diff --git a/c/backtrace.c b/c/backtrace.c
index 7aa73b8..57011e0 100644
--- a/c/backtrace.c
+++ b/c/backtrace.c
@@ -6,6 +6,7 @@
 
 struct backtrace * backtrace_allocate(struct vm * vm)
 {
+  assert(false); // backtrace must become an objectref
   struct backtrace * backtrace = memory_allocate((sizeof (struct backtrace)));
   backtrace->num_entries = vm->frame_stack.ix;
   int backtrace_entries_size = (sizeof (struct backtrace_entry)) * backtrace->num_entries;
diff --git a/c/class_resolver.c b/c/class_resolver.c
index dea31a5..56b7322 100644
--- a/c/class_resolver.c
+++ b/c/class_resolver.c
@@ -151,6 +151,7 @@ static int32_t class_resolver_create_fields_hash_table(int class_hash_table_leng
   class_entry->fields.length = fields_hash_table_length;
   class_entry->fields.entry = fields_hash_table;
   class_entry->instance_fields_count = instance_index;
+  class_entry->static_fields_count = static_index;
 
   return static_index;
 }
diff --git a/c/class_resolver.h b/c/class_resolver.h
index 48c8e36..bd798e3 100644
--- a/c/class_resolver.h
+++ b/c/class_resolver.h
@@ -37,6 +37,7 @@ struct class_entry {
   struct class_file * class_file;
   enum initialization_state initialization_state;
   union attribute_entry * attribute_entry;
+  int32_t static_fields_count;
   int32_t * static_fields;
   int32_t instance_fields_count;
 
diff --git a/c/gc.c b/c/gc.c
new file mode 100644
index 0000000..86a65f9
--- /dev/null
+++ b/c/gc.c
@@ -0,0 +1,119 @@
+#include "frame.h"
+#include "memory_allocator.h"
+
+static void walk_address(void * address);
+
+static void walk_object(struct objectref * objectref)
+{
+  if (arrayref->tag->mark != 0)
+    return;
+  objectref->tag->mark = 1;
+
+  for (int i = 0; i < objectref->class_entry->instance_fields_count; i++) {
+    void * address = objectref->aref[i];
+    walk_address(address);
+  }
+}
+
+static void walk_ref_array(struct arrayref * arrayref)
+{
+  if (arrayref->tag->mark != 0)
+    return;
+  arrayref->tag->mark = 1;
+
+  if (arrayref->class_entry == nullptr) {
+    // this is an array of arrayrefs
+    for (int i = 0; i < arrayref->length; i++) {
+      if (arrayref->aref[i] != nullptr) {
+        walk_ref_array(arrayref->aref[i]);
+      }
+    }
+  } else {
+    // this is an array of objectrefs
+    for (int i = 0; i < arrayref->length; i++) {
+      if (arrayref->oref[i] != nullptr) {
+        walk_object(arrayref->oref[i]);
+      }
+    }
+  }
+}
+
+static void walk_address(void * address)
+{
+  if (address == nullptr)
+    return;
+
+  if (!memory_is_allocated(address))
+    return;
+
+  struct tag * tag = (struct tag *)address;
+  if (tag->type == TAG_TYPE_OBJECT) {
+    walk_object((struct objectref *)address);
+  }
+  if (tag->type == TAG_TYPE_REF_ARRAY) {
+    walk_ref_array((struct arrayref *)address);
+  }
+}
+
+static void walk_frame(struct frame * frame)
+{
+  int num_local_variables = frame->code_attribute->max_locals;
+  int num_stack_variables = frame->operand_stack_ix;
+
+  for (int i = 0; i < num_local_variables; i++) {
+    void * address = frame->local_variable[i];
+    walk_address(address);
+  }
+
+  for (int i = 0; i < num_stack_variables; i++) {
+    void * address = frame->operand_stack[i];
+    walk_address(address);
+  }
+}
+
+static void walk_static_fields(struct class_entry * class_entry)
+{
+  for (int i = 0; i < class_entry->static_fields_count; i++) {
+    void * address = class_entry->static_fields[i];
+    walk_address(address);
+  }
+}
+
+static void walk_class_hash_table_entry(struct hash_table_entry * entry)
+{
+  if (entry->key == nullptr)
+    return;
+
+  struct class_entry * class_entry = (struct class_entry *)e->value;
+  assert(class_entry != nullptr);
+  walk_static_fiends(class_entry);
+
+  if (entry->next != nullptr)
+    walk_class_hash_table_entry(entry->next);
+}
+
+static void walk_class_hash_table(int length, struct hash_table_entry * entry)
+{
+  for (int i = 0; i < length; i++)
+    walk_class_hash_table_entry(&entry[i]);
+}
+
+static void walk_vm(struct vm * vm)
+{
+  walk_class_hash_table(vm->class_hash_table.length,
+                        vm->class_hash_table.entry);
+
+  for (int i = 0; i < vm->frame_stack->ix; i++) {
+    walk_frame(&vm->frame_stack->frame[i]);
+  }
+}
+
+void gc_mark(struct vm * vm)
+{
+  walk_vm(vm);
+}
+
+void gc_sweep()
+{
+
+}
diff --git a/c/gc.h b/c/gc.h
new file mode 100644
index 0000000..af070f6
--- /dev/null
+++ b/c/gc.h
@@ -0,0 +1,5 @@
+#pragma once
+
+#include "frame.h"
+
+void gc_mark(struct vm * vm);
diff --git a/c/memory_allocator.c b/c/memory_allocator.c
index ef34cf0..a37c6ae 100644
--- a/c/memory_allocator.c
+++ b/c/memory_allocator.c
@@ -72,6 +72,7 @@ void memory_free(void * p)
   assert(buf >= memory);
   uint32_t address_index = buf - memory;
   assert(address_index < (sizeof (memory)));
+  assert((address_index & (~(block_size - 1))) != address_index);
   uint32_t free_list_index = address_index >> block_power;
   assert((free_list[free_list_index] & START) != 0);
 
