54278



     200 POKE SCRNLOW, LOBYTE: POKE SCRNHI
         , HIBYTE
     210 GOTO 40

     Coarse scrolling is relatively easy to implement in the Atari: one
     basically alters the screen RAM to display the new material. Fine
     scrolling is more difficult: each scroll register must be POKEd
     with the number of units to be scrolled -- color clocks or scan
     lines -- and the corresponding display list instructions must have
     the proper bits set. This means you can selectively fine scroll any
     mode lines you wish by setting only those bits of the lines you
     intend to scroll. Other lines will be displayed normally. You can
     set a DL instruction for both horizontal and vertical scroll enable.
     See the Hardware Manual for a discussion of the problems in fine
     scrolling.

     Fine scrolling will allow only a certain amount of data to be
     scrolled before the register must be reset (16 clock bits or scan
     lines maximum). In order to make the scrolling activity
     continuous, the register involved must be reset to zero when the
     desired value is reached, a coarse scroll must be implemented
     (usually during a DLI or VBLANK interval) and a new fine scroll
     begun. This is not easily done in BASIC since it is too slow, and
     changing registers during ANTIC's display process usually
     causes rough or jerky motion. Assembly routines are suggested
     for smooth display. See De Re Atari, Micro, November 1981,
     BYTE, January 1982, and Santa Cruz's Tricky Tutorial #2 for
     more information.

54278          D406           ....
     Unused.

54279          D407           PMBASE
     (W) MSB of the player/missile base address used to locate the
     graphics for your players and missiles (the address equals
     PMBASE * 256. P/M graphics are tricky to use since there are no
     direct Atari 8K BASIC commands to either create or move them
     (there are, however, commands for P/M graphics in BASIC A+
     and in valFORTH utilities).

     Your P/M graphics must always begin on a 1K boundary
     (PEEK(RAMTOP)-4 for double line resolution players) or 2K
     boundary (PEEK(RAMTOP)-5 for single line resolution), so the
     LSB is always zero (page numbers always end in $XX00). For
     example:

     10   POKE 106, PEEK(106) - 8: GRAPHIC
        S 8: SETCOLOR 2,3,4
     20   POKE 559,62: POKE 53248,100: POK