The enhanced Denise chip provides the following four  new genlock 
features:

   *  Chromakey
   *  BitPlaneKey
   *  BorderBlank
   *  BorderNotTransparent

ChromaKey allows any color register to control the video overlay.
BitPlaneKey allows any bitplane to enable the video overlay. BorderBlank
creates a transparent "frame" surrounding the active area.
BorderNotTransparent makes an opaque "frame" surrounding the active area.