diff --git a/vm/allot.hpp b/vm/allot.hpp
index 2c2c58c278..9a7c898efa 100644
--- a/vm/allot.hpp
+++ b/vm/allot.hpp
@@ -7,6 +7,10 @@ namespace factor
  */
 inline object *factor_vm::allot_object(cell type, cell size)
 {
+#ifdef FACTOR_DEBUG
+	assert(!current_gc);
+#endif
+
 	/* If the object is smaller than the nursery, allocate it in the nursery,
 	after a GC if needed */
 	if(nursery.size > size)
diff --git a/vm/booleans.cpp b/vm/booleans.cpp
index a7871dcbcb..34955cec8c 100644
--- a/vm/booleans.cpp
+++ b/vm/booleans.cpp
@@ -15,7 +15,7 @@ VM_C_API void box_boolean(bool value, factor_vm *parent)
 
 VM_C_API bool to_boolean(cell value, factor_vm *parent)
 {
-	return parent->to_boolean(value);
+	return to_boolean(value);
 }
 
 }
diff --git a/vm/booleans.hpp b/vm/booleans.hpp
index 375c8e3756..5654f445ad 100644
--- a/vm/booleans.hpp
+++ b/vm/booleans.hpp
@@ -4,12 +4,7 @@ namespace factor
 VM_C_API void box_boolean(bool value, factor_vm *vm);
 VM_C_API bool to_boolean(cell value, factor_vm *vm);
 
-inline cell factor_vm::tag_boolean(cell untagged)
-{
-	return (untagged ? true_object : false_object);
-}
-
-inline bool factor_vm::to_boolean(cell value)
+inline static bool to_boolean(cell value)
 {
 	return value != false_object;
 }
diff --git a/vm/callbacks.cpp b/vm/callbacks.cpp
index 121913eeea..36efbad450 100644
--- a/vm/callbacks.cpp
+++ b/vm/callbacks.cpp
@@ -19,35 +19,44 @@ void factor_vm::init_callbacks(cell size)
 	callbacks = new callback_heap(size,this);
 }
 
-void callback_heap::update(callback *stub)
+void callback_heap::update(code_block *stub)
 {
 	tagged<array> code_template(parent->special_objects[CALLBACK_STUB]);
 
 	cell rel_class = untag_fixnum(array_nth(code_template.untagged(),1));
+	cell rel_type = untag_fixnum(array_nth(code_template.untagged(),2));
 	cell offset = untag_fixnum(array_nth(code_template.untagged(),3));
 
-	instruction_operand op(rel_class,offset + (cell)(stub + 1));
-	op.store_value((cell)(stub->compiled + 1));
+	relocation_entry rel(
+		(relocation_type)rel_type,
+		(relocation_class)rel_class,
+		offset);
 
-	flush_icache((cell)stub,stub->size);
+	instruction_operand op(rel,stub,0);
+	op.store_value((cell)callback_xt(stub));
+
+	stub->flush_icache();
 }
 
-callback *callback_heap::add(code_block *compiled)
+code_block *callback_heap::add(cell owner)
 {
 	tagged<array> code_template(parent->special_objects[CALLBACK_STUB]);
 	tagged<byte_array> insns(array_nth(code_template.untagged(),0));
 	cell size = array_capacity(insns.untagged());
 
-	cell bump = align(size,sizeof(cell)) + sizeof(callback);
+	cell bump = align(size + sizeof(code_block),data_alignment);
 	if(here + bump > seg->end) fatal_error("Out of callback space",0);
 
-	callback *stub = (callback *)here;
-	stub->compiled = compiled;
-	memcpy(stub + 1,insns->data<void>(),size);
-
-	stub->size = align(size,sizeof(cell));
+	free_heap_block *free_block = (free_heap_block *)here;
+	free_block->make_free(bump);
 	here += bump;
 
+	code_block *stub = (code_block *)free_block;
+	stub->owner = owner;
+	stub->literals = false_object;
+	stub->relocation = false_object;
+
+	memcpy(stub->xt(),insns->data<void>(),size);
 	update(stub);
 
 	return stub;
@@ -57,9 +66,7 @@ void factor_vm::primitive_callback()
 {
 	tagged<word> w(dpop());
 	w.untag_check(this);
-
-	callback *stub = callbacks->add(w->code);
-	box_alien(stub + 1);
+	box_alien(callbacks->add(w.value())->xt());
 }
 
 }
diff --git a/vm/callbacks.hpp b/vm/callbacks.hpp
index c499ad4719..c5900f4618 100644
--- a/vm/callbacks.hpp
+++ b/vm/callbacks.hpp
@@ -1,11 +1,28 @@
 namespace factor
 {
 
-struct callback {
-	cell size;
-	code_block *compiled;
-	void *code() { return (void *)(this + 1); }
-};
+/* The callback heap is used to store the machine code that alien-callbacks
+actually jump to when C code invokes them.
+
+The callback heap has entries that look like code_blocks from the code heap,
+but callback heap entries are allocated contiguously, never deallocated, and all
+fields but the owner are set to false_object. The owner points to the callback
+bottom word, whose XT is the callback body itself, generated by the optimizing
+compiler. The machine code that follows a callback stub consists of a single
+CALLBACK_STUB machine code template, which performs a jump to a "far" address
+(on PowerPC and x86-64, its loaded into a register first).
+
+GC updates the CALLBACK_STUB code if the code block of the callback bottom word
+is ever moved. The callback stub itself won't move, though, and is never
+deallocated. This means that the callback stub itself is a stable function
+pointer that C code can hold on to until the associated Factor VM exits.
+
+Since callback stubs are GC roots, and are never deallocated, the associated
+callback code in the code heap is also never deallocated.
+
+The callback heap is not saved in the image. Running GC in a new session after
+saving the image will deallocate any code heap entries that were only reachable
+from the callback heap in the previous session when the image was saved. */
 
 struct callback_heap {
 	segment *seg;
@@ -15,18 +32,24 @@ struct callback_heap {
 	explicit callback_heap(cell size, factor_vm *parent);
 	~callback_heap();
 
