diff --git a/gc.c b/gc.c
index b45af75..2616cce 100644
--- a/gc.c
+++ b/gc.c
@@ -3,6 +3,7 @@
 #include <assert.h>
 #include <ctype.h>
 #include <inttypes.h>
+#include <setjmp.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -27,21 +28,13 @@ gc_stats_t gc_stats;
 /* Alignment must ensure each object has enough room to hold a forwarding object */
 #define GC_ALIGNMENT  ((size_t)(sizeof(object_t)))
 
+/* Smaller block sizes allow for more precise GC, but require more memory for the bitmap. */
+/* The block bitmap will be allocated at startup assuming a max. heap size of 2GB. */
+/* The default of 128 KB/block should require a 2 KB bitmap. */
+#define GC_DIRTY_BLOCK_SIZE (128UL << 10)
+
 /****************************************************************************/
 
-static char   *gc_ranges[2];
-static size_t  gc_min_size;
-static size_t  gc_max_size;
-static size_t  gc_soft_limit;
-static bool    gc_enabled;
-
-static int     gc_current_range;
-static char   *gc_free_ptr;
-static char   *gc_range_end;
-
-/* A convenient shorthand */
-#define gc_other_range() (1-gc_current_range)
-
 static value_t gc_weak_box_list;
 static value_t gc_will_list;
 static value_t gc_will_active_list;
@@ -343,51 +336,61 @@ builtin_fn_t *get_builtin_fn(value_t v)
   return _get_builtin_fn(v);
 }
 
-/****************************************************************************/
+/*************************** Common Collector Code **************************/
+
+static bool gc_enabled;
+static bool gc_in_gen0_collection;
+static bool gc_in_gen1_collection;
+
+/* Also used from Gen-0 code to track new Gen-1 objects */
+static char      *gc_gen1_ranges[2];
+static size_t     gc_gen1_min_size;
+static size_t     gc_gen1_max_size;
+static size_t     gc_gen1_soft_limit;
+
+static int        gc_gen1_current_range;
+static char      *gc_gen1_free_ptr;
+static char      *gc_gen1_range_end;
+
+static size_t     gc_gen1_max_blocks;
+static uint32_t  *gc_gen1_dirty_bits;
+static char     **gc_gen1_block_starts;
+
+/* A convenient shorthand */
+#define gc_gen1_other_range() (1-gc_gen1_current_range)
 
 static inline size_t gc_align(size_t nbytes) __attribute__ ((const));
-static int gc_range_of(void *object) __attribute__ ((const,unused));
-static void transfer_object(value_t *value);
-static size_t transfer_children(object_t *object);
-static void _collect_garbage(size_t min_free);
+static void transfer_roots(void);
+static size_t transfer_children(object_t *obj);
+static void process_weak_boxes(void);
+static void process_wills(void);
+static void update_weak_box_list(void);
+
+static void gc_gen0_init(size_t gen0_size);
+static void gc_gen1_init(size_t min_size, size_t max_size);
+
+static int gen1_gc_range_of(void *object) __attribute__ ((const));
+static size_t gc_gen1_block_of(void *obj) __attribute__ ((const));
+static inline size_t gc_gen1_free_space(void);
+
+static void *gc_alloc_gen1(size_t nbytes);
+static void collect_gen1_garbage(size_t min_free);
+static void gc_gen1_clear_dirty_bits(void);
+
+static inline size_t gc_gen1_free_space(void)
+{
+  return gc_gen1_range_end - gc_gen1_free_ptr;
+}
 
 static inline size_t gc_align(size_t nbytes)
 {
   return ((nbytes + GC_ALIGNMENT - 1) & ~(GC_ALIGNMENT - 1));
 }
 
