+static int designate_modified(void *p);
 # endif
 
 # define TEST 0
@@ -3920,6 +3920,11 @@ void *GC_malloc_allow_interior(size_t size_in_bytes)
   return malloc_bigblock(size_in_bytes, MTYPE_ARRAY, 1);
 }
 
+void *GC_malloc_tagged_allow_interior(size_t size_in_bytes)
+{
+  return malloc_bigblock(size_in_bytes, MTYPE_TAGGED, 1);
+}
+
 void *GC_malloc_array_tagged(size_t size_in_bytes)
 {
   return malloc_untagged(size_in_bytes, MTYPE_TAGGED_ARRAY, &tagged_array);
diff --git a/src/mzscheme/gc2/gc2.h b/src/mzscheme/gc2/gc2.h
index 03a5663f4f..82b6d4e655 100644
--- a/src/mzscheme/gc2/gc2.h
+++ b/src/mzscheme/gc2/gc2.h
@@ -179,6 +179,10 @@ GC2_EXTERN void *GC_malloc_allow_interior(size_t size_in_bytes);
    pointers into the middle of the array, or just past the end of the
    array. */
 
+GC2_EXTERN void *GC_malloc_tagged_allow_interior(size_t size_in_bytes);
+/*
+   Like GC_malloc_allow_interior(), but for a tagged object. */
+
 GC2_EXTERN void *GC_malloc_weak_array(size_t size_in_bytes, void *replace_val);
 /*
    Alloc an array of weak pointers, initially zeroed.  When a value in
diff --git a/src/mzscheme/gc2/newgc.c b/src/mzscheme/gc2/newgc.c
index a623e0ef7c..027aa6b4b3 100644
--- a/src/mzscheme/gc2/newgc.c
+++ b/src/mzscheme/gc2/newgc.c
@@ -510,6 +510,7 @@ void *GC_malloc_array_tagged(size_t s) { return allocate(s, PAGE_TARRAY); }
 void *GC_malloc_atomic(size_t s) { return allocate(s, PAGE_ATOMIC); }
 void *GC_malloc_atomic_uncollectable(size_t s) { return malloc(s); }
 void *GC_malloc_allow_interior(size_t s) {return allocate_big(s, PAGE_ARRAY);}
+void *GC_malloc_tagged_allow_interior(size_t s) {return allocate_big(s, PAGE_TAGGED);}
 void GC_free(void *p) {}
 
 void *GC_malloc_one_small_tagged(size_t sizeb)
diff --git a/src/mzscheme/src/eval.c b/src/mzscheme/src/eval.c
index 2a38b56752..cda1c5dbb7 100644
--- a/src/mzscheme/src/eval.c
+++ b/src/mzscheme/src/eval.c
@@ -716,16 +716,16 @@ void *scheme_enlarge_runstack(long size, void *(*k)())
       size = 1000;
   }
 
-  p->runstack_saved = saved;
   if (p->spare_runstack && (size <= p->spare_runstack_size)) {
     size = p->spare_runstack_size;
     MZ_RUNSTACK_START = p->spare_runstack;
     p->spare_runstack = NULL;
   } else {
-    MZ_RUNSTACK_START = scheme_malloc_allow_interior(sizeof(Scheme_Object*) * size);
+    MZ_RUNSTACK_START = scheme_alloc_runstack(size);
   }
   p->runstack_size = size;
   MZ_RUNSTACK = MZ_RUNSTACK_START + size;
+  p->runstack_saved = saved;
   
   cont_count = scheme_cont_capture_count;
 
