diff --git a/vm/code_blocks.cpp b/vm/code_blocks.cpp
index bc3171a239..4e1944b792 100755
--- a/vm/code_blocks.cpp
+++ b/vm/code_blocks.cpp
@@ -433,7 +433,7 @@ code_block *factor_vm::add_code_block(code_block_type type, cell code_, cell lab
 	if(to_boolean(labels.value()))
 		fixup_labels(labels.as<array>().untagged(),compiled);
 
-	compiled->stack_frame_size = frame_size_untagged;
+	compiled->set_stack_frame_size(frame_size_untagged);
 
 	/* Once we are ready, fill in literal and word references in this code
 	block's instruction operands. In most cases this is done right after this
diff --git a/vm/code_blocks.hpp b/vm/code_blocks.hpp
index 099b793866..d14a55c479 100644
--- a/vm/code_blocks.hpp
+++ b/vm/code_blocks.hpp
@@ -5,15 +5,17 @@ namespace factor
 struct code_block
 {
 	// header format (bits indexed with least significant as zero):
-	// bit   0   : free?
-	// bits  1- 2: type (as a code_block_type)
-	// bits  4-  : code size / 16
+	// bit   0  : free?
+	// bits  1-2: type (as a code_block_type)
+	// if not free:
+	//   bits  3-23: code size / 8
+	//   bits 24-31: stack frame size / 16
+	// if free:
+	//   bits  3-end: code size / 8
 	cell header;
 	cell owner; /* tagged pointer to word, quotation or f */
 	cell parameters; /* tagged pointer to array or f */
 	cell relocation; /* tagged pointer to byte-array or f */
-	cell stack_frame_size;
-	cell pad;
 
 	bool free_p() const
 	{
@@ -42,11 +44,32 @@ struct code_block
 
 	cell size() const
 	{
-		cell size = header & ~7;
+		cell size;
+		if (free_p())
+			size = header & ~7;
+		else
+			size = header & 0xFFFFF8;
 		FACTOR_ASSERT(size > 0);
 		return size;
 	}
 
+	cell stack_frame_size() const
+	{
+		if (free_p())
+			return 0;
+		else
+			return (header >> 20) & 0xFF0;
+	}
+
+	void set_stack_frame_size(cell frame_size)
+	{
+		FACTOR_ASSERT(size() < 0xFFFFFF);
+		FACTOR_ASSERT(!free_p());
+		FACTOR_ASSERT(frame_size % 16 == 0);
+		FACTOR_ASSERT(frame_size <= 0xFF0);
+		header = (header & 0xFFFFFF) | (frame_size << 20);
+	}
+
 	template<typename Fixup> cell size(Fixup fixup) const
 	{
 		return size();
diff --git a/vm/debug.cpp b/vm/debug.cpp
index 17363a526c..54a48897e5 100755
--- a/vm/debug.cpp
+++ b/vm/debug.cpp
@@ -418,7 +418,7 @@ struct code_block_printer {
 			std::cout << std::hex << (cell)scan << std::dec << " ";
 			std::cout << std::hex << size << std::dec << " ";
 			std::cout << status << " ";
-			std::cout << "stack frame " << scan->stack_frame_size;
+			std::cout << "stack frame " << scan->stack_frame_size();
 			std::cout << std::endl;
 		}
 	}