The present invention includes dynamically analyzing look-up requests from a cache look-up algorithm to look-up data block tags corresponding to blocks of data previously inserted into a cache memory, to determine a cache related parameter. The lithographic apparatus further includes an autofocus system having a light source, a position sensitive detector, and a light directing arrangement for directing a first portion of light from the light source to the position sensitive detector. chassis is mounted for movement along a longitudinal path by a motor which rotates a longitudinal drive screw selectively coupled to the drive chassis by a threaded clamp.