-	callback *add(code_block *compiled);
-	void update(callback *stub);
-
-	callback *next(callback *stub)
+	void *callback_xt(code_block *stub)
 	{
-		return (callback *)((cell)stub + stub->size + sizeof(callback));
+		word *w = (word *)UNTAG(stub->owner);
+		return w->xt;
 	}
 
-	template<typename Iterator> void iterate(Iterator &iter)
+	void update(code_block *stub);
+	code_block *add(cell owner);
+
+	code_block *next(code_block *stub)
 	{
-		callback *scan = (callback *)seg->start;
-		callback *end = (callback *)here;
+		return (code_block *)((cell)stub + stub->size());
+	}
+
+	template<typename Iterator> void each_callback(Iterator &iter)
+	{
+		code_block *scan = (code_block *)seg->start;
+		code_block *end = (code_block *)here;
 		while(scan < end)
 		{
 			iter(scan);
diff --git a/vm/code_block_visitor.hpp b/vm/code_block_visitor.hpp
index dbc3018491..17ade71d2d 100644
--- a/vm/code_block_visitor.hpp
+++ b/vm/code_block_visitor.hpp
@@ -1,6 +1,17 @@
 namespace factor
 {
 
+/* Code block visitors iterate over sets of code blocks, applying a functor to
+each one. The functor returns a new code_block pointer, which may or may not
+equal the old one. This is stored back to the original location.
+
+This is used by GC's sweep and compact phases, and the implementation of the
+modify-code-heap primitive.
+
+Iteration is driven by visit_*() methods. Some of them define GC roots:
+- visit_context_code_blocks()
+- visit_callback_code_blocks() */
+ 
 template<typename Visitor> struct code_block_visitor {
 	factor_vm *parent;
 	Visitor visitor;
@@ -12,7 +23,6 @@ template<typename Visitor> struct code_block_visitor {
 	void visit_object_code_block(object *obj);
 	void visit_embedded_code_pointers(code_block *compiled);
 	void visit_context_code_blocks();
-	void visit_callback_code_blocks();
 };
 
 template<typename Visitor>
@@ -81,14 +91,11 @@ struct embedded_code_pointers_visitor {
 
 	explicit embedded_code_pointers_visitor(Visitor visitor_) : visitor(visitor_) {}
 
-	void operator()(relocation_entry rel, cell index, code_block *compiled)
+	void operator()(instruction_operand op)
 	{
-		relocation_type type = rel.rel_type();
+		relocation_type type = op.rel_type();
 		if(type == RT_XT || type == RT_XT_PIC || type == RT_XT_PIC_TAIL)
-		{
-			instruction_operand op(rel.rel_class(),rel.rel_offset() + (cell)compiled->xt());
 			op.store_code_block(visitor(op.load_code_block()));
-		}
 	}
 };
 
@@ -98,7 +105,7 @@ void code_block_visitor<Visitor>::visit_embedded_code_pointers(code_block *compi
 	if(!parent->code->needs_fixup_p(compiled))
 	{
 		embedded_code_pointers_visitor<Visitor> visitor(this->visitor);
-		parent->iterate_relocations(compiled,visitor);
+		compiled->each_instruction_operand(visitor);
 	}
 }
 
@@ -109,26 +116,4 @@ void code_block_visitor<Visitor>::visit_context_code_blocks()
 	parent->iterate_active_frames(call_frame_visitor);
 }
 
-template<typename Visitor>
-struct callback_code_block_visitor {
-	callback_heap *callbacks;
-	Visitor visitor;
-
-	explicit callback_code_block_visitor(callback_heap *callbacks_, Visitor visitor_) :
-		callbacks(callbacks_), visitor(visitor_) {}
-
-	void operator()(callback *stub)
-	{
-		stub->compiled = visitor(stub->compiled);
-		callbacks->update(stub);
-	}
-};
-
-template<typename Visitor>
-void code_block_visitor<Visitor>::visit_callback_code_blocks()
-{
-	callback_code_block_visitor<Visitor> callback_visitor(parent->callbacks,visitor);
-	parent->callbacks->iterate(callback_visitor);
-}
-
 }
diff --git a/vm/code_blocks.cpp b/vm/code_blocks.cpp
index 8ede7af654..d4f0f0a2f4 100755
--- a/vm/code_blocks.cpp
+++ b/vm/code_blocks.cpp
@@ -150,12 +150,9 @@ struct update_word_references_relocation_visitor {
 
 	explicit update_word_references_relocation_visitor(factor_vm *parent_) : parent(parent_) {}
 
