diff --git a/vm/debug.cpp b/vm/debug.cpp
index da8c603254..4cb4444d41 100755
--- a/vm/debug.cpp
+++ b/vm/debug.cpp
@@ -296,7 +296,7 @@ void factor_vm::dump_code_heap()
 		const char *status;
 		if(scan->type() == FREE_BLOCK_TYPE)
 			status = "free";
-		else if(scan->marked_p())
+		else if(code->state->is_marked_p(scan))
 		{
 			reloc_size += object_size(((code_block *)scan)->relocation);
 			literal_size += object_size(((code_block *)scan)->literals);
diff --git a/vm/full_collector.cpp b/vm/full_collector.cpp
index 86f3216e9c..59f87bb5a5 100644
--- a/vm/full_collector.cpp
+++ b/vm/full_collector.cpp
@@ -134,6 +134,8 @@ void factor_vm::collect_full_impl(bool trace_contexts_p)
 {
 	full_collector collector(this);
 
+	code->state->clear_mark_bits();
+
 	collector.trace_roots();
         if(trace_contexts_p)
 	{
@@ -148,16 +150,6 @@ void factor_vm::collect_full_impl(bool trace_contexts_p)
 	nursery.here = nursery.start;
 }
 
-/* In both cases, compact code heap before updating code blocks so that
-XTs are correct after */
-
-void factor_vm::big_code_heap_update()
-{
-	big_code_heap_updater updater(this);
-	code->free_unmarked(updater);
-	code->clear_remembered_set();
-}
-
 void factor_vm::collect_growing_heap(cell requested_bytes,
 	bool trace_contexts_p,
 	bool compact_code_heap_p)
@@ -168,15 +160,18 @@ void factor_vm::collect_growing_heap(cell requested_bytes,
 	collect_full_impl(trace_contexts_p);
 	delete old;
 
-	if(compact_code_heap_p) compact_code_heap(trace_contexts_p);
+	if(compact_code_heap_p)
+	{
+		compact_code_heap(trace_contexts_p);
+		big_code_heap_updater updater(this);
+		iterate_code_heap(updater);
+	}
+	else
+	{
+		big_code_heap_updater updater(this);
+		code->free_unmarked(updater);
+	}
 
-	big_code_heap_update();
-}
-
-void factor_vm::small_code_heap_update()
-{
-	small_code_heap_updater updater(this);
-	code->free_unmarked(updater);
 	code->clear_remembered_set();
 }
 
@@ -190,10 +185,16 @@ void factor_vm::collect_full(bool trace_contexts_p, bool compact_code_heap_p)
 	if(compact_code_heap_p)
 	{
 		compact_code_heap(trace_contexts_p);
-		big_code_heap_update();
+		big_code_heap_updater updater(this);
+		iterate_code_heap(updater);
 	}
 	else
-		small_code_heap_update();
+	{
+		small_code_heap_updater updater(this);
+		code->free_unmarked(updater);
+	}
+
+	code->clear_remembered_set();
 }
 
 }
diff --git a/vm/gc.cpp b/vm/gc.cpp
index c89add6066..95a6857db6 100755
--- a/vm/gc.cpp
+++ b/vm/gc.cpp
@@ -54,9 +54,6 @@ void factor_vm::gc(gc_op op,
 			current_gc->op = collect_full_op;
 			break;
 		case collect_full_op:
-			/* Since we start tracing again, any previously
-			marked code blocks must be re-marked and re-traced */
-			code->clear_mark_bits();
 			current_gc->op = collect_growing_heap_op;
 			break;
 		default:
diff --git a/vm/heap.cpp b/vm/heap.cpp
index 0f0da63df0..71aac62704 100644
--- a/vm/heap.cpp
+++ b/vm/heap.cpp
@@ -16,9 +16,18 @@ heap::heap(bool secure_gc_, cell size, bool executable_p) : secure_gc(secure_gc_
 	if(size > (1L << (sizeof(cell) * 8 - 6))) fatal_error("Heap too large",size);
 	seg = new segment(align_page(size),executable_p);
 	if(!seg) fatal_error("Out of memory in heap allocator",size);
+	state = new mark_bits<heap_block,block_size_increment>(seg->start,size);
 	clear_free_list();
 }
 
