diff --git a/simulation.lisp b/simulation.lisp
index 59db1f2..ce2c793 100644
--- a/simulation.lisp
+++ b/simulation.lisp
@@ -4,22 +4,25 @@
 (in-package :ham)
 
 (defclass simulation ()
-  ((update :initarg :update)
-   (render :initarg :render)
-   (state :initarg :start-state)
+  ((state :initarg :start)
+   (params :initarg :params)
    (width :initarg :width :initform 800)
    (height :initarg :height :initform 600)
    (running :initform nil)
    simulation-thread))
 
+(defgeneric update (simulation dt))
+(defgeneric render (simulation))
+
 (defmethod run ((sim simulation))
   (sdl2:make-this-thread-main
    (lambda ()
 	 (with-slots (width height running) sim
 	   (with-graphics-context (ctx :width width :height height)
 		 (loop while running do
-		   (run-update sim (/ 1.0 60.0)) ; assume 60 Hz
-		   (run-render sim ctx)))))))
+		   (update sim (/ 1.0 60.0))	; assume 60 Hz
+		   (let ((frame (render sim)))
+			 (display-frame ctx frame))))))))
 
 (defmethod start ((sim simulation))
   (with-slots (running simulation-thread) sim
@@ -32,12 +35,3 @@
   (with-slots (running simulation-thread) sim
 	(setf running nil)
 	(sb-thread:join-thread simulation-thread)))
-
-(defmethod run-update ((sim simulation) dt)
-  (with-slots (update state) sim
-	(setf state (funcall update state dt))))
-
-(defmethod run-render ((sim simulation) ctx)
-  (with-slots (render state) sim
-	(let ((frame (apply render state)))
-	  (display-frame ctx frame))))