-static int gc_range_of(void *object)
-{
-  if (((value_t)object >= (value_t)gc_ranges[0]) &&
-      ((value_t)object <  (value_t)gc_ranges[1]))
-    return 0;
-  if (((value_t)object >= (value_t)gc_ranges[1]) &&
-      ((value_t)object <  (value_t)gc_ranges[2]))
-    return 1;
-  return -1;
-}
-
-static inline size_t gc_free_space(void)
-{
-  return gc_range_end - gc_free_ptr;
-}
-
 void gc_init(size_t min_size, size_t max_size)
 {
-  assert(min_size <= max_size);
-
-  gc_ranges[0] = (char*)malloc(max_size);
-  gc_ranges[1] = (char*)malloc(max_size);
-
-  assert(gc_ranges[0] && gc_ranges[1]);
-
-  gc_current_range = 0;
-  gc_free_ptr = gc_ranges[gc_current_range];
-
-  gc_min_size   = min_size;
-  gc_max_size   = max_size;
-  gc_soft_limit = gc_min_size;
-  gc_range_end  = gc_free_ptr + gc_soft_limit;
+  gc_gen0_init(min_size);
+  gc_gen1_init(min_size, max_size);
 
   gc_weak_box_list    = NIL;
   gc_will_list        = NIL;
@@ -400,46 +403,269 @@ void gc_init(size_t min_size, size_t max_size)
 
 void clear_gc_stats(void)
 {
-  gc_stats.collections = 0;
+  gc_stats.gen0_passes = 0;
+  gc_stats.gen1_passes = 0;
   gc_stats.total_ns    = 0;
   gc_stats.total_freed = 0;
   gc_stats.high_water  = 0;
   gc_stats.max_ns      = 0;
 }
 
-/* Preconditions: nbytes pre-aligned a la gc_align(), and space exists. */
-static inline void *_gc_alloc(size_t nbytes)
+/****************************** Gen-0 Collector *****************************/
+
+/* These private variables are exported ONLY for use by is_gen0_object(). */
+char *gc_gen0_range;
+size_t gc_gen0_size;
+
+static char *gc_gen0_free_ptr;
+static char *gc_gen0_range_end;
+
+/* Used to signal that Gen-0 pass has been obviated by Gen-1 collection. */
+static jmp_buf gc_gen0_end_ctx;
+
+static inline size_t gc_gen0_free_space(void)
 {
-  void *p = gc_free_ptr;
-  assert(nbytes == gc_align(nbytes));
-  assert(nbytes <= gc_free_space());
-  gc_free_ptr += nbytes;
-  return p;
+  return gc_gen0_range_end - gc_gen0_free_ptr;
+}
+
+static void gc_gen0_init(size_t gen0_size)
+{
+  assert(gen0_size >= GC_ALIGNMENT);
+
+  gc_gen0_size = gen0_size;
+  gc_gen0_range = (char*)malloc(gc_gen0_size);
+  release_assert(gc_gen0_range);
+
+  gc_gen0_free_ptr = gc_gen0_range;
+  gc_gen0_range_end = gc_gen0_range + gc_gen0_size;
+}
+
+static void collect_gen0_garbage(void)
+{
+#ifndef NO_STATS
+  size_t initial_free_space;
+#ifndef NO_TIMING_STATS
+  struct timespec start_time;
+  clock_gettime(TIMING_CLOCK, &start_time);
+#endif
+
+  initial_free_space = gc_gen0_free_space() + gc_gen1_free_space();
+#endif
+
+  //debug(("Performing Gen-0 garbage collection pass...\n"));
+
+  assert(!gc_in_gen0_collection);
+  assert(!gc_in_gen1_collection);
+
+  gc_in_gen0_collection = true;
+
+  /* If we trigger a Gen-1 collection at any point then we are done. */
+  /* Full collection will pull in any current Gen-0 objects. */
+  if (setjmp(gc_gen0_end_ctx) == 0)