diff --git a/src/mzscheme/src/fun.c b/src/mzscheme/src/fun.c
index 65c551f686..a44270f23a 100644
--- a/src/mzscheme/src/fun.c
+++ b/src/mzscheme/src/fun.c
@@ -4277,7 +4277,7 @@ static void restore_continuation(Scheme_Cont *cont, Scheme_Thread *p, int for_pr
   MZ_MARK_STACK_TYPE copied_cms = 0;
   Scheme_Object **mv, *sub_conts = NULL;
   int mc;
-  
+
   if (SAME_OBJ(result, SCHEME_MULTIPLE_VALUES)) {
     /* Get values out before GC */
     mv = p->ku.multiple.array;
@@ -4349,6 +4349,7 @@ static void restore_continuation(Scheme_Cont *cont, Scheme_Thread *p, int for_pr
       mc = cont->meta_continuation;
     p->meta_continuation = mc;
   }
+
   if (shortcut_prompt) {
     /* In shortcut mode, we need to preserve saved runstacks
        that were pruned when capturing the continuation. */
@@ -4370,14 +4371,19 @@ static void restore_continuation(Scheme_Cont *cont, Scheme_Thread *p, int for_pr
     p->runstack_saved = rs;
   } else
     p->runstack_saved = cont->runstack_saved;
+
   MZ_RUNSTACK_START = cont->runstack_start;
   p->runstack_size = cont->runstack_size;
 
   scheme_restore_env_stack_w_thread(cont->ss, p);
 
   if (p->runstack_owner
-      && (*p->runstack_owner == p))
+      && (*p->runstack_owner == p)) {
     *p->runstack_owner = NULL;
+  }
+
+  if (resume)
+    p->meta_prompt = NULL; /* in case there's a GC before we can set it */
 
   p->runstack_owner = cont->runstack_owner;
   if (p->runstack_owner && (*p->runstack_owner != p)) {
diff --git a/src/mzscheme/src/mkmark.ss b/src/mzscheme/src/mkmark.ss
index 7fe6d00c2d..267d0220b7 100644
--- a/src/mzscheme/src/mkmark.ss
+++ b/src/mzscheme/src/mkmark.ss
@@ -6,6 +6,8 @@
 
 (define re:mark "^ mark:")
 (define re:size "^ size:")
+(define re:size-or-more "^ (?:size|more):")
+(define re:fixup-start "^ fixup:")
 (define re:close "^}")
 
 (define re:const-size (regexp "^[ \t]*gcBYTES_TO_WORDS[(]sizeof[(][A-Za-z0-9_]*[)][)];[ \t]*$"))
@@ -24,7 +26,8 @@
 		    [(regexp-match re:done l)
 		     null]
 		    [(or (regexp-match re:mark l)
-			 (regexp-match re:size l))
+                         (regexp-match re:size-or-more l)
+			 (regexp-match re:fixup-start l))
 		     (error 'mkmark.ss "unexpected label: ~a at ~a" l
 			    (file-position (current-input-port)))]
 		    [(regexp-match re:close l)
@@ -36,7 +39,12 @@
 			  (printf "~a~n" s))
 			l))])
     (let ([prefix (read-lines re:mark)]
-	  [mark (read-lines re:size)]
+	  [mark (read-lines re:size-or-more)]
+	  [fixup (if (regexp-match-peek re:fixup-start (current-input-port))
+                     (begin
+                       (regexp-match re:fixup-start (current-input-port))
+                       (read-lines re:size))
+                     null)]
 	  [size (read-lines re:close)])
       (printf "static int ~a_SIZE(void *p) {~n" name)
       (print-lines prefix)
@@ -69,7 +77,9 @@
 			    s 
 			    "FIXUP")
 			   "\\1"))
-			mark))
+			(append
+                         mark
+                         fixup)))
       (printf "  return~n")
       (print-lines size)
       (printf "}~n~n")
diff --git a/src/mzscheme/src/mzmark.c b/src/mzscheme/src/mzmark.c
index 7808351577..fbfd32ebe6 100644
--- a/src/mzscheme/src/mzmark.c
+++ b/src/mzscheme/src/mzmark.c
@@ -1752,6 +1752,57 @@ static int thread_val_FIXUP(void *p) {
 #define thread_val_IS_CONST_SIZE 1
 