diff --git a/c/memory_allocator.h b/c/memory_allocator.h
index 2ace29e..de455df 100644
--- a/c/memory_allocator.h
+++ b/c/memory_allocator.h
@@ -1,5 +1,8 @@
+#pragma once
+
 #include <stdint.h>
 
 void memory_reset_free_list();
 void * memory_allocate(uint32_t size);
 void memory_free(void * p);
+bool memory_is_allocated(void * p);
diff --git a/c/native_types.h b/c/native_types.h
index 6a60e26..c7260ea 100644
--- a/c/native_types.h
+++ b/c/native_types.h
@@ -5,10 +5,22 @@
 
 #include "class_resolver.h"
 
-struct object_arrayref;
+enum tag_type {
+  TAG_TYPE_OBJECT = -30741,
+  TAG_TYPE_REF_ARRAY = 23240,
+  TAG_TYPE_PRIM_ARRAY = -5251,
+};
+
+struct tag {
+  int8_t mark;
+  int8_t _res;
+  int16_t type;
+};
+
+static_assert((sizeof (struct tag)) == 4);
 
 struct objectref {
-  int32_t _res;
+  struct tag tag;
   struct class_entry * class_entry;
   union {
     struct objectref * oref[0];
@@ -17,11 +29,12 @@ struct objectref {
   };
 };
 
-static_assert((sizeof (struct objectref)) == 8);
+static_assert((sizeof (struct objectref)) == 12);
 
 struct arrayref {
-  int32_t length; // length position must match primitive_arrayref
+  struct tag tag;
   struct class_entry * class_entry;
+  int32_t length;
   union {
     // object array:
     struct objectref * oref[0];
@@ -34,7 +47,7 @@ struct arrayref {
   };
 };
 
-static_assert((sizeof (struct arrayref)) == 8);
+static_assert((sizeof (struct arrayref)) == 12);
 
 enum ARRAY_TYPE {
   T_BOOLEAN = 4, // 1 byte