+  {
+    char *object_ptr = gc_gen1_free_ptr;
+    const size_t used_bytes = gc_gen1_free_ptr - gc_gen1_ranges[gc_gen1_current_range];
+    const int current_block_groups = (used_bytes + GC_DIRTY_BLOCK_SIZE - 1) / GC_DIRTY_BLOCK_SIZE;
+    int group;
+    int bit;
+
+    /* 1. Transfer Gen-0 roots (ignore Gen-1). */
+    transfer_roots();
+
+    /* 2. Locate and transfer Gen-0 references from dirty Gen-1 blocks. */
+    for (group = 0; group < current_block_groups; ++group)
+    {
+      for (bit = 0; bit < 32; ++bit)
+      {
+        if (gc_gen1_dirty_bits[group] & (1UL << bit))
+        {
+          /* Find first object in block */
+          const int block = group * 32 + bit;
+          char *block_obj = gc_gen1_block_starts[block];
+
+          /* For each object in block: transfer children */
+          do {
+            block_obj += gc_align(transfer_children((object_t*)block_obj));
+          } while (gen1_gc_range_of(block_obj) == block);
+        }
+      }
+    }
+
+    /* Transfer Gen-0 children of objects newly moved to Gen-1 */
+    while (object_ptr < gc_gen1_free_ptr)
+    {
+      object_ptr += gc_align(transfer_children((object_t*)object_ptr));
+    }
+
+    /* These have to be examined after normal reachability has been determined */
+    process_weak_boxes();
+    process_wills();
+
+    /* Keep transferring until no more objects in the new range refer to the old one.
+     * This is so that values which are otherwise unreachable, but have finalizers which
+     * may be able to reach them, are not collected prematurely. process_wills() transfers
+     * the value of any will newly placed on the active list. Note that these values may
+     * be finalized in any order, and that any weak references have already been cleared. */
+    while (object_ptr < gc_gen1_free_ptr)
+    {
+      object_ptr += gc_align(transfer_children((object_t*)object_ptr));
+    }
+
+    update_weak_box_list();
+
+    /* 4. Reset Gen-0 range to 'empty' state. */
+    gc_gen1_clear_dirty_bits();
+    gc_gen0_free_ptr = gc_gen0_range;
+
+#ifndef NO_STATS
+#ifndef NO_TIMING_STATS
+    {
+      struct timespec end_time;
+      nsec_t nsec;
+
+      clock_gettime(TIMING_CLOCK, &end_time);
+
+      nsec  = (end_time.tv_sec - start_time.tv_sec) * 1000000000LL;
+      nsec += (end_time.tv_nsec - start_time.tv_nsec);
+
+      gc_stats.total_ns += nsec;
+
+      if (nsec > gc_stats.max_ns)
+        gc_stats.max_ns = nsec;
+    }
+#endif
+    gc_stats.total_freed -= initial_free_space;
+    gc_stats.total_freed += gc_gen0_free_space();
+    gc_stats.total_freed += gc_gen1_free_space();
+    ++gc_stats.gen0_passes;
+#endif
+  }
+  else
+  {
+    //debug(("Gen-0 pass was interrupted by Gen-1 (full) collection.\n"));
+  }
+
+#ifndef NDEBUG
+  /* Clear old range, to make it easier to detect bugs. */
+  memset(gc_gen0_range, 0, gc_gen0_size);
+#endif
+
+  gc_in_gen0_collection = false;
 }
 