-	void operator()(relocation_entry rel, cell index, code_block *compiled)
+	void operator()(instruction_operand op)
 	{
-		relocation_type type = rel.rel_type();
-		instruction_operand op(rel.rel_class(),rel.rel_offset() + (cell)compiled->xt());
-
-		switch(type)
+		switch(op.rel_type())
 		{
 		case RT_XT:
 			{
@@ -201,7 +198,7 @@ void factor_vm::update_word_references(code_block *compiled)
 	else
 	{
 		update_word_references_relocation_visitor visitor(this);
-		iterate_relocations(compiled,visitor);
+		compiled->each_instruction_operand(visitor);
 		compiled->flush_icache();
 	}
 }
@@ -218,13 +215,13 @@ struct relocate_code_block_relocation_visitor {
 
 	explicit relocate_code_block_relocation_visitor(factor_vm *parent_) : parent(parent_) {}
 
-	void operator()(relocation_entry rel, cell index, code_block *compiled)
+	void operator()(instruction_operand op)
 	{
-		instruction_operand op(rel.rel_class(),rel.rel_offset() + (cell)compiled->xt());
-		array *literals = (parent->to_boolean(compiled->literals)
-			? untag<array>(compiled->literals) : NULL);
+		code_block *compiled = op.parent_code_block();
+		array *literals = (to_boolean(compiled->literals) ? untag<array>(compiled->literals) : NULL);
+		cell index = op.parameter_index();
 
-		switch(rel.rel_type())
+		switch(op.rel_type())
 		{
 		case RT_PRIMITIVE:
 			op.store_value(parent->compute_primitive_relocation(array_nth(literals,index)));
@@ -245,7 +242,7 @@ struct relocate_code_block_relocation_visitor {
 			op.store_value(parent->compute_xt_pic_tail_relocation(array_nth(literals,index)));
 			break;
 		case RT_HERE:
-			op.store_value(parent->compute_here_relocation(array_nth(literals,index),rel.rel_offset(),compiled));
+			op.store_value(parent->compute_here_relocation(array_nth(literals,index),op.rel_offset(),compiled));
 			break;
 		case RT_THIS:
 			op.store_value((cell)compiled->xt());
@@ -269,7 +266,7 @@ struct relocate_code_block_relocation_visitor {
 			op.store_value(parent->decks_offset);
 			break;
 		default:
-			critical_error("Bad rel type",rel.rel_type());
+			critical_error("Bad rel type",op.rel_type());
 			break;
 		}
 	}
@@ -280,23 +277,24 @@ void factor_vm::relocate_code_block(code_block *compiled)
 {
 	code->needs_fixup.erase(compiled);
 	relocate_code_block_relocation_visitor visitor(this);
-	iterate_relocations(compiled,visitor);
+	compiled->each_instruction_operand(visitor);
 	compiled->flush_icache();
 }
 
 /* Fixup labels. This is done at compile time, not image load time */
 void factor_vm::fixup_labels(array *labels, code_block *compiled)
 {
-	cell i;
 	cell size = array_capacity(labels);
 
-	for(i = 0; i < size; i += 3)
+	for(cell i = 0; i < size; i += 3)
 	{
-		cell rel_class = untag_fixnum(array_nth(labels,i));
+		relocation_class rel_class = (relocation_class)untag_fixnum(array_nth(labels,i));
 		cell offset = untag_fixnum(array_nth(labels,i + 1));
 		cell target = untag_fixnum(array_nth(labels,i + 2));
 
-		instruction_operand op(rel_class,offset + (cell)compiled->xt());
+		relocation_entry new_entry(RT_HERE,rel_class,offset);
+
+		instruction_operand op(new_entry,compiled,0);
 		op.store_value(target + (cell)compiled->xt());
 	}
 }
diff --git a/vm/code_blocks.hpp b/vm/code_blocks.hpp
index 9fbd5ee749..199b0afff0 100644
--- a/vm/code_blocks.hpp
+++ b/vm/code_blocks.hpp
@@ -48,6 +48,24 @@ struct code_block
 	{
 		factor::flush_icache((cell)this,size());
 	}
+
+	template<typename Iterator> void each_instruction_operand(Iterator &iter)
+	{
+		if(to_boolean(relocation))
+		{
+			byte_array *rels = (byte_array *)UNTAG(relocation);
+
+			cell index = 0;
+			cell length = (rels->capacity >> TAG_BITS) / sizeof(relocation_entry);
+
+			for(cell i = 0; i < length; i++)
+			{
+				relocation_entry rel = rels->data<relocation_entry>()[i];
+				iter(instruction_operand(rel,this,index));
+				index += rel.number_of_parameters();
+			}
+		}
+	}
 };
 
 }
diff --git a/vm/code_heap.cpp b/vm/code_heap.cpp
index bd8e200e36..5086e3359c 100755
--- a/vm/code_heap.cpp
+++ b/vm/code_heap.cpp
@@ -59,6 +59,11 @@ void code_heap::code_heap_free(code_block *compiled)
 	allocator->free(compiled);
 }
 
+void code_heap::flush_icache()
+{
+	factor::flush_icache(seg->start,seg->size);
+}
+
 /* Allocate a code heap during startup */
 void factor_vm::init_code_heap(cell size)
 {
@@ -86,7 +91,7 @@ defining a new word. */
 void factor_vm::update_code_heap_words()
 {
 	word_updater updater(this);
-	iterate_code_heap(updater);
+	each_code_block(updater);
 }
 
 void factor_vm::primitive_modify_code_heap()
@@ -171,7 +176,7 @@ struct stack_trace_stripper {
 void factor_vm::primitive_strip_stack_traces()
 {
 	stack_trace_stripper stripper;
-	iterate_code_heap(stripper);
+	each_code_block(stripper);
 }
 