+heap::~heap()
+{
+	delete seg;
+	seg = NULL;
+	delete state;
+	state = NULL;
+}
+
 void heap::add_to_free_list(free_heap_block *block)
 {
 	if(block->size() < free_list_count * block_size_increment)
@@ -34,52 +43,15 @@ void heap::add_to_free_list(free_heap_block *block)
 	}
 }
 
-/* Called after reading the code heap from the image file, and after code GC.
-
-In the former case, we must add a large free block from compiling.base + size to
-compiling.limit. */
+/* Called after reading the code heap from the image file, and after code heap
+compaction. Makes a free list consisting of one free block, at the very end. */
 void heap::build_free_list(cell size)
 {
-	heap_block *prev = NULL;
-
 	clear_free_list();
-
-	size = (size + block_size_increment - 1) & ~(block_size_increment - 1);
-
-	heap_block *scan = first_block();
 	free_heap_block *end = (free_heap_block *)(seg->start + size);
-
-	/* Add all free blocks to the free list */
-	while(scan && scan < (heap_block *)end)
-	{
-		if(scan->type() == FREE_BLOCK_TYPE)
-			add_to_free_list((free_heap_block *)scan);
-
-		prev = scan;
-		scan = next_block(scan);
-	}
-
-	/* If there is room at the end of the heap, add a free block. This
-	branch is only taken after loading a new image, not after code GC */
-	if((cell)(end + 1) <= seg->end)
-	{
-		end->set_marked_p(false);
-		end->set_type(FREE_BLOCK_TYPE);
-		end->set_size(seg->end - (cell)end);
-
-		/* add final free block */
-		add_to_free_list(end);
-	}
-	/* This branch is taken if the newly loaded image fits exactly, or
-	after code GC */
-	else
-	{
-		/* even if there's no room at the end of the heap for a new
-		free block, we might have to jigger it up by a few bytes in
-		case prev + prev->size */
-		if(prev) prev->set_size(seg->end - (cell)prev);
-	}
-
+	end->set_type(FREE_BLOCK_TYPE);
+	end->set_size(seg->end - (cell)end);
+	add_to_free_list(end);
 }
 
 void heap::assert_free_block(free_heap_block *block)
@@ -154,7 +126,6 @@ heap_block *heap::heap_allot(cell size, cell type)
 	{
 		block = split_free_block(block,size);
 		block->set_type(type);
-		block->set_marked_p(false);
 		return block;
 	}
 	else
@@ -170,18 +141,7 @@ void heap::heap_free(heap_block *block)
 
 void heap::mark_block(heap_block *block)
 {
-	block->set_marked_p(true);
-}
-
-void heap::clear_mark_bits()
-{
-	heap_block *scan = first_block();
-
-	while(scan)
-	{
-		scan->set_marked_p(false);
-		scan = next_block(scan);
-	}
+	state->set_marked_p(block,true);
 }
 
 /* Compute total sum of sizes of free blocks, and size of largest free block */
@@ -210,20 +170,21 @@ void heap::heap_usage(cell *used, cell *total_free, cell *max_free)
 	}
 }
 
-/* The size of the heap, not including the last block if it's free */
+/* The size of the heap after compaction */
 cell heap::heap_size()
 {
 	heap_block *scan = first_block();
+	
+	while(scan)
+	{
+		if(scan->type() == FREE_BLOCK_TYPE) break;
+		else scan = next_block(scan);
+	}
 
-	while(next_block(scan) != NULL)
-		scan = next_block(scan);
+	assert(scan->type() == FREE_BLOCK_TYPE);
+	assert((cell)scan + scan->size() == seg->end);
 
-	/* this is the last block in the heap, and it is free */
-	if(scan->type() == FREE_BLOCK_TYPE)
-		return (cell)scan - seg->start;
-	/* otherwise the last block is allocated */
-	else
-		return seg->size;
+	return (cell)scan - (cell)first_block();
 }
 
 void heap::compact_heap()
@@ -238,7 +199,7 @@ void heap::compact_heap()
 	{
 		heap_block *next = next_block(scan);
  