 
+static int runstack_val_SIZE(void *p) {
+  long *s = (long *)p;
+  return
+  s[1];
+}
+
+static int runstack_val_MARK(void *p) {
+  long *s = (long *)p;
+  void **a, **b;
+  a = (void **)s + 4 + s[2];
+  b = (void **)s + 4 + s[3];
+  while (a < b) {
+    gcMARK(*a);
+    a++;
+  }
+  return
+  s[1];
+}
+
+static int runstack_val_FIXUP(void *p) {
+  long *s = (long *)p;
+  void **a, **b;
+  a = (void **)s + 4 + s[2];
+  b = (void **)s + 4 + s[3];
+  while (a < b) {
+    gcFIXUP(*a);
+    a++;
+  }
+
+  /* Zero out the part that we didn't mark, in case it becomes
+     live later. */
+  a = (void **)s + 4;
+  b = (void **)s + 4 + s[2];
+  while (a < b) {
+    *a = NULL;
+    a++;
+  }
+  a = (void **)s + 4 + s[3];
+  b = (void **)s + 4 + (s[1] - 4);
+  while (a < b) {
+    *a = NULL;
+    a++;
+  }
+  return
+  s[1];
+}
+
+#define runstack_val_IS_ATOMIC 0
+#define runstack_val_IS_CONST_SIZE 0
+
+
 static int prompt_val_SIZE(void *p) {
   return
   gcBYTES_TO_WORDS(sizeof(Scheme_Prompt));
diff --git a/src/mzscheme/src/mzmarksrc.c b/src/mzscheme/src/mzmarksrc.c
index a0dbd4755e..82cf367b03 100644
--- a/src/mzscheme/src/mzmarksrc.c
+++ b/src/mzscheme/src/mzmarksrc.c
@@ -684,6 +684,36 @@ thread_val {
   gcBYTES_TO_WORDS(sizeof(Scheme_Thread));
 }
 
+runstack_val {
+  long *s = (long *)p;
+ mark:
+  void **a, **b;
+  a = (void **)s + 4 + s[2];
+  b = (void **)s + 4 + s[3];
+  while (a < b) {
+    gcMARK(*a);
+    a++;
+  }
+ more:
+ fixup:
+  /* Zero out the part that we didn't mark, in case it becomes
+     live later. */
+  a = (void **)s + 4;
+  b = (void **)s + 4 + s[2];
+  while (a < b) {
+    *a = NULL;
+    a++;
+  }
+  a = (void **)s + 4 + s[3];
+  b = (void **)s + 4 + (s[1] - 4);
+  while (a < b) {
+    *a = NULL;
+    a++;
+  }
+ size:
+  s[1];
+}
+
 prompt_val {
  mark: 
   Scheme_Prompt *pr = (Scheme_Prompt *)p;
diff --git a/src/mzscheme/src/schpriv.h b/src/mzscheme/src/schpriv.h
index 2f38261d21..8f8a2b55cc 100644
--- a/src/mzscheme/src/schpriv.h
+++ b/src/mzscheme/src/schpriv.h
@@ -324,6 +324,9 @@ void scheme_start_itimer_thread(long usec);
 void scheme_block_child_signals(int block);
 #endif
 
+Scheme_Object **scheme_alloc_runstack(long len);
+void scheme_set_runstack_limits(Scheme_Object **rs, long len, long start, long end);
+
 void scheme_alloc_list_stack(Scheme_Thread *p);
 void scheme_clean_list_stack(Scheme_Thread *p);
 
diff --git a/src/mzscheme/src/stypes.h b/src/mzscheme/src/stypes.h
index aa2f3db7f9..303d468097 100644
--- a/src/mzscheme/src/stypes.h
+++ b/src/mzscheme/src/stypes.h
@@ -227,6 +227,7 @@ enum {
   scheme_rt_delay_load_info,            /* 205 */
   scheme_rt_marshal_info,               /* 206 */
   scheme_rt_unmarshal_info,             /* 207 */
+  scheme_rt_runstack,                   /* 208 */
 #endif
 
