IO (SOUND LIST)                                      PAGE  I5
          -------------------------------------------------------------
           If the first byte is hex FF or decimal 255, the next two
          bytes indicate an address in the other memory space. (That
          means one block points to another block but in another type
          of memory.) These allow switching sound lists from GROM/GRAM
          to VDP or VDP to GRAM/GROM. By making this the beginning of
          the entire table, the sound sequence can be made to repeat
          indefinitely.
           The type 0 indicates sound lists in GROM or GRAM and type 1
          indicates sound lists in VDP.
           Executing a sound list while table-driven sound control is
          already in progress (from a previous sound list) causes the
          old sound control to be totally supplanted by the new sound
          instruction. (That means any sound chip command will over-
          ride old sound chip commands).
           The SGC has 3 tone (square wave) generators - 0, 1, and 2
          all of which can be working simultaneously or in combination.
          The frequency (pitch) and attenuation (volume) of each
          generator can be independently controlled. In addition, there
          is a noise generator which can output white or periodic
          noise. For more information on controlling the SGC, see the             
          TSM9919 SGC specification.                                                
 
          ATTENUATION CONTROL (for generators 0, 1, 2 or 3)
 
          One byte must be transmitted to the SGC:
 
          Binary     1-REG#-1-Attenuation
 
                REG# = register number (0,1,2,3)
                Attenuation = Attenuation/2
                      (e.g. A=0000 0  db = highest volume;
                            A=1000 16 db = medium volume;
                            A=1111 30 db = off. )
 
          EXAMPLE: 1 10 1 0000 : turn on gen. #2 highest volume.
                   1 01 1 0100 : turn on gen. #1 medium high volume.
                   1 11 1 1111 | turn off gen. #3 (noise generator).