A new register in the enhanced Agnus, BEAMCON0, provides a programmable
signal generator.

   BEAMCON0  W   A     Beam counter control register

           Bit     Use
           ---     ---
           15
           14    HARDDIS    Disable hardwired vertical/horizontal blank
           13    LPENDIS    Ignore latched pen value on vertical pos read
           12    VARVBEN    Use VBSTRT/STOP disable hard window stop
           11    LOLDIS     Disable long line/short line toggle
           10    CSCBEN     Composite sync redirection
            9    VARVSYEN   Variable vertical sync enable
            8    VARHSYEN   Variable horizontal sync enable
            7    VARBEAMEN  Variable beam counter comparator enable
            6    DUAL       Special ultra resolution mode enable
            5    PAL        Programmable pal mode enable
            4    VARCSYEN   Variable composite sync
            3    BLANKEN    Composite blank redirection
            2    CSYTRUE    Polarity control for C sync pin
            1    VSYTRUE    Polarity control for V sync pin
            0    HSYTRUE    Polarity control for H sync pin

   Warning:
   --------
   Programmable changes between PAL and NTSC modes are new for V2.0.
   They rely on hardware sync and blank in the Agnus/Denise chip set to
   guarantee necessary signals for a correctly displayed picture.

   Other modes, such as VGA (31 KHz programmable mode) disable the hard
   stops on display sync and blank.  Do not write to this register.

   Incorrectly writing directly to BEAMCON0 has the (remote) possibility
   of destroying your multisync monitor.