 }
diff --git a/vm/code_heap.hpp b/vm/code_heap.hpp
index 8f4790d2f9..f19319a26c 100755
--- a/vm/code_heap.hpp
+++ b/vm/code_heap.hpp
@@ -26,6 +26,7 @@ struct code_heap {
 	void set_marked_p(code_block *compiled);
 	void clear_mark_bits();
 	void code_heap_free(code_block *compiled);
+	void flush_icache();
 };
 
 struct code_heap_room {
diff --git a/vm/compaction.cpp b/vm/compaction.cpp
index 52814b8351..27922ec2b1 100644
--- a/vm/compaction.cpp
+++ b/vm/compaction.cpp
@@ -114,17 +114,15 @@ struct code_block_compaction_relocation_visitor {
 		slot_forwarder(slot_forwarder_),
 		code_forwarder(code_forwarder_) {}
 
-	void operator()(relocation_entry rel, cell index, code_block *compiled)
+	void operator()(instruction_operand op)
 	{
-		relocation_type type = rel.rel_type();
-		instruction_operand op(rel.rel_class(),rel.rel_offset() + (cell)compiled->xt());
+		code_block *compiled = op.parent_code_block();
+		array *literals = (to_boolean(compiled->literals) ? untag<array>(compiled->literals) : NULL);
+		cell index = op.parameter_index();
 
-		array *literals = (parent->to_boolean(compiled->literals)
-			? untag<array>(compiled->literals) : NULL);
+		cell old_offset = op.rel_offset() + (cell)old_address->xt();
 
-		cell old_offset = rel.rel_offset() + (cell)old_address->xt();
-
-		switch(type)
+		switch(op.rel_type())
 		{
 		case RT_IMMEDIATE:
 			op.store_value(slot_forwarder.visit_pointer(op.load_value(old_offset)));
@@ -135,7 +133,7 @@ struct code_block_compaction_relocation_visitor {
 			op.store_code_block(code_forwarder.visit_code_block(op.load_code_block(old_offset)));
 			break;
 		case RT_HERE:
-			op.store_value(parent->compute_here_relocation(array_nth(literals,index),rel.rel_offset(),compiled));
+			op.store_value(parent->compute_here_relocation(array_nth(literals,index),op.rel_offset(),compiled));
 			break;
 		case RT_THIS:
 			op.store_value((cell)compiled->xt());
@@ -173,7 +171,7 @@ struct code_block_compaction_updater {
 		slot_forwarder.visit_code_block_objects(new_address);
 
 		code_block_compaction_relocation_visitor<SlotForwarder> visitor(parent,old_address,slot_forwarder,code_forwarder);
-		parent->iterate_relocations(new_address,visitor);
+		new_address->each_instruction_operand(visitor);
 	}
 };
 
@@ -215,7 +213,6 @@ void factor_vm::collect_compact_impl(bool trace_contexts_p)
 	{
 		slot_forwarder.visit_contexts();
 		code_forwarder.visit_context_code_blocks();
-		code_forwarder.visit_callback_code_blocks();
 	}
 
 	update_code_roots_for_compaction();
@@ -252,10 +249,7 @@ void factor_vm::collect_compact_code_impl(bool trace_contexts_p)
 	code_block_visitor<forwarder<code_block> > code_forwarder(this,forwarder<code_block>(code_forwarding_map));
 
 	if(trace_contexts_p)
-	{
 		code_forwarder.visit_context_code_blocks();
-		code_forwarder.visit_callback_code_blocks();
-	}
 
 	/* Update code heap references in data heap */
 	object_grow_heap_updater updater(code_forwarder);
@@ -270,4 +264,22 @@ void factor_vm::collect_compact_code_impl(bool trace_contexts_p)
 	update_code_roots_for_compaction();
 }
 
+void factor_vm::collect_compact(bool trace_contexts_p)
+{
+	collect_mark_impl(trace_contexts_p);
+	collect_compact_impl(trace_contexts_p);
+	code->flush_icache();
+}
+
+void factor_vm::collect_growing_heap(cell requested_bytes, bool trace_contexts_p)
+{
+	/* Grow the data heap and copy all live objects to the new heap. */
+	data_heap *old = data;
+	set_data_heap(data->grow(requested_bytes));
+	collect_mark_impl(trace_contexts_p);
+	collect_compact_code_impl(trace_contexts_p);
+	code->flush_icache();
+	delete old;
+}
+
 }
diff --git a/vm/free_list.cpp b/vm/free_list.cpp
index ed961e3dab..9b4b06683d 100644
--- a/vm/free_list.cpp
+++ b/vm/free_list.cpp
@@ -30,8 +30,8 @@ void free_list::add_to_free_list(free_heap_block *block)
 	free_block_count++;
 	free_space += size;
 
