diff --git a/lib/waterfoul/gpu.rb b/lib/waterfoul/gpu.rb
index 3599416..dd1aa6f 100644
--- a/lib/waterfoul/gpu.rb
+++ b/lib/waterfoul/gpu.rb
@@ -9,7 +9,7 @@ module Waterfoul
     SCREEN_WIDTH = 160
     SCREEN_HEIGHT = 144
 
-    FRAMEBUFFER_SIZE = SCREEN_WIDTH * SCREEN_HEIGHT
+    FRAMEBUFFER_SIZE = (Screen::SCREEN_WIDTH - 1) * (Screen::SCREEN_HEIGHT - 1)
 
     H_BLANK_STATE     = 0
     V_BLANK_STATE     = 1