 void *gc_alloc(size_t nbytes)
 {
   nbytes = gc_align(nbytes);
 
-  if (nbytes > gc_free_space())
-    _collect_garbage(nbytes);
+  assert(!gc_in_gen0_collection);
+  assert(!gc_in_gen1_collection);
 
-  return _gc_alloc(nbytes);
+  if (nbytes >= gc_gen0_size)
+  {
+    //debug(("Allocating directly from Gen-0...\n"));
+    return gc_alloc_gen1(nbytes);
+  }
+  else
+  {
+    if (nbytes > gc_gen0_free_space())
+      collect_gen0_garbage();
+
+    assert(nbytes <= gc_gen0_free_space());
+
+    {
+      void *const p = gc_gen0_free_ptr;
+      gc_gen0_free_ptr += nbytes;
+      return p;
+    }
+  }
+}
+
+/****************************** Gen-1 Collector *****************************/
+
+static int gen1_gc_range_of(void *object)
+{
+  if (((value_t)object >= (value_t)gc_gen1_ranges[0]) &&
+      ((value_t)object <  (value_t)(gc_gen1_ranges[0] + gc_gen1_max_size)))
+    return 0;
+  if (((value_t)object >= (value_t)gc_gen1_ranges[1]) &&
+      ((value_t)object <  (value_t)(gc_gen1_ranges[1] + gc_gen1_max_size)))
+    return 1;
+  return -1;
+}
+
+static void gc_gen1_init(size_t min_size, size_t max_size)
+{
+  release_assert(min_size <= ((max_size+1)/2));
+
+  gc_gen1_ranges[0] = (char*)malloc(max_size);
+  gc_gen1_ranges[1] = (char*)malloc(max_size);
+
+  release_assert(gc_gen1_ranges[0] && gc_gen1_ranges[1]);
+
+  gc_gen1_current_range = 0;
+  gc_gen1_free_ptr = gc_gen1_ranges[gc_gen1_current_range];
+
+  gc_gen1_min_size   = min_size;
+  gc_gen1_max_size   = max_size;
+  gc_gen1_soft_limit = 2*gc_gen1_min_size;
+  gc_gen1_range_end  = gc_gen1_free_ptr + gc_gen1_soft_limit;
+
+  {
+    gc_gen1_max_blocks = ((size_t)2 << 30) / GC_DIRTY_BLOCK_SIZE;
+    gc_gen1_dirty_bits = (uint32_t*)malloc(4 * ((gc_gen1_max_blocks + 31) / 32));
+    release_assert(gc_gen1_dirty_bits);
+    gc_gen1_block_starts = (char**)malloc(gc_gen1_max_blocks * sizeof(char*));
+    gc_gen1_clear_dirty_bits();
+    memset(gc_gen1_block_starts, 0, gc_gen1_max_blocks * sizeof(char*));
+  }
+}
+
+static void gc_gen1_clear_dirty_bits(void)
+{
+  memset(gc_gen1_dirty_bits, 0, 4 * ((gc_gen1_max_blocks + 31) / 32));
+}
+
+static void *gc_alloc_gen1(size_t nbytes)
+{
+  nbytes = gc_align(nbytes);
+
+  if ((nbytes + gc_gen0_size) > gc_gen1_free_space())
+    collect_gen1_garbage(nbytes);
+
+  assert(nbytes <= gc_gen1_free_space());
+
+  {
+    void *const p = gc_gen1_free_ptr;
+    const size_t block = gc_gen1_block_of(p);
+
+    if (!gc_gen1_block_starts[block])
+      gc_gen1_block_starts[block] = (char*)p;
+
+    gc_gen1_free_ptr += nbytes;
+    return p;
+  }
 }
 
-/* Precondition: *value refers to an object (or pair). */
 static void transfer_object(value_t *value)
 {
-  if (is_object(*value))
+  /* During Gen-0 collection pass, leave Gen-1 objects alone. Always ignore non-objects. */
+  if (is_object(*value) && (gc_in_gen1_collection || is_gen0_object(*value)))
   {
     object_t *obj = _get_object(*value);
     size_t nbytes;
     void *newobj;
 
-    assert(gc_range_of(obj) != gc_current_range);
+    assert(is_gen0_object(*value) ||
+           (gen1_gc_range_of(obj) == gc_gen1_other_range()));
 
     if (obj->tag == BROKEN_HEART)
     {
+      if (gen1_gc_range_of(_get_object(obj->forward)) != gc_gen1_current_range)
+      {
+        /* Gen-0 object was transferred into old range; needs to move to current range */
+        transfer_object(&obj->forward);
+      }
+
       /* Object has already been moved; just update the reference */
       *value = obj->forward;
       return;
@@ -476,7 +702,7 @@ static void transfer_object(value_t *value)
       break;
     }
 
-    newobj = _gc_alloc(gc_align(nbytes));
+    newobj = gc_alloc_gen1(nbytes);
     memcpy(newobj, obj, nbytes);
 
     /* Keep the original tag bits (pair or object) */
@@ -558,11 +784,11 @@ static size_t transfer_children(object_t *obj)
   }
 }
 
-static void swap_gc_ranges(void)
+static void swap_gen1_gc_ranges(void)
 {
-  gc_current_range = gc_other_range();
-  gc_free_ptr      = gc_ranges[gc_current_range];
-  gc_range_end     = gc_free_ptr + gc_soft_limit;
+  gc_gen1_current_range = gc_gen1_other_range();
+  gc_gen1_free_ptr      = gc_gen1_ranges[gc_gen1_current_range];
+  gc_gen1_range_end     = gc_gen1_free_ptr + gc_gen1_soft_limit;
 }
 
 static void transfer_roots(void)
@@ -599,7 +825,7 @@ static void process_weak_boxes(void)
     }
     else
     {
-      /* Box hasn't been moved yet, but may live on as the value of a will. */
+      /* Box hasn't been moved. Could be Gen-0 pass, or may live on as the value of a will. */
       assert(is_weak_box(wb));
       box = _get_weak_box(wb);
     }
@@ -609,15 +835,21 @@ static void process_weak_boxes(void)
       /* The value in the box is reachable; update w/ new location. */
       box->value = _get_object(box->value)->forward;
     }
-    else if (is_object(box->value))
+    else if (is_object(box->value) && (gc_in_gen1_collection || is_gen0_object(box->value)))
     {
       /* The value in the box is an unreachable object; change to #f. */
 