-	if(size < free_list_count * block_granularity)
-		small_blocks[size / block_granularity].push_back(block);
+	if(size < free_list_count * data_alignment)
+		small_blocks[size / data_alignment].push_back(block);
 	else
 		large_blocks.insert(block);
 }
@@ -39,9 +39,9 @@ void free_list::add_to_free_list(free_heap_block *block)
 free_heap_block *free_list::find_free_block(cell size)
 {
 	/* Check small free lists */
-	if(size / block_granularity < free_list_count)
+	if(size / data_alignment < free_list_count)
 	{
-		std::vector<free_heap_block *> &blocks = small_blocks[size / block_granularity];
+		std::vector<free_heap_block *> &blocks = small_blocks[size / data_alignment];
 		if(blocks.size() == 0)
 		{
 			/* Round up to a multiple of 'size' */
diff --git a/vm/free_list_allocator.hpp b/vm/free_list_allocator.hpp
index 62e4e09758..4c725bcf4f 100644
--- a/vm/free_list_allocator.hpp
+++ b/vm/free_list_allocator.hpp
@@ -84,7 +84,7 @@ template<typename Block> bool free_list_allocator<Block>::can_allot_p(cell size)
 
 template<typename Block> Block *free_list_allocator<Block>::allot(cell size)
 {
-	size = align(size,block_granularity);
+	size = align(size,data_alignment);
 
 	free_heap_block *block = free_blocks.find_free_block(size);
 	if(block)
diff --git a/vm/full_collector.cpp b/vm/full_collector.cpp
index 10a6f53c41..13546460d1 100644
--- a/vm/full_collector.cpp
+++ b/vm/full_collector.cpp
@@ -27,11 +27,6 @@ void full_collector::trace_context_code_blocks()
 	code_visitor.visit_context_code_blocks();
 }
 
-void full_collector::trace_callback_code_blocks()
-{
-	code_visitor.visit_callback_code_blocks();
-}
-
 void full_collector::trace_object_code_block(object *obj)
 {
 	code_visitor.visit_object_code_block(obj);
@@ -51,7 +46,7 @@ void factor_vm::update_code_roots_for_sweep()
 	for(; iter < end; iter++)
 	{
 		code_root *root = *iter;
-		code_block *block = (code_block *)(root->value & -block_granularity);
+		code_block *block = (code_block *)(root->value & -data_alignment);
 		if(root->valid && !state->marked_p(block))
 			root->valid = false;
 	}
@@ -70,7 +65,7 @@ void factor_vm::update_code_roots_for_compaction()
 	for(; iter < end; iter++)
 	{
 		code_root *root = *iter;
-		code_block *block = (code_block *)(root->value & -block_granularity);
+		code_block *block = (code_block *)(root->value & -data_alignment);
 
 		/* Offset of return address within 16-byte allocation line */
 		cell offset = root->value - (cell)block;
@@ -99,7 +94,6 @@ void factor_vm::collect_mark_impl(bool trace_contexts_p)
 	{
 		collector.trace_contexts();
 		collector.trace_context_code_blocks();
-		collector.trace_callback_code_blocks();
 	}
 
 	while(!mark_stack.empty())
@@ -149,25 +143,7 @@ void factor_vm::collect_full(bool trace_contexts_p)
 		current_gc->event->op = collect_compact_op;
 		collect_compact_impl(trace_contexts_p);
 	}
-	flush_icache(code->seg->start,code->seg->size);
-}
-
-void factor_vm::collect_compact(bool trace_contexts_p)
-{
-	collect_mark_impl(trace_contexts_p);
-	collect_compact_impl(trace_contexts_p);
-	flush_icache(code->seg->start,code->seg->size);
-}
-
-void factor_vm::collect_growing_heap(cell requested_bytes, bool trace_contexts_p)
-{
-	/* Grow the data heap and copy all live objects to the new heap. */
-	data_heap *old = data;
-	set_data_heap(data->grow(requested_bytes));
-	collect_mark_impl(trace_contexts_p);
-	collect_compact_code_impl(trace_contexts_p);
-	flush_icache(code->seg->start,code->seg->size);
-	delete old;
+	code->flush_icache();
 }
 
 }
diff --git a/vm/full_collector.hpp b/vm/full_collector.hpp
index e4f1147cbc..a126ad4d96 100644
--- a/vm/full_collector.hpp
+++ b/vm/full_collector.hpp
@@ -49,7 +49,6 @@ struct full_collector : collector<tenured_space,full_policy> {
 	explicit full_collector(factor_vm *parent_);
 	void trace_code_block(code_block *compiled);
 	void trace_context_code_blocks();
-	void trace_callback_code_blocks();
 	void trace_object_code_block(object *obj);
 };
 
diff --git a/vm/image.cpp b/vm/image.cpp
index e31d50a463..8c283863ae 100755
--- a/vm/image.cpp
+++ b/vm/image.cpp
@@ -122,10 +122,10 @@ struct object_fixupper {
 
 				alien *ptr = (alien *)obj;
 
-				if(!parent->to_boolean(ptr->base))
-					ptr->expired = parent->true_object;
-				else
+				if(to_boolean(ptr->base))
 					ptr->update_address();
+				else
+					ptr->expired = parent->true_object;
 				break;
 			}
 		case DLL_TYPE:
@@ -175,17 +175,15 @@ struct code_block_fixup_relocation_visitor {
 		data_visitor(slot_visitor<data_fixupper>(parent_,data_fixupper(data_offset_))),
 		code_visitor(code_fixupper(code_offset_)) {}
 
-	void operator()(relocation_entry rel, cell index, code_block *compiled)
+	void operator()(instruction_operand op)
 	{
-		relocation_type type = rel.rel_type();
-		instruction_operand op(rel.rel_class(),rel.rel_offset() + (cell)compiled->xt());
-	
-		array *literals = (parent->to_boolean(compiled->literals)
-			? untag<array>(compiled->literals) : NULL);
+		code_block *compiled = op.parent_code_block();
+		array *literals = (to_boolean(compiled->literals) ? untag<array>(compiled->literals) : NULL);
+		cell index = op.parameter_index();
 
