Agnus/
                  Read/   Denise/
Register Address  Write   Paula         Function
-------- -------  -----   -------       --------
BLTAFWM    044      W       A      Blitter first-word mask for source A
BLTALWM    046      W       A      Blitter last-word mask for source A

                 The patterns in these two registers are ANDed with
                 the first and last words of each line of data from
                 source A into the blitter. A zero in any bit
                 overrides data from source A.  These registers
                 should be set to all 1s for fill mode or for
                 line-drawing mode.