-      /* Note that an object is considered unreachable via weak box when it could be finalized,
-       * even though it will be kept alive until the finalizer(s) is/are removed from the 'active'
-       * list and the finalizer(s) itself/themselves may restore the object to a reachable state. */
-
-      /* This last behavior is not recommended. */
+      /*
+       * NOTE: An object is considered unreachable via weak box when it could be finalized, even
+       * though it will be kept alive until any finalizers are removed from the 'active' list.
+       * The finalizer(s) may restore the object to a reachable state, in which case it will not
+       * be collected--but the weak reference will remain broken.
+       *
+       * Restoring references to otherwise GC'able objects is not recommended.
+       *
+       * The only known alternative would have been to invoke the finalizer while other objects
+       * may still be able to access the object (and create new references) via the weak box.
+       */
 
       box->value = FALSE_VALUE;
     }
@@ -650,7 +882,7 @@ static void process_wills(void)
       /* Move on to this will's 'next' pointer */
       will = &w->next;
     }
-    else
+    else if (gc_in_gen1_collection || is_gen0_object(w->value))
     {
       assert(is_object(w->value));
 
@@ -679,6 +911,8 @@ static void update_weak_box_list(void)
   {
     if (is_broken_heart(*wb))
     {
+      assert(gen1_gc_range_of(_get_object(_get_object(*wb)->forward)) == gc_gen1_current_range);
+
       /* The box itself is reachable; need to update 'next' pointer to new location */
       *wb = _get_object(*wb)->forward;
 
@@ -699,36 +933,51 @@ static void update_weak_box_list(void)
 
 static void update_soft_limit(size_t min_free)
 {
-  size_t bytes_used = gc_free_ptr - gc_ranges[gc_current_range];
+  size_t bytes_used = gc_gen1_free_ptr - gc_gen1_ranges[gc_gen1_current_range];
   size_t min_limit = bytes_used + min_free;
-  size_t new_limit = (4 * min_limit) / 3;
+  size_t new_limit = 2 * min_limit;
 
-  if (gc_soft_limit > GC_DEFLATE_SIZE)
+  if (gc_gen1_soft_limit > GC_DEFLATE_SIZE)
   {
-    size_t deflate_limit = gc_soft_limit - GC_DEFLATE_SIZE;
+    size_t deflate_limit = gc_gen1_soft_limit - GC_DEFLATE_SIZE;
 
     if (new_limit < deflate_limit)
       new_limit = deflate_limit;
   }
 
-  if (new_limit > gc_max_size)
-    new_limit = gc_max_size;
-  else if (new_limit < gc_min_size)
-    new_limit = gc_min_size;
+  if (new_limit > gc_gen1_max_size)
+    new_limit = gc_gen1_max_size;
+  else if (new_limit < gc_gen1_min_size)
+    new_limit = gc_gen1_min_size;
 
-  gc_soft_limit = new_limit;
+  gc_gen1_soft_limit = new_limit;
 
   /* Update end of range to reflect new limit */
-  gc_range_end = gc_ranges[gc_current_range] + gc_soft_limit;
+  gc_gen1_range_end = gc_gen1_ranges[gc_gen1_current_range] + gc_gen1_soft_limit;
 
 #ifndef NO_STATS
-  if (gc_soft_limit > gc_stats.high_water)
+  if (gc_gen1_soft_limit > gc_stats.high_water)
   {
-    gc_stats.high_water = gc_soft_limit;
+    gc_stats.high_water = gc_gen1_soft_limit;
   }
 #endif
 }
 
+static size_t gc_gen1_block_of(void *obj)
+{
+  const intptr_t offset = (intptr_t)obj
+                        - (intptr_t)gc_gen1_ranges[gc_gen1_current_range];
+
+  return (offset & ((intptr_t)2 << 30)) / GC_DIRTY_BLOCK_SIZE;
+}
+
+void _gc_mark_updated_gen1_object(value_t v)
+{
+  const size_t block = gc_gen1_block_of(_get_object(v));
+  assert(is_object(v) && !is_gen0_object(v));
+  gc_gen1_dirty_bits[block / 32] |= (1UL << (block % 32));
+}
+
 static void _out_of_memory(void) __attribute__ ((noreturn));
 static void _out_of_memory(void)
 {
@@ -736,35 +985,41 @@ static void _out_of_memory(void)
   abort();
 }
 