-		cell old_offset = (cell)rel.rel_offset() + (cell)compiled->xt() - code_offset;
+		cell old_offset = op.rel_offset() + (cell)compiled->xt() - code_offset;
 
-		switch(type)
+		switch(op.rel_type())
 		{
 		case RT_IMMEDIATE:
 			op.store_value(data_visitor.visit_pointer(op.load_value(old_offset)));
@@ -202,7 +200,7 @@ struct code_block_fixup_relocation_visitor {
 			op.store_value(parent->compute_dlsym_relocation(literals,index));
 			break;
 		case RT_HERE:
-			op.store_value(parent->compute_here_relocation(array_nth(literals,index),rel.rel_offset(),compiled));
+			op.store_value(parent->compute_here_relocation(array_nth(literals,index),op.rel_offset(),compiled));
 			break;
 		case RT_THIS:
 			op.store_value((cell)compiled->xt());
@@ -226,7 +224,7 @@ struct code_block_fixup_relocation_visitor {
 			op.store_value(parent->decks_offset);
 			break;
 		default:
-			critical_error("Bad rel type",rel.rel_type());
+			critical_error("Bad rel type",op.rel_type());
 			break;
 		}
 	}
@@ -248,7 +246,7 @@ struct code_block_fixupper {
 		data_visitor.visit_code_block_objects(compiled);
 
 		code_block_fixup_relocation_visitor code_visitor(parent,data_offset,code_offset);
-		parent->iterate_relocations(compiled,code_visitor);
+		compiled->each_instruction_operand(code_visitor);
 	}
 };
 
diff --git a/vm/instruction_operands.cpp b/vm/instruction_operands.cpp
index b55598c5aa..e00954b67c 100644
--- a/vm/instruction_operands.cpp
+++ b/vm/instruction_operands.cpp
@@ -3,6 +3,9 @@
 namespace factor
 {
 
+instruction_operand::instruction_operand(relocation_entry rel_, code_block *compiled_, cell index_) :
+	rel(rel_), compiled(compiled_), index(index_), pointer((cell)compiled_->xt() + rel_.rel_offset()) {}
+
 /* Load a 32-bit value from a PowerPC LIS/ORI sequence */
 fixnum instruction_operand::load_value_2_2()
 {
@@ -22,7 +25,7 @@ fixnum instruction_operand::load_value_masked(cell mask, fixnum shift)
 
 fixnum instruction_operand::load_value(cell relative_to)
 {
-	switch(rel_class)
+	switch(rel.rel_class())
 	{
 	case RC_ABSOLUTE_CELL:
 		return *(cell *)pointer;
@@ -45,7 +48,7 @@ fixnum instruction_operand::load_value(cell relative_to)
 	case RC_INDIRECT_ARM_PC:
 		return load_value_masked(rel_indirect_arm_mask,0) + relative_to + sizeof(cell) * 2;
 	default:
-		critical_error("Bad rel class",rel_class);
+		critical_error("Bad rel class",rel.rel_class());
 		return 0;
 	}
 }
@@ -90,7 +93,7 @@ void instruction_operand::store_value(fixnum absolute_value)
 {
 	fixnum relative_value = absolute_value - pointer;
 
-	switch(rel_class)
+	switch(rel.rel_class())
 	{
 	case RC_ABSOLUTE_CELL:
 		*(cell *)pointer = absolute_value;
@@ -123,7 +126,7 @@ void instruction_operand::store_value(fixnum absolute_value)
 		store_value_masked(relative_value - sizeof(cell) * 2,rel_indirect_arm_mask,0);
 		break;
 	default:
-		critical_error("Bad rel class",rel_class);
+		critical_error("Bad rel class",rel.rel_class());
 		break;
 	}
 }
diff --git a/vm/instruction_operands.hpp b/vm/instruction_operands.hpp
index ab7e482e3e..788a64ac60 100644
--- a/vm/instruction_operands.hpp
+++ b/vm/instruction_operands.hpp
@@ -67,7 +67,7 @@ static const cell rel_relative_arm_3_mask = 0xffffff;
 struct relocation_entry {
 	u32 value;
 
-	relocation_entry(u32 value_) : value(value_) {}
+	explicit relocation_entry(u32 value_) : value(value_) {}
 
 	relocation_entry(relocation_type rel_type,
 		relocation_class rel_class,
@@ -120,11 +120,32 @@ struct relocation_entry {
 };
 
 struct instruction_operand {
-	cell rel_class;
+	relocation_entry rel;
+	code_block *compiled;
+	cell index;
 	cell pointer;
 
