diff --git a/world.lua b/world.lua
index 421489f..4e6df71 100644
--- a/world.lua
+++ b/world.lua
@@ -1,4 +1,5 @@
---- Procedural generation methods
+--- Procedural generation methods, and other aspects of generating the
+--- environment
 
 function init_world()
    -- Metadata for different biomes
@@ -156,14 +157,8 @@ end
 
 
 ---
---- mod buffer functions - these handle parts of the world map that have
---- changed from their 'default' position
----
---- todo: the mod buffer eventually needs to be a bit more elaborate.
----   we need to make it possible to cull only old and unimportant changes,
----   so each entry needs a 'critical' flag and something to indicate order
----   added. We could keep 2 buffers for the latter, one that's just keys
----   in an array... expensive though.
+--- mod buffer functions - these handle locations on the world map that have
+--- changed from their 'default' state
 ---
 
 -- x and y are global coords
@@ -173,24 +168,34 @@ end
 
 -- x and y are map-local coords
 function write_map_change(new_sprite, x, y, perm)
-   if #mod_buffer >= 8192 then
-      cull_mod_buffer()
-   end
    local key = get_mod_key(x, y)
    mod_buffer[key] = new_sprite
 
    -- the queue gives us a time-ordered list of items to delete.
    -- anything that should persist is simply not added to the queue,
-   -- making it un-deletable.
+   -- making it un-deletable. It also doesn't count against the maximum
+   -- size of the mod queue before deletion, meaning it permanently inflates
+   -- the size of ram.
    -- obviously if we end up with a *very large number* of persistent
    -- objects we can run into trouble, but this is functionally a design
-   -- limitation.
+   -- limitation. We also only save 32 of these on exit, further limiting
+   -- the possible size of the 'memory leak' pool, as it were.
    if not perm then
       add(mod_queue, key)
    end
+
+   if #mod_queue >= 8192 then
+      cull_mod_buffer()
+   end
 end
 
 function cull_mod_buffer()
    -- we cull 512 entries at a time.
-   -- stub: implement me!
+   local count = 0
+   for i=1,511 do
+      local key = mod_queue[1]
+      if (not key) return -- check that we're not out of items for some reason
+      mod_buffer[key] = nil
+      del(mod_queue, key)
+   end
 end