Atari 400 Technical Reference Manual page 531

Mixing Graphics Modes, cont.
III. PROCEDURE TO PRINT AND PLOT IN MIXED MODES, cont.
2. Some modes may have mode line #'s outside of their normal
range.
example: Mode 2 normally has mode line #'s 1 through 12
(full screen). These are modified to #63 and
#64 in example #1.
To prevent the computer from giving a "cursor out of range"
error message the following procedure can be used:
a. Set a variable labelled "MEMST" to be the display
memory start pointer.
MEMST
=
PEEK(START) + PEEK(START + 1) * 256
b. Set a variable labelled CHRPOS to position characters
to be printed on the target line.
CHRPOS = MEMST + [(MI-l)*R-X 2 *(R-20)-M
3
*(R-I0)]+X
Hhere:
X horizontal position of character on the target
line.
R the RAM per line of the Max RAM Mode (table #1).
MI
=
the Mode Line # of the target line.
M
2
=
the number of mode lines of 20 bytes of RAM per
line above the target line.
M3=
the number of mode lines of 10 bytes of RAM per
line above the target line.
Example: calculate CHRPOS for Mode Line #64 (the
last line of the Mode 2 area) at horizontal
position 5.
X 5
R 40
MI 64
M2 7 (6 from Mode 1 area, 1 from Mode 2 area).
M3 0
CHRPOS MEMST + [(64-1)*40-7*(40-20)-0*(40-10)]+5
CHRPOS
HEMST + [(63)*40-7*(20)-0*(30)]+5
CHRPOS MEMST
+
[2520
-
140]
+
5
CHRPOS MEMST
+
[2380] + 5
CHRPOS HEMST + 2385
B
-6-