   _scheme_last_type_
diff --git a/src/mzscheme/src/thread.c b/src/mzscheme/src/thread.c
index 345eee70f4..2cdba5f41b 100644
--- a/src/mzscheme/src/thread.c
+++ b/src/mzscheme/src/thread.c
@@ -2037,7 +2037,7 @@ static Scheme_Thread *make_thread(Scheme_Config *config,
     process->runstack_size = init_stack_size;
     {
       Scheme_Object **sa;
-      sa = scheme_malloc_allow_interior(sizeof(Scheme_Object*) * init_stack_size);
+      sa = scheme_alloc_runstack(init_stack_size);
       process->runstack_start = sa;
     }
     process->runstack = process->runstack_start + init_stack_size;
@@ -2162,6 +2162,48 @@ void *scheme_tls_get(int pos)
     return p->user_tls[pos];
 }
 
+#ifdef MZ_XFORM
+START_XFORM_SKIP;
+#endif
+
+Scheme_Object **scheme_alloc_runstack(long len)
+{
+#ifdef MZ_PRECISE_GC
+  long sz;
+  void **p;
+  sz = sizeof(Scheme_Object*) * (len + 4);
+  p = (void **)GC_malloc_tagged_allow_interior(sz);
+  *(Scheme_Type *)(void *)p = scheme_rt_runstack;
+  ((long *)(void *)p)[1] = gcBYTES_TO_WORDS(sz);
+  ((long *)(void *)p)[2] = 0;
+  ((long *)(void *)p)[3] = len;
+  return (Scheme_Object **)(p + 4);
+#else
+  return (Scheme_Object **)scheme_malloc_allow_interior(sizeof(Scheme_Object*) * len);
+#endif
+}
+
+void scheme_set_runstack_limits(Scheme_Object **rs, long len, long start, long end)
+/* With 3m, we can tell the GC not to scan the unused parts, and we
+   can have the fixup function zero out the unused parts; that avoids
+   writing and scanning pages that could be skipped for a minor
+   GC. For CGC, we have to just clear out the unused part. */
+{
+#ifdef MZ_PRECISE_GC
+  if (((long *)(void *)rs)[-2] != start)
+    ((long *)(void *)rs)[-2] = start;
+  if (((long *)(void *)rs)[-1] != end)
+    ((long *)(void *)rs)[-1] = end;
+#else
+  memset(rs, 0, start * sizeof(Scheme_Object *));
+  memset(rs + end, 0, (len - end) * sizeof(Scheme_Object *));
+#endif
+}
+
+#ifdef MZ_XFORM
+END_XFORM_SKIP;
+#endif
+
 /*========================================================================*/
 /*                     thread creation and swapping                       */
 /*========================================================================*/
@@ -2371,10 +2413,12 @@ static void remove_thread(Scheme_Thread *r)
   if (r->runstack_owner) {
     /* Drop ownership, if active, and clear the stack */
     if (r == *(r->runstack_owner)) {
-      memset(r->runstack_start, 0, r->runstack_size * sizeof(Scheme_Object*));
-      r->runstack_start = NULL;
+      if (r->runstack_start) {
+        scheme_set_runstack_limits(r->runstack_start, r->runstack_size, 0, 0);
+        r->runstack_start = NULL;
+      }
       for (saved = r->runstack_saved; saved; saved = saved->prev) {
-	memset(saved->runstack_start, 0, saved->runstack_size * sizeof(Scheme_Object*));
+        scheme_set_runstack_limits(saved->runstack_start, saved->runstack_size, 0, 0);
       }
       r->runstack_saved = NULL;
       *(r->runstack_owner) = NULL;
@@ -6596,39 +6640,41 @@ static void prepare_thread_for_GC(Scheme_Object *t)
   /* zero ununsed part of env stack in each thread */
 
   if (!p->nestee) {
-    Scheme_Object **o, **e, **e2;
     Scheme_Saved_Stack *saved;
 # define RUNSTACK_TUNE(x) /* x   - Used for performance tuning */
     RUNSTACK_TUNE( long size; );
 
     if (!p->runstack_owner
 	|| (p == *p->runstack_owner)) {
-      o = p->runstack_start;
-      e = p->runstack;
-      e2 = p->runstack_tmp_keep;
-
-      while (o < e && (o != e2)) {
-	*(o++) = NULL;
-      }
+      long rs_end;
 
