Appendix B: Mixing Graphics Modes - Atari 400 Technical Reference Manual

APPENDIX B
MIXING GRAPHICS MODES
I. GENERAL
This procedure describes how to mix several graphic modes on
the TV screen at the same time using BASIC commands. Each graphics
mode has a different number of scan lines per "Mode Line" (one line
of a graphics mode). The TV screen must consist of 192 scan lines,
so when mixing modes, they must be combined in· such a way as to get
192 scan lines. This is accomplished by modifying the Display
List.
When a graphics mode is set on the computer, the
ols
allocates
RAM space for the graphics mode, then builds
th~
display list
adjacent to the graphics RAM, and sets a pointer to the beginning
of the display list. Each "mode line" is constructed from a "mode
byte" in the display list that determines how many scan lines in
each mode line. The display list describes the screen display from
top to bottom.
A Display List must be built for the "max RAM mode" (the
graphi~s
mode that requires the most RAM) then modified with POKES
to mix the other modes with it. This "max RAM mode" cannot be a
s p 1 i t s c r e e n mod e (t ext win dow), the ref
0
r e " m a x RAM mod e"
+
1 6 mu s t
be used. If the max RAM mode will be at the top of the screen, then
the "LMS byte" (load memory scan byte) at the top of the Display
List will already be correct. If not, the "LMS byte" will have
to be modified.
The Display List is modified by POKING a new mode byte for
each mode line that is not a max
~A~
mode line. At the end of the
display list is a JUMP instruction pointing to the top of the
Display list. When the Display List is modified, the JUMP instruc-
tion must be placed immediately after the last mode byte.
Example #1 will be used throughout this procedure to
illustrate each step.
B
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