Head                   Body
+---------------+      +---------------+
| Ptr to body -------> | Unused        |
+---------------+      +---------------+
| Ref count     |      | Unused        |
+---------------+      +---------------+
| Flags & type  |      | Unused        |
+---------------+      +---------------+
| Unused        |      | Unused        |
+---------------+      +---------------+
                       | IV/UV slot    |
                       +---------------+