560,561



     logical unit which varies in size with the GRAPHICS mode. Due
     to the limitations of most TV sets, you will not be able to see all of
     the wide playfield unless you scroll into the offscreen portions.
     BIT 5 must be set to enable ANTIC operation; it enables DMA for
     fetching the display list instructions.

560,561        230,231        SDLSTL
     Starting address of the display list. The display list is an
     instruction set to tell ANTIC where the screen data is and how to
     display it. These locations are the shadow for 54274 and 54275
     ($D402, $D403). You can also find the address of the DL by
     PEEKing one byte above the top of free memory:

     PRINT PEEK(741) + PEEK(742) * 256 + 1.

     However, 560 and 561 are more reliable pointers since custom
     DL's can be elsewhere in memory. Atari standard display lists
     simply instruct the ANTIC chip as to which types of mode lines to
     use for a screen and where the screen data may be found in
     memory. Normally, a DL is between 24 and 256 bytes long (most
     are less than 100 bytes, however), depending on your
     GRAPHICS mode (see location 88,89 for a chart of DL sizes and
     screen display use).

     By altering the DL, you can mix graphics modes on the same
     screen; enable fine scrolling; change the location of the screen
     data; and force interrupts (DLI's) in order to perform short
     machine language routines.

     DL bytes five and six are the addresses of the screen memory
     data, the same as in locations 88 and 89 ($58, $59). Bytes four,
     five, and six are the first Load Memory Scan (LMS) instruction.
     Byte four tells ANTIC what mode to use; the next two bytes are
     the location of the first byte of the screen RAM (LSB/MSB).
     Knowing this location allows you to write directly to the screen by
     using POKE commands (you POKE the internal character codes,
     not the ATASCII codes -- see the BASIC Reference Manual, p.
     55).

     For example, the program below will POKE the internal codes to
     the various screen modes. You can see not only how each screen
     mode handles the codes, but also roughly where the text window
     is in relation to the display screen (the 160 bytes below
     RAMTOP). Note that the GTIA modes have no text window. If
     you don't have the GTIA chip, your Atari will default to
     GRAPHICS 8, but with GTIA formatting.

     1   TRAP 10: GRAPHICS Z
     5   SCREEN = PEEK(560) + PEEK(561) *
       256