diff --git a/lib/openmsx/boot_env.tcl b/lib/openmsx/boot_env.tcl
index b26216e..e44a32d 100644
--- a/lib/openmsx/boot_env.tcl
+++ b/lib/openmsx/boot_env.tcl
@@ -9,6 +9,9 @@
 # RENDERER=SDL
 # Override video output from command line.
 #
+# SCALE_FACTOR=3
+# Override video scale factor for SDL renderer.
+#
 # THROTTLE=off
 # Disables msx speed emulation.
 #