-		if(scan->type() != FREE_BLOCK_TYPE && scan->marked_p())
+		if(state->is_marked_p(scan))
 		{
 			cell size = scan->size();
 			memmove(address,scan,size);
diff --git a/vm/heap.hpp b/vm/heap.hpp
index ef09c2b238..bc8851bab1 100644
--- a/vm/heap.hpp
+++ b/vm/heap.hpp
@@ -13,9 +13,11 @@ struct heap {
 	bool secure_gc;
 	segment *seg;
 	heap_free_list free;
+	mark_bits<heap_block,block_size_increment> *state;
 	unordered_map<heap_block *, char *> forwarding;
 
 	explicit heap(bool secure_gc_, cell size, bool executable_p);
+	~heap();
 
 	inline heap_block *next_block(heap_block *block)
 	{
@@ -46,7 +48,6 @@ struct heap {
 	heap_block *heap_allot(cell size, cell type);
 	void heap_free(heap_block *block);
 	void mark_block(heap_block *block);
-	void clear_mark_bits();
 	void heap_usage(cell *used, cell *total_free, cell *max_free);
 	cell heap_size();
 	void compact_heap();
@@ -71,11 +72,10 @@ struct heap {
 				else
 					prev = scan;
 			}
-			else if(scan->marked_p())
+			else if(state->is_marked_p(scan))
 			{
 				if(prev && prev->type() == FREE_BLOCK_TYPE)
 					add_to_free_list((free_heap_block *)prev);
-				scan->set_marked_p(false);
 				prev = scan;
 				iter(scan);
 			}
diff --git a/vm/image.cpp b/vm/image.cpp
index 05e0d66724..527ed3901b 100755
--- a/vm/image.cpp
+++ b/vm/image.cpp
@@ -67,86 +67,6 @@ void factor_vm::load_code_heap(FILE *file, image_header *h, vm_parameters *p)
 	code->build_free_list(h->code_size);
 }
 
-/* Save the current image to disk */
-bool factor_vm::save_image(const vm_char *filename)
-{
-	FILE* file;
-	image_header h;
-
-	file = OPEN_WRITE(filename);
-	if(file == NULL)
-	{
-		print_string("Cannot open image file: "); print_native_string(filename); nl();
-		print_string(strerror(errno)); nl();
-		return false;
-	}
-
-	h.magic = image_magic;
-	h.version = image_version;
-	h.data_relocation_base = data->tenured->start;
-	h.data_size = data->tenured->here - data->tenured->start;
-	h.code_relocation_base = code->seg->start;
-	h.code_size = code->heap_size();
-
-	h.t = T;
-	h.bignum_zero = bignum_zero;
-	h.bignum_pos_one = bignum_pos_one;
-	h.bignum_neg_one = bignum_neg_one;
-
-	for(cell i = 0; i < USER_ENV; i++)
-		h.userenv[i] = (save_env_p(i) ? userenv[i] : F);
-
-	bool ok = true;
-
-	if(fwrite(&h,sizeof(image_header),1,file) != 1) ok = false;
-	if(fwrite((void*)data->tenured->start,h.data_size,1,file) != 1) ok = false;
-	if(fwrite(code->first_block(),h.code_size,1,file) != 1) ok = false;
-	if(fclose(file)) ok = false;
-
-	if(!ok)
-	{
-		print_string("save-image failed: "); print_string(strerror(errno)); nl();
-	}
-
-	return ok;
-}
-
-void factor_vm::primitive_save_image()
-{
-	/* do a full GC to push everything into tenured space */
-	primitive_compact_gc();
-
-	gc_root<byte_array> path(dpop(),this);
-	path.untag_check(this);
-	save_image((vm_char *)(path.untagged() + 1));
-}
-
-void factor_vm::primitive_save_image_and_exit()
-{
-	/* We unbox this before doing anything else. This is the only point
-	where we might throw an error, so we have to throw an error here since
-	later steps destroy the current image. */
-	gc_root<byte_array> path(dpop(),this);
-	path.untag_check(this);
-
-	/* strip out userenv data which is set on startup anyway */
-	for(cell i = 0; i < USER_ENV; i++)
-	{
-		if(!save_env_p(i)) userenv[i] = F;
-	}
-
-	gc(collect_full_op,
-		0, /* requested size */
-		false, /* discard objects only reachable from stacks */
-		true /* compact the code heap */);
-
-	/* Save the image */
-	if(save_image((vm_char *)(path.untagged() + 1)))
-		exit(0);
-	else
-		exit(1);
-}
-
 void factor_vm::data_fixup(cell *handle, cell data_relocation_base)
 {
 	if(immediate_p(*handle))
@@ -353,4 +273,84 @@ void factor_vm::load_image(vm_parameters *p)
 	userenv[IMAGE_ENV] = allot_alien(F,(cell)p->image_path);
 }
 