-static void _collect_garbage(size_t min_free)
+static void collect_gen1_garbage(size_t min_free)
 {
+  bool collected_garbage = false;
+
+  gc_in_gen1_collection = true;
+
   if (gc_enabled)
   {
     char *object_ptr;
 
 #ifndef NO_STATS
+    size_t initial_free_space = gc_gen0_free_space() + gc_gen1_free_space();
 #ifndef NO_TIMING_STATS
     struct timespec start_time;
     clock_gettime(TIMING_CLOCK, &start_time);
 #endif
-
-    gc_stats.total_freed -= gc_free_space();
-    ++gc_stats.collections;
 #endif
 
-    //debug(("Collecting garbage...\n"));
+    //debug(("Performing Gen-1 garbage collection pass...\n"));
 
-    swap_gc_ranges();
+    gc_enabled = false;
+    swap_gen1_gc_ranges();
+
+    /* Record the start of each Gen-1 block as we go. */
+    memset(gc_gen1_block_starts, 0, gc_gen1_max_blocks * sizeof(char*));
 
     /* New "current" range is initially empty, old one is full */
-    object_ptr = gc_free_ptr;
+    object_ptr = gc_gen1_free_ptr;
 
     /* Prime the pump */
     transfer_roots();
 
     /* Keep transferring until no more objects in the new range refer to the old one,
      * other than pending wills and weak boxes. */
-    while (object_ptr < gc_free_ptr)
+    while (object_ptr < gc_gen1_free_ptr)
     {
       object_ptr += gc_align(transfer_children((object_t*)object_ptr));
     }
@@ -778,7 +1033,7 @@ static void _collect_garbage(size_t min_free)
      * may be able to reach them, are not collected prematurely. process_wills() transfers
      * the value of any will newly placed on the active list. Note that these values may
      * be finalized in any order, and that any weak references have already been cleared. */
-    while (object_ptr < gc_free_ptr)
+    while (object_ptr < gc_gen1_free_ptr)
     {
       object_ptr += gc_align(transfer_children((object_t*)object_ptr));
     }
@@ -787,10 +1042,21 @@ static void _collect_garbage(size_t min_free)
 
 #ifndef NDEBUG
     /* Clear old range, to make it easier to detect bugs. */
-    memset(gc_ranges[gc_other_range()], 0, gc_soft_limit);
+    memset(gc_gen1_ranges[gc_gen1_other_range()], 0, gc_gen1_soft_limit);
 #endif
+
+    /*
+     * Gen-0 should be empty at this point; all active objects
+     * have been moved to the Gen-1 memory region.
+     */
+
+    gc_gen1_clear_dirty_bits();
+    gc_gen0_free_ptr = gc_gen0_range;
+    collected_garbage = true;
     
-    //debug(("Finished collection with %d bytes to spare (out of %d bytes).\n", gc_free_space(), gc_soft_limit));
+    //debug(("Finished collection with %d bytes to spare (out of %d bytes).\n", gc_gen1_free_space(), gc_gen1_soft_limit));
+
+    gc_enabled = true;
 
 #ifndef NO_STATS
 #ifndef NO_TIMING_STATS
@@ -808,17 +1074,19 @@ static void _collect_garbage(size_t min_free)
       if (nsec > gc_stats.max_ns)
         gc_stats.max_ns = nsec;
     }
+#endif
+    gc_stats.total_freed -= initial_free_space;
+    gc_stats.total_freed += gc_gen0_free_space();
+    gc_stats.total_freed += gc_gen1_free_space();
+    ++gc_stats.gen1_passes;
+#endif
   }
-#endif
 
-  gc_stats.total_freed += gc_free_space();
-#endif
+  update_soft_limit(min_free + gc_gen0_size);
 
-  update_soft_limit(min_free);
-
-  if (gc_free_space() < min_free)
+  if (gc_gen1_free_space() < min_free)
   {
-    size_t bytes_used = gc_free_ptr - gc_ranges[gc_current_range];
+    size_t bytes_used = gc_gen1_free_ptr - gc_gen1_ranges[gc_gen1_current_range];
     size_t need_bytes = bytes_used + min_free;
 
