HCENTER

         BIT#  15  14  13  12  11  10  09  08  07  06  05  04  03  02  01  00
                x   x   x   x   x   x   x   x  h8  h7  h6  h5  h4  h3  h2  h1


         This is necessary for interlace mode with variable beam
         counters. See BEAMCON0 for when it affects chip outputs.