Agnus/
                  Read/   Denise/
Register Address  Write   Paula         Function
-------- -------  -----   -------       --------
SPRxCTL    142      W     A D( e ) sprite x vert stop position and
                                       control data
SPRxPOS    140      W      A D     Sprite x vert-horiz start position data

                 These two registers work together as position, size and
                 feature sprite-control registers.  They are usually loaded
                 by the sprite DMA channel during horizontal blank;
                 however, they may be loaded by either processor at any
                 time.  SPRxPOS register:

                 BIT#   SYM      FUNCTION
                 ----   ----     -----------------------------
                 15-08  SV7-SV0  Start vertical value. High bit(SV8) is
                                 in SPRxCTL register below.
                 07-00  SH8-SH1  Start horizontal value. Low bit(SH0) is
                                 in SPRxCTL register below.

                 SPRxCTL register (writing this address disables sprite
                                   horizontal comparator circuit):

                 BIT#    SYM       FUNCTION
                 ----    --------  -----------------------------
                 15-08   EV7-EV0   End (stop) vertical value low 8 bits
                 07      ATT       Sprite attach control bit (odd sprites)
                 06-04    X        Not used
                 02      SV8       Start vertical value high bit
                 01      EV8       End (stop) vertical value high bit
                 00      SH0       Start horizontal value low bit