Atari 400 Technical Reference Manual page 201

colorllum change will be effected every 8.3 seconds
(512160).
810 ATRACT [004D, 1] -- Attract mode timer and flag
ATRACT is the timer (and flag) that controls the initiation and
termination of attract mode. Whenever a keyboard key is pressed,
the keyboard IRO service routine sets ATRACT to zero, thus
terminating attract mode; the [BREAKJ key logic behaves
accordingl~.
As part of the stage 1 VBLANK process, ATRACT is
incremented every 4 seconds; if the value exceeds 127 (after 9
minutes without keyboard activity), the value of ATRACT will
be set to $FE and will retain that value until attract mode is
terminated.
Since the attract mQde is prevented and terminated by the
as
based only upon keyboard activity, some users can want to reset
ATRACT based upon Atari-controller event detection,
user-controlled Serial 1/0 bus activity or any other signs of
life.
Bll COLRSH* [004F, lJ -- Color shift mask
COLRSH has the value $00 when attract mode is inactive, thus
effecting no change to the screen colors; when attract mode is
active, COLRSH contains the current value of the timer variable
middle digit (RTCLOK+1).
B12 DRKMSK* [004E,1] -- Dark (luminance) mask
DRKMSK has the value $FE when attract mode is inactive, which does not
alter the luminance; and has the value $F6 when attract mode
is active, which forces the most significant bit of the luminance
field to zero, thus guaranteeing that the luminance will never
exceed 50 percent.
Tabbing
See Section 5 for a discussion of the use of tabs in conJunction
with the Screen Editor.
B13 TABMAP [02A3,15] -- Tab stop setting map
The tab settings are retained in a 15-byte (120 bit) map, where a
bit value of 1 indicates a tab setting; the diagram below shows
the mapping of the individual bits to tab positions.
OPERATING SYSTEM C016555 -- Appendix L
216