+/* Save the current image to disk */
+bool factor_vm::save_image(const vm_char *filename)
+{
+	FILE* file;
+	image_header h;
+
+	file = OPEN_WRITE(filename);
+	if(file == NULL)
+	{
+		print_string("Cannot open image file: "); print_native_string(filename); nl();
+		print_string(strerror(errno)); nl();
+		return false;
+	}
+
+	h.magic = image_magic;
+	h.version = image_version;
+	h.data_relocation_base = data->tenured->start;
+	h.data_size = data->tenured->here - data->tenured->start;
+	h.code_relocation_base = code->seg->start;
+	h.code_size = code->heap_size();
+
+	h.t = T;
+	h.bignum_zero = bignum_zero;
+	h.bignum_pos_one = bignum_pos_one;
+	h.bignum_neg_one = bignum_neg_one;
+
+	for(cell i = 0; i < USER_ENV; i++)
+		h.userenv[i] = (save_env_p(i) ? userenv[i] : F);
+
+	bool ok = true;
+
+	if(fwrite(&h,sizeof(image_header),1,file) != 1) ok = false;
+	if(fwrite((void*)data->tenured->start,h.data_size,1,file) != 1) ok = false;
+	if(fwrite(code->first_block(),h.code_size,1,file) != 1) ok = false;
+	if(fclose(file)) ok = false;
+
+	if(!ok)
+	{
+		print_string("save-image failed: "); print_string(strerror(errno)); nl();
+	}
+
+	return ok;
+}
+
+void factor_vm::primitive_save_image()
+{
+	/* do a full GC to push everything into tenured space */
+	primitive_compact_gc();
+
+	gc_root<byte_array> path(dpop(),this);
+	path.untag_check(this);
+	save_image((vm_char *)(path.untagged() + 1));
+}
+
+void factor_vm::primitive_save_image_and_exit()
+{
+	/* We unbox this before doing anything else. This is the only point
+	where we might throw an error, so we have to throw an error here since
+	later steps destroy the current image. */
+	gc_root<byte_array> path(dpop(),this);
+	path.untag_check(this);
+
+	/* strip out userenv data which is set on startup anyway */
+	for(cell i = 0; i < USER_ENV; i++)
+	{
+		if(!save_env_p(i)) userenv[i] = F;
+	}
+
+	gc(collect_full_op,
+		0, /* requested size */
+		false, /* discard objects only reachable from stacks */
+		true /* compact the code heap */);
+
+	/* Save the image */
+	if(save_image((vm_char *)(path.untagged() + 1)))
+		exit(0);
+	else
+		exit(1);
+}
+
 }
diff --git a/vm/layouts.hpp b/vm/layouts.hpp
index 2fba97d747..72b37249fe 100644
--- a/vm/layouts.hpp
+++ b/vm/layouts.hpp
@@ -201,15 +201,6 @@ struct heap_block
 {
 	cell header;
 
