Atari 800

Xixing Graphics Xodes, cont.
III. PROCEDURE T0 PRI~T AND PLOT IN MIXED MO~ES, cont.
2. Some modes may have mode line #'s outside of their normal
range.
example:
~ ~ de 2 normaljy has mode line #'s 1 through 12
(full screen). Th ~ se are modif ' ed to #63 and
#64 in exaruple #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.
~EMST = PEEK(START) + PEEK(START + 1) * 256
b. Set a variable labelled CHRPOS to position characters
to be printed on the target line.
CHRPOS = ~1EMST + [(Ml-1)*R-~
2
*(R-20)-~
3
*(R-10)]+X
Hhere:
X = horizontal position of character on the target
line.
R the RAX per line of the Max RAM Mode (table #1).
M
1
= the Mode Line If of the target 1 in e.
M
2
= the number of mode lines of 20 bytes of RAM per
line above the target line.
~
3
= the number of mode lines of
line above the target line.
10 bytes of RAM per
Example: calculate CHRPO~ for Mode Line #64 (the
last line of the Mode 2 area) at horizontal
position 5.
X 5
R 40
M
1
6 4
M
2
7 (6 from Mode 1 area, 1 from Mode 2 area).
M
3
0
CHRPOS
CHRPOS
CHRPOS
CHRPOS
CHRPOS
=
~EMST
+
HEMST
+
MEMST
+
MEMST
+
MEMST
+
[(64-1)*40-7*(40-20)-0*(40-10)]+5
[ (63) *40-7* (20)-0* (30) J +5
[2520
-
14 0 J
+
5
[2380] + 5
2385
B -6-
-