Agnus/
                  Read/   Denise/
Register Address  Write   Paula         Function
-------- -------  -----   -------       --------
VHPOSR     006      R       A     Read vertical and horizontal position of
                                      beam or lightpen
VHPOSW     02C      W       A     Write vertical and horizontal position
                                      of beam or lightpen

                 BIT# 15,14,13,12,11,10,09,08,07,06,05,04,03,02,01,00
                      -----------------------------------------------
                 USE  V7 V6 V5 V4 V3 V2 V1 V0,H8 H7 H6 H5 H4 H3 H2 H1

                 RESOLUTION = 1/160 of screen width (280 ns)