Atari 400 Hardware 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 0/S allocates
RAM space for the graphics mode, then builds the 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
split screen mode (text window), therefore "max RAM mode" +16 must
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 -1-

Atari 400 Specifications

General

Manufacturer	Atari
Model	400
Type	Home Computer
Release Year	1979
CPU	MOS Technology 6502
CPU Speed	1.79 MHz
ROM	10 KB
Operating System	Atari OS
Keyboard	Membrane keyboard
RAM	8 KB (expandable to 48 KB)
Graphics	ANTIC and GTIA chips
Sound	4 channels
Display	RF output for connection to TV
Storage	Optional cassette tape drive or floppy disk drive
Ports	Cartridge
Display Resolution	320x192 (16 colors)