       /* If there's a meta-prompt, we can also zero out past the unused part */
       if (p->meta_prompt && (p->meta_prompt->runstack_boundary_start == p->runstack_start)) {
-        e = p->runstack_start + p->runstack_size;
-        o = p->runstack_start + p->meta_prompt->runstack_boundary_offset;
-        while (o < e) {
-          *(o++) = NULL;
-        }
+        rs_end = p->meta_prompt->runstack_boundary_offset;
+      } else {
+        rs_end = p->runstack_size;
       }
+
+      scheme_set_runstack_limits(p->runstack_start, 
+                                 p->runstack_size,
+                                 p->runstack - p->runstack_start,
+                                 rs_end);
       
       RUNSTACK_TUNE( size = p->runstack_size - (p->runstack - p->runstack_start); );
       
       for (saved = p->runstack_saved; saved; saved = saved->prev) {
-	o = saved->runstack_start;
-	e = o + saved->runstack_offset;
 	RUNSTACK_TUNE( size += saved->runstack_size; );
-	while (o < e) {
-	  *(o++) = NULL;
-	}
+
+        if (p->meta_prompt && (p->meta_prompt->runstack_boundary_start == saved->runstack_start)) {
+          rs_end = p->meta_prompt->runstack_boundary_offset;
+        } else {
+          rs_end = saved->runstack_size;
+        }
+
+        scheme_set_runstack_limits(saved->runstack_start,
+                                   saved->runstack_size,
+                                   saved->runstack_offset,
+                                   rs_end);
       }
     }
 
@@ -6662,11 +6708,16 @@ static void prepare_thread_for_GC(Scheme_Object *t)
     
     if (segpos < p->cont_mark_seg_count) {
       Scheme_Cont_Mark *seg = p->cont_mark_stack_segments[segpos];
-      int stackpos = ((long)p->cont_mark_stack & SCHEME_MARK_SEGMENT_MASK), i;
+      int stackpos = ((long)p->cont_mark_stack & SCHEME_MARK_SEGMENT_MASK);
       for (i = stackpos; i < SCHEME_MARK_SEGMENT_SIZE; i++) {
-	seg[i].key = NULL;
-	seg[i].val = NULL;
-	seg[i].cache = NULL;
+        if (seg[i].key) {
+          seg[i].key = NULL;
+          seg[i].val = NULL;
+          seg[i].cache = NULL;
+        } else {
+          /* NULL means we already cleared from here on. */
+          break;
+        }
       }
     }
 
@@ -6729,9 +6780,7 @@ static void get_ready_for_GC()
   scheme_current_thread->cont_mark_pos = MZ_CONT_MARK_POS;
 #endif
 
-  if (scheme_fuel_counter) {
-    for_each_managed(scheme_thread_type, prepare_thread_for_GC);
-  }
+  for_each_managed(scheme_thread_type, prepare_thread_for_GC);
 
 #ifdef MZ_PRECISE_GC
   scheme_flush_stack_copy_cache();
diff --git a/src/mzscheme/src/type.c b/src/mzscheme/src/type.c
index 7c55bc9ce6..79459c77a2 100644
--- a/src/mzscheme/src/type.c
+++ b/src/mzscheme/src/type.c
@@ -540,6 +540,8 @@ void scheme_register_traversers(void)
   GC_REG_TRAV(scheme_global_ref_type, small_object);
 
   GC_REG_TRAV(scheme_delay_syntax_type, small_object);
+
+  GC_REG_TRAV(scheme_rt_runstack, runstack_val);
 }
 
 END_XFORM_SKIP;