74



74             4A             CKEY
     Cassette boot request flag on coldstart. Checks to see if the
     START key is pressed and, if so, CKEY is set. Autoboot cassettes
     are loaded by pressing the START console key while turning the
     power on. In response to the beep, press the PLAY button on the
     recorder.

75             4B             CASSBT
     Cassette boot flag. The Atari attempts both a disk and a cassette
     boot simultaneously. Zero here means no cassette boot was suc-
     cessful. See location 9

76             4C             DSTAT
     Display status and keyboard register used by the display handler.
     Also used to indicate memory is too small for the screen mode,
     cursor out of range error, and the BREAK abort status.

77             4D             ATRACT
     Attract mode timer and flag. Attract mode rotates colors on your
     screen at low luminance levels when the computer is on but no
     keyboard input is read for a long time (seven to nine minutes).
     This helps to save your TV screen from "burn-out" damage suf-
     fered from being left on and not used. It is set to zero by IRQ
     whenever a key is pressed, otherwise incremented every four
     seconds by VBLANK (see locations 18 - 20; $12 - $14). When the
     value in ATRACT reaches 127 ($7F), it is then set to 254 ($FE) un-
     til attract mode is terminated. This sets the flag to reduce the
     luminance and rotate the colors when the Atari is sitting idle.
     POKE with 128 ($80) to see this effect immediately: it normally
     takes seven to nine minutes to enable the attract mode. The OS
     cannot "attract" color generated by DLI's, although your DLI
     routine can, at a loss of time.

     Joysticks alone will not reset location 77 to zero. You will have to
     add a POKE 77,0 to your program periodically or frequently call
     in a subroutine to prevent the Atari from entering attract mode if
     you are not using any keyboard input.

78             4E             DRKMSK
     Dark attract mask; set to 254 ($FE) for normal brightness when
     the attract mode is inactive (see location 77). Set to 246 ($F6)
     when the attract mode is active to guarantee screen color
     luminance will not exceed 50% . Initialized to 254 ($FE).

79             4F             COLRSH
     Color shift mask; attract color shifter; the color registers are
     EORd with locations 78 and 79 at the stage two VBLANK (see
     locations 18 - 20; $12 - $14). When set to zero and location 78
     equals 246, color luminance is reduced 50%. COLRSH contains