-extern int nships,score,initticks,ticks_since_last, last_ticks;
-extern int gameover;
-extern int maneuver;
+// All movement is based on t_frame.
+// All speeds are pixels/tick, with 20 ticks per second.
+extern float t_frame; // length of this frame (in ticks = 1/20th second)
+
+extern int nships,score;