Atari 400 Technical Reference Manual page 207

DMASK is a mask that contains zeros for all bits that do not
correspond to the specific pixel to be operated upon, and
l's for all bits that do correspond. DMASK can contain
the values shown below in binary notation:
11111111
screen modes 1 and 2; one pixel per byte.
11110000
00001111
screen modes 9-11; two pixels per byte.
11000000
00110000
00001100
00000011
screen modes
3,
5 and 7; four pixels per byte.
10000000
01000000
screen modes 4, 6 and 8; eight pixels per byte.
00000010
00000001
929 SHFAMT* [006F, II -- Pixel Justification
SHFAMT indicates the amount to shift the right-Justified pixel
data on output, or the amount to shift the input data to right
Justify it on input. The value is always the same as for DMASK
prior to the Justification process.
Internal Working Variables
B30 HOLD1* [0051,ll Temporary storage
B31 HOLD2* [029F,ll Temporary storage
832 HOLD3* [029D,11 Temporary storage
B33 TMPCHR* [0050,ll -- Temporary storage
B34 DSTAT* [004C, 11 Display status
935 DINDEX [0057,11 Display mode
DINDEX contains the current screen mode obtained from the low
order four bits of the most recent
OPEN
AUXl byte.
936 SAVMSC [0058,21 -- Screen Memory Address
SAVMSC contains the lowest address of the screen data regipn; the
data at that address is displayed at the upper left corner of the
screen.
OPERATING SYSTEM C016555 -- Appendix
L
222