-	bool marked_p() { return header & 1; }
-	void set_marked_p(bool marked)
-	{
-		if(marked)
-			header |= 1;
-		else
-			header &= ~1;
-	}
-
 	cell type() { return (header >> 1) & 0x1f; }
 	void set_type(cell type)
 	{
diff --git a/vm/mark_bits.hpp b/vm/mark_bits.hpp
new file mode 100644
index 0000000000..7945be1850
--- /dev/null
+++ b/vm/mark_bits.hpp
@@ -0,0 +1,103 @@
+namespace factor
+{
+
+const int forwarding_granularity = 128;
+
+template<typename Block, int Granularity> struct mark_bits {
+	cell start;
+	cell size;
+	cell bits_size;
+	unsigned int *marked;
+	unsigned int *freed;
+	cell forwarding_size;
+	cell *forwarding;
+
+	void clear_mark_bits()
+	{
+		memset(marked,0,bits_size * sizeof(unsigned int));
+	}
+
+	void clear_free_bits()
+	{
+		memset(freed,0,bits_size * sizeof(unsigned int));
+	}
+
+	void clear_forwarding()
+	{
+		memset(forwarding,0,forwarding_size * sizeof(cell));
+	}
+
+	explicit mark_bits(cell start_, cell size_) :
+		start(start_),
+		size(size_),
+		bits_size(size / Granularity / 32),
+		marked(new unsigned int[bits_size]),
+		freed(new unsigned int[bits_size]),
+		forwarding_size(size / Granularity / forwarding_granularity),
+		forwarding(new cell[forwarding_size])
+	{
+		clear_mark_bits();
+		clear_free_bits();
+		clear_forwarding();
+	}
+
+	~mark_bits()
+	{
+		delete[] marked;
+		marked = NULL;
+		delete[] freed;
+		freed = NULL;
+		delete[] forwarding;
+		forwarding = NULL;
+	}
+
+	std::pair<cell,cell> bitmap_deref(Block *address)
+	{
+		cell word_number = (((cell)address - start) / Granularity);
+		cell word_index = (word_number >> 5);
+		cell word_shift = (word_number & 31);
+
+#ifdef FACTOR_DEBUG
+		assert(word_index < bits_size);
+#endif
+
+		return std::make_pair(word_index,word_shift);
+	}
+
+	bool bitmap_elt(unsigned int *bits, Block *address)
+	{
+		std::pair<cell,cell> pair = bitmap_deref(address);
+		return (bits[pair.first] & (1 << pair.second)) != 0;
+	}
+
+	void set_bitmap_elt(unsigned int *bits, Block *address, bool flag)
+	{
+		std::pair<cell,cell> pair = bitmap_deref(address);
+		if(flag)
+			bits[pair.first] |= (1 << pair.second);
+		else
+			bits[pair.first] &= ~(1 << pair.second);
+	}
+
+	bool is_marked_p(Block *address)
+	{
+		return bitmap_elt(marked,address);
+	}
+
+	void set_marked_p(Block *address, bool marked_p)
+	{
+		set_bitmap_elt(marked,address,marked_p);
+	}
+
+	bool is_free_p(Block *address)
+	{
+		return bitmap_elt(freed,address);
+	}
+
+	void set_free_p(Block *address, bool free_p)
+	{
+		set_bitmap_elt(freed,address,free_p);
+	}
+};
+
+}
diff --git a/vm/master.hpp b/vm/master.hpp
index c5aed5e983..847980fac6 100755
--- a/vm/master.hpp
+++ b/vm/master.hpp
@@ -78,6 +78,7 @@ namespace factor
 #include "words.hpp"
 #include "float_bits.hpp"
 #include "io.hpp"
+#include "mark_bits.hpp"
 #include "heap.hpp"
 #include "image.hpp"
 #include "alien.hpp"
diff --git a/vm/vm.hpp b/vm/vm.hpp
index c3d71dc6f2..5b949287b6 100755
--- a/vm/vm.hpp
+++ b/vm/vm.hpp
@@ -253,8 +253,6 @@ struct factor_vm
 	void collect_nursery();
 	void collect_aging();
 	void collect_to_tenured();
-	void big_code_heap_update();
-	void small_code_heap_update();
 	void collect_full_impl(bool trace_contexts_p);
 	void collect_growing_heap(cell requested_bytes, bool trace_contexts_p, bool compact_code_heap_p);
 	void collect_full(bool trace_contexts_p, bool compact_code_heap_p);