-	instruction_operand(cell rel_class_, cell pointer_) :
-		rel_class(rel_class_), pointer(pointer_) {}
+	instruction_operand(relocation_entry rel_, code_block *compiled_, cell index_);
+
+	relocation_type rel_type()
+	{
+		return rel.rel_type();
+	}
+
+	cell rel_offset()
+	{
+		return rel.rel_offset();
+	}
+
+	cell parameter_index()
+	{
+		return index;
+	}
+
+	code_block *parent_code_block()
+	{
+		return compiled;
+	}
 
 	fixnum load_value_2_2();
 	fixnum load_value_masked(cell mask, fixnum shift);
diff --git a/vm/jit.cpp b/vm/jit.cpp
index 2fe9384630..1bdbbc6171 100644
--- a/vm/jit.cpp
+++ b/vm/jit.cpp
@@ -28,14 +28,11 @@ void jit::emit_relocation(cell code_template_)
 	cell capacity = array_capacity(code_template.untagged());
 	for(cell i = 1; i < capacity; i += 3)
 	{
-		cell rel_class = array_nth(code_template.untagged(),i);
-		cell rel_type = array_nth(code_template.untagged(),i + 1);
+		relocation_class rel_class = (relocation_class)untag_fixnum(array_nth(code_template.untagged(),i));
+		relocation_type rel_type = (relocation_type)untag_fixnum(array_nth(code_template.untagged(),i + 1));
 		cell offset = array_nth(code_template.untagged(),i + 2);
 
-		relocation_entry new_entry(
-			(relocation_type)untag_fixnum(rel_type),
-			(relocation_class)untag_fixnum(rel_class),
-			code.count + untag_fixnum(offset));
+		relocation_entry new_entry(rel_type,rel_class,code.count + untag_fixnum(offset));
 		relocation.append_bytes(&new_entry,sizeof(relocation_entry));
 	}
 }
diff --git a/vm/mark_bits.hpp b/vm/mark_bits.hpp
index d4b1dcda8d..5115f9a821 100644
--- a/vm/mark_bits.hpp
+++ b/vm/mark_bits.hpp
@@ -1,7 +1,6 @@
 namespace factor
 {
 
-const int block_granularity = 16;
 const int mark_bits_granularity = sizeof(cell) * 8;
 const int mark_bits_mask = sizeof(cell) * 8 - 1;
 
@@ -25,7 +24,7 @@ template<typename Block> struct mark_bits {
 	explicit mark_bits(cell size_, cell start_) :
 		size(size_),
 		start(start_),
-		bits_size(size / block_granularity / mark_bits_granularity),
+		bits_size(size / data_alignment / mark_bits_granularity),
 		marked(new cell[bits_size]),
 		forwarding(new cell[bits_size])
 	{
@@ -43,12 +42,12 @@ template<typename Block> struct mark_bits {
 
 	cell block_line(Block *address)
 	{
-		return (((cell)address - start) / block_granularity);
+		return (((cell)address - start) / data_alignment);
 	}
 
 	Block *line_block(cell line)
 	{
-		return (Block *)(line * block_granularity + start);
+		return (Block *)(line * data_alignment + start);
 	}
 
 	std::pair<cell,cell> bitmap_deref(Block *address)
diff --git a/vm/master.hpp b/vm/master.hpp
index 7c8598f6e3..3059ea8b42 100755
--- a/vm/master.hpp
+++ b/vm/master.hpp
@@ -49,6 +49,7 @@ namespace factor
 #include "errors.hpp"
 #include "bignumint.hpp"
 #include "bignum.hpp"
+#include "booleans.hpp"
 #include "instruction_operands.hpp"
 #include "code_blocks.hpp"
 #include "bump_allocator.hpp"
@@ -92,7 +93,6 @@ namespace factor
 #include "callstack.hpp"
 #include "arrays.hpp"
 #include "math.hpp"
-#include "booleans.hpp"
 #include "byte_arrays.hpp"
 #include "jit.hpp"
 #include "quotations.hpp"
diff --git a/vm/object_start_map.cpp b/vm/object_start_map.cpp
index 3159313dd5..105f934f99 100644
--- a/vm/object_start_map.cpp
+++ b/vm/object_start_map.cpp
@@ -60,7 +60,7 @@ void object_start_map::update_card_for_sweep(cell index, u16 mask)
 	cell offset = object_start_offsets[index];
 	if(offset != card_starts_inside_object)
 	{
-		mask >>= (offset / block_granularity);
+		mask >>= (offset / data_alignment);
 
 		if(mask == 0)
 		{
@@ -70,7 +70,7 @@ void object_start_map::update_card_for_sweep(cell index, u16 mask)
 		else
 		{
 			/* Move the object start forward if necessary */
-			object_start_offsets[index] = offset + (rightmost_set_bit(mask) * block_granularity);
+			object_start_offsets[index] = offset + (rightmost_set_bit(mask) * data_alignment);
 		}
 	}
 }
diff --git a/vm/quotations.cpp b/vm/quotations.cpp
index c476c06c69..ab2f549063 100755
--- a/vm/quotations.cpp
+++ b/vm/quotations.cpp
@@ -93,7 +93,7 @@ bool quotation_jit::stack_frame_p()
 		switch(tagged<object>(obj).type())
 		{
 		case WORD_TYPE:
-			if(!parent->to_boolean(untag<word>(obj)->subprimitive))
+			if(!to_boolean(untag<word>(obj)->subprimitive))
 				return true;
 			break;
 		case QUOTATION_TYPE:
@@ -154,7 +154,7 @@ void quotation_jit::iterate_quotation()
 		{
 		case WORD_TYPE:
 			/* Intrinsics */
-			if(parent->to_boolean(obj.as<word>()->subprimitive))
+			if(to_boolean(obj.as<word>()->subprimitive))
 				emit_subprimitive(obj.value());
 			/* The (execute) primitive is special-cased */
 			else if(obj.value() == parent->special_objects[JIT_EXECUTE_WORD])
diff --git a/vm/slot_visitor.hpp b/vm/slot_visitor.hpp
index 72fe5e8860..a423d74a54 100644
--- a/vm/slot_visitor.hpp
+++ b/vm/slot_visitor.hpp
@@ -1,6 +1,22 @@
 namespace factor
 {
 