     /* If GC is disabled then we can't move anything, so reallocating is impossible. */
@@ -832,39 +1100,45 @@ static void _collect_garbage(size_t min_free)
     debug(("Ran out of free memory; will try to allocate more...\n"));
 
     do {
-      release_assert(gc_max_size < (SIZE_MAX/2));
-      gc_max_size *= 2;
-    } while (gc_max_size < need_bytes);
+      release_assert(gc_gen1_max_size < (SIZE_MAX/2));
+      gc_gen1_max_size *= 2;
+    } while (gc_gen1_max_size < need_bytes);
 
     /* Reallocate the unused space. */
     {
-      char *unused_range = gc_ranges[gc_other_range()];
-      gc_ranges[gc_other_range()] = (char*)malloc(gc_max_size);
+      char *unused_range = gc_gen1_ranges[gc_gen1_other_range()];
+      gc_gen1_ranges[gc_gen1_other_range()] = (char*)malloc(gc_gen1_max_size);
       free(unused_range);
     }
 
     /* See if reallocation succeeded. */
-    if (!gc_ranges[gc_other_range()])
+    if (!gc_gen1_ranges[gc_gen1_other_range()])
       _out_of_memory();
 
     /* Move everything into the newly enlarged space.
      * Will update the soft-limit. */
-    _collect_garbage(0);
+    collect_gen1_garbage(0);
 
     /* Reallocate the other space, now unused. */
-    free(gc_ranges[gc_other_range()]);
-    gc_ranges[gc_other_range()] = (char*)malloc(gc_max_size);
+    free(gc_gen1_ranges[gc_gen1_other_range()]);
+    gc_gen1_ranges[gc_gen1_other_range()] = (char*)malloc(gc_gen1_max_size);
 
     /* Ensure second reallocation succeeded. */
-    if (!gc_ranges[gc_other_range()])
+    if (!gc_gen1_ranges[gc_gen1_other_range()])
       _out_of_memory();
   }
+
+  gc_in_gen1_collection = false;
+
+  /* If Gen-1 was invoked within Gen-0, skip the rest: we're done. */
+  if (gc_in_gen0_collection && collected_garbage)
+    longjmp(gc_gen0_end_ctx, 1);
 }
 
 void collect_garbage(size_t min_free)
 {
   bool was_enabled = set_gc_enabled(true);
-  _collect_garbage(min_free);
+  collect_gen1_garbage(min_free);
   set_gc_enabled(was_enabled);
 }
 
@@ -1075,22 +1349,23 @@ void fprint_value(FILE *f, value_t v)
 
 void fprint_gc_stats(FILE *f)
 {
-  if (gc_stats.collections > 0)
+  if ((gc_stats.gen0_passes + gc_stats.gen1_passes) > 0)
   {
     const double total_time = gc_stats.total_ns / 1.0e9;
     const double max_time = gc_stats.max_ns / 1.0e9;
 
-    fprintf(f, "GC: %lld bytes freed by %d GCs in %0.6f sec => %0.3f MB/sec.\n",
+    fprintf(f, "GC: %lld bytes freed by %d+%d GCs in %0.6f sec => %0.3f MB/sec.\n",
         gc_stats.total_freed,
-        gc_stats.collections,
+        gc_stats.gen0_passes,
+        gc_stats.gen1_passes,
         total_time,
         (gc_stats.total_freed / total_time) / (1024*1024));
 
     fprintf(f, "GC: Avg. time was %0.6f sec, max %0.6f.\n",
-        (total_time / gc_stats.collections), max_time);
+        (total_time / (gc_stats.gen0_passes + gc_stats.gen1_passes)), max_time);
 
     fprintf(f, "GC: The soft-limit peaked at %d bytes out of %d allocated.\n",
-        gc_stats.high_water, gc_max_size);
+        gc_stats.high_water, gc_gen1_max_size);
   }
   else
   {
diff --git a/gc.h b/gc.h
index 87750f1..309228a 100644
--- a/gc.h
+++ b/gc.h
@@ -82,7 +82,7 @@ typedef void (builtin_fn_t)(struct interp_state *state);
 /* Invoke this macro after creating any reference from a Gen-1 GC object to a Gen-0 object. */
 /* If unsure, invoke the macro; at most there will be a slight cost in performance. */
 /* Failing to invoke the macro before the next Gen-0 GC can lead to incorrect behavior. */
-#define WRITE_BARRIER(gen1_value) ((void)0)
+#define WRITE_BARRIER(value) ((void)_gc_write_barrier((value)))
 
