-extern int nships,score,initticks,ticks_since_last, last_ticks;
+// all movement is based on s_frame.
+extern float t_frame; // length of this frame (in ticks = 1/20th second)
+extern float s_frame; // length of this frame (seconds)
+extern int ms_frame; // length of this frame (milliseconds)
+extern int ms_end; // end of this frame (milliseconds)
+
+extern int nships,score;