Atari XL User Manual

154
The Ateri XL Handbook
reaches the edge of the screen it has completed one
Scan Line and is turned off and moved back to a
posi
tion just below the last starting posi tion, before
the process begins again. This scanning continues
until the electron beam reaches the bottom of the
screen, at which point it starts allover again at the
top. This process must happen many times a second
to create a picture that doesn't flicker.
To
create a colour picture, the screen is coated with
three different types of phosphor which emit the
colours red, blue or green when struck by the
electron beam. The different phosphors are
distributed in tiny dots or blocks over the screen
and the colour TV signal must contain information
about which of these points to direct the electron
beam at, as well as luminance information.
In creating its displays the XL uses the same
principle as the TV transmitter, with the ANTIC
chip feeding data to the GTIA chip, which controls
the electron beam and supplies colour and
luminance information for each dot on the display.
THE DISPLAY LIST
The ANTIC is a microprocessor, like the 6502, and
has its own program in RAM telling it how the data
in screen memory is to be organised to create a
display. This program is called the display list, and
is placed in memory (immediately below screen
memory) by the 6502 processor. The contents of the
display list depend upon the graphics mode
curren tly selected.
The Atari display is divided into groups of scan
lines known as display blocks. The number of scan
lines per display block depends upon the graphics
mode. In mode 0a display block is eight scan lines
high, since a graphics 0 character is eight scan
lines high. In mode 8a display block is only one

Atari XL Specifications

General

Release Year	1983
CPU	MOS Technology 6502C
Graphics	ANTIC, GTIA
Sound	POKEY (4 channels)
Storage	Cassette tape
Ports	Cartridge
Operating System	Atari BASIC
CPU Speed	1.79 MHz
RAM	64 KB