 typedef struct object
 {
@@ -166,7 +166,8 @@ typedef unsigned long long llsize_t;
 
 typedef struct gc_stats
 {
-  int      collections;
+  int      gen0_passes;
+  int      gen1_passes;
   nsec_t   total_ns;
   llsize_t total_freed;
   size_t   high_water;
@@ -392,6 +393,7 @@ void unregister_gc_root(gc_root_t *root);
 void *gc_alloc(size_t nbytes);
 void collect_garbage(size_t min_free);
 bool set_gc_enabled(bool enable);
+void _gc_mark_updated_gen1_object(value_t v);
 
 void fprint_value(FILE *f, value_t v);
 void fprint_gc_stats(FILE *f);
@@ -406,6 +408,28 @@ static inline void print_gc_stats(void)
   fprint_gc_stats(stderr);
 }
 
+static inline bool is_gen0_object(value_t v)
+{
+  /* These private variables are exported ONLY for use by this inline function. */
+  extern char *gc_gen0_range;
+  extern size_t gc_gen0_size;
+
+  const char const *obj = (const char*)_get_object(v);
+  return is_object(v)
+      && (obj >= (char*)gc_gen0_range)
+      && ((obj - (char*)gc_gen0_range) < gc_gen0_size);
+}
+
+/* Don't call this directly; use the WRITE_BARRIER macro. */
+static inline void _gc_write_barrier(value_t v)
+{
+  if (is_object(v) && !is_gen0_object(v))
+  {
+    _gc_mark_updated_gen1_object(v);
+  }
+}
+
+/* Implements the release_assert() macro */
 void _release_assert(bool expr, const char *str, const char *file, int line);
 
 /* To be provided by the main application */
diff --git a/rosella.c b/rosella.c
index e42296b..401ccfd 100644
--- a/rosella.c
+++ b/rosella.c
@@ -34,8 +34,20 @@ void out_of_memory(void)
 int main(int argc, char **argv)
 {
   srand((unsigned int)time(NULL));
+#ifdef __linux__
+  {
+    FILE *f = fopen("/dev/urandom", "rb");
+    if (f)
+    {
+      unsigned int seed;
+      if (fread(&seed, sizeof(seed), 1, f)==1)
+        srand(seed);
+      fclose(f);
+    }
+  }
+#endif
 
-  gc_init(256*1024, 1024*1024);
+  gc_init(12*1024*1024, 64*1024*1024);
   builtin_init();
   interpreter_init();
 
@@ -142,9 +154,17 @@ static void test_weak_boxes_and_wills(void)
 static void test_garbage_collection(bool keep_going)
 {
   gc_root_t root;
+  gc_root_t root2;
   int count = 0;
 
   register_gc_root(&root, NIL);
+  register_gc_root(&root2, NIL);
+
+  /* Construct a large, static tree w/ many links. */
+  for (int i = 0; i < 1000000; ++i)
+  {
+    root2.value = cons(root2.value, root2.value);
+  }
 
   while (1)
   {
@@ -156,7 +176,7 @@ static void test_garbage_collection(bool keep_going)
     }
     else
     {
-      switch (r & 7)
+      switch (r & 15)
       {
       case 0:
         root.value = cons(fixnum_value(rand()), root.value);
@@ -173,19 +193,19 @@ static void test_garbage_collection(bool keep_going)
         WRITE_BARRIER(_CDR(root.value));
         break;
       case 4:
-      case 5:
-      case 6:
-      case 7:
         {
           value_t s = make_vector(4, FALSE_VALUE);
           _get_vector(s)->elements[r & 3] = root.value;
           root.value = s;
         }
         break;
+      default:
+        (void)cons(make_box(NIL), cons(NIL, cons(NIL, NIL)));
+        break;
       }
     }
 
-    if (++count >= 50000000)
+    if (++count >= 80000000)
     {
       print_gc_stats();
       nl();
@@ -199,6 +219,7 @@ static void test_garbage_collection(bool keep_going)
   }
 
   unregister_gc_root(&root);
+  unregister_gc_root(&root2);
 }
 
 static void test_reader(void)