+/* Slot visitors iterate over the slots of an object, applying a functor to
+each one that is a non-immediate slot. The pointer is untagged first. The
+functor returns a new untagged object pointer. The return value may or may not equal the old one,
+however the new pointer receives the same tag before being stored back to the
+original location.
+
+Slots storing immediate values are left unchanged and the visitor does inspect
+them.
+
+This is used by GC's copying, sweep and compact phases, and the implementation
+of the become primitive.
+
+Iteration is driven by visit_*() methods. Some of them define GC roots:
+- visit_roots()
+- visit_contexts() */
+
 template<typename Visitor> struct slot_visitor {
 	factor_vm *parent;
 	Visitor visitor;
@@ -15,6 +31,7 @@ template<typename Visitor> struct slot_visitor {
 	void visit_stack_elements(segment *region, cell *top);
 	void visit_data_roots();
 	void visit_bignum_roots();
+	void visit_callback_roots();
 	void visit_roots();
 	void visit_contexts();
 	void visit_code_block_objects(code_block *compiled);
@@ -92,6 +109,28 @@ void slot_visitor<Visitor>::visit_bignum_roots()
 	}
 }
 
+template<typename Visitor>
+struct callback_slot_visitor {
+	callback_heap *callbacks;
+	slot_visitor<Visitor> *visitor;
+
+	explicit callback_slot_visitor(callback_heap *callbacks_, slot_visitor<Visitor> *visitor_) :
+		callbacks(callbacks_), visitor(visitor_) {}
+
+	void operator()(code_block *stub)
+	{
+		visitor->visit_handle(&stub->owner);
+		callbacks->update(stub);
+	}
+};
+
+template<typename Visitor>
+void slot_visitor<Visitor>::visit_callback_roots()
+{
+	callback_slot_visitor<Visitor> callback_visitor(parent->callbacks,this);
+	parent->callbacks->each_callback(callback_visitor);
+}
+
 template<typename Visitor>
 void slot_visitor<Visitor>::visit_roots()
 {
@@ -102,6 +141,7 @@ void slot_visitor<Visitor>::visit_roots()
 
 	visit_data_roots();
 	visit_bignum_roots();
+	visit_callback_roots();
 
 	for(cell i = 0; i < special_object_count; i++)
 		visit_handle(&parent->special_objects[i]);
@@ -130,13 +170,10 @@ struct literal_references_visitor {
 
 	explicit literal_references_visitor(slot_visitor<Visitor> *visitor_) : visitor(visitor_) {}
 
-	void operator()(relocation_entry rel, cell index, code_block *compiled)
+	void operator()(instruction_operand op)
 	{
-		if(rel.rel_type() == RT_IMMEDIATE)
-		{
-			instruction_operand op(rel.rel_class(),rel.rel_offset() + (cell)compiled->xt());
+		if(op.rel_type() == RT_IMMEDIATE)
 			op.store_value(visitor->visit_pointer(op.load_value()));
-		}
 	}
 };
 
@@ -154,7 +191,7 @@ void slot_visitor<Visitor>::visit_embedded_literals(code_block *compiled)
 	if(!parent->code->needs_fixup_p(compiled))
 	{
 		literal_references_visitor<Visitor> visitor(this);
-		parent->iterate_relocations(compiled,visitor);
+		compiled->each_instruction_operand(visitor);
 	}
 }
 
diff --git a/vm/vm.hpp b/vm/vm.hpp
index 0f25412609..57d098b650 100755
--- a/vm/vm.hpp
+++ b/vm/vm.hpp
@@ -374,8 +374,11 @@ struct factor_vm
 
 	//booleans
 	void box_boolean(bool value);
-	bool to_boolean(cell value);
-	inline cell tag_boolean(cell untagged);
+
+	inline cell tag_boolean(cell untagged)
+	{
+		return (untagged ? true_object : false_object);
+	}
 
 	//byte arrays
 	byte_array *allot_byte_array(cell size);
@@ -511,25 +514,6 @@ struct factor_vm
 	cell compute_context_relocation();
 	cell compute_vm_relocation(cell arg);
 	cell code_block_owner(code_block *compiled);
-
-	template<typename Iterator> void iterate_relocations(code_block *compiled, Iterator &iter)
-	{
-		if(to_boolean(compiled->relocation))
-		{
-			byte_array *relocation = (byte_array *)UNTAG(compiled->relocation);
-
-			cell index = 0;
-			cell length = (relocation->capacity >> TAG_BITS) / sizeof(relocation_entry);
-
-			for(cell i = 0; i < length; i++)
-			{
-				relocation_entry rel = relocation->data<relocation_entry>()[i];
-				iter(rel,index,compiled);
-				index += rel.number_of_parameters();
-			}
-		}
-	}
-
 	void update_word_references(code_block *compiled);
 	void check_code_address(cell address);
 	void relocate_code_block(code_block *compiled);
@@ -553,8 +537,7 @@ struct factor_vm
 	void primitive_code_room();
 	void primitive_strip_stack_traces();
 
-	/* Apply a function to every code block */
-	template<typename Iterator> void iterate_code_heap(Iterator &iter)
+	template<typename Iterator> void each_code_block(Iterator &iter)
 	{
 		code->allocator->iterate(iter);
 	}