This is the sprite 0 position register. The  word  written into this
register controls the position on the screen at which the upper left-hand
corner of the sprite is to be placed. The most significant bit of the
first  data word  will be placed in this position on the screen.

   Sprite placement resolution.
   ----------------------------
   The sprites have a placement resolution on a full screen of 320 by
   200 NTSC (320 by 256 PAL). The sprite resolution is independent of
   the bitplane resolution.

Bit positions:

   *  Bits 15-8 specify the vertical start position, bits V7 - V0.
   *  Bits 7-0 specify the horizontal start position, bits H8 - H1.

   Warning:
   --------
   This register is normally only written by the sprite  DMA channel 
   itself. See the details above regarding the organization of the
   sprite data.  This register is usually updated directly by DMA.