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Atari 400 Hardware Manual

Atari 400
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II.
DESCRIPTION
OF
HARDWARE
A.
ANTIC
AND
CTIA
TV
Display:
The
ANTIC
and
CTIA
chips
generate
the
television
display
at
the
rate
of
60
frames
per
second
on
the
NTSC
(US)
system.
The
PAL
(European)
system
is
different
and
is
described
in
the
section
on
NTSC
vs
PAL.
Each
frame
consists
of
262
horizontal
TV
lines
and
each
line
is
made up
of
228
color
clocks,
as
shown
in
figure
VI-3.
The 6502
microprocessor
runs
at
1.79
MHz.
This
rate
was
chosen
so
that
one
machine
cycle
is
equivalent
in
length
to
two
color
clocks.
One
clock
is
approximately
equal
in
width
to
two
TV
lines.
In
any
graphics
mode,
the
display
is
divided
up
into
small
squares
or
rectangles
called
pixels
(picture
elements).
The
highest
resolution
graphics
mode
has
a
pixel
size
of
1/2
color
clock
by
1
TV
line.
A
sample
display
list
is
given
in
section
IV.
The
current
TV
line
may
be
determined
by
reading
the
vertical
counter
(VCOUNT).
This
register
gives
the
line
count
divided
by
2.
There
are
262
lines
per
frame
so
VCOUNT
runs
from 0
to
130 (0
to
155 on
the
PAL
system).
The 0
point
occurs
near
the
end
of
vertical
blank
(see
figure
VI.5).
Vertical
blank
(VBLANK)
is
the
time
during
which
the
electron
beam
returns
back
to
the
top
of
the
screen
in
preparation
for
the
next
frame.
The
Atari
800
does
not
do
interlacing,
so
each
frame
is
identical
unless
the
program
which
is
being
executed
changes
the
display.
Vertical
sync
(VSYNC)
occurs
during
the
fourth
through
sixth
lines
of
vertical
blank
(VCOUNT
•
hex
7D
through
7F).
This
tells
the
TV
set
where
each
frame
starts.
After
VSYNC,
there
are
16 more
lines
of
VBLANK
for
a
total
of
22
lines
of
VBLANK.
The
display
list
jump and
wait
instruction
(to
be
described
later)
causes
the
display
list
graphics
to
start
at
the
end
of
VB
LANK.
Operating
System
(OS): The
ATARI
400/800
comes
with
a
lOK
Operating
System
(OS)
in
ROM.
The
OS
affects
some
of
the
hardware
registers,
so
it
will
be
mentioned
from
time
to
time
in
this
manual.
Refer
to
the
OS
manual
for
more
details.
The
OS
descriptions
in
this
manual
apply
to
the
version
that
was
being
distributed
when
this
manual
was
written.
The
OS
supports
most
of
the
hardware
graphics
modes (BASICS,
GRAPHICS,
PLOT,
and
DRAWTO
commands). The
OS
always
displays
24
background
lines
after
the
end
of
vertical
blank.
This
convention
is
used
at
Atari
to
compensate
for
television
sets
which
overscan.
Most
TV's
are
designed
so
that
the
edges
of
the
picture
are
cut
off.
This
is
fine
for
ordinary
broadcasts,
but
with
a
computer
it
is
essential
for
all
important
information
to
be
displayed
on
the
screen.
It
is
fairly
common
for
four
to
eight
color
clocks
at
the
right
or
left
edge
of
the
picture
to
overscan.
A
TV
set
that
has
excessive
overscan
may
have
to
readjusted
to
obtain
a
satisfactory
display.
II.1

Table of Contents

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Atari 400 Specifications

General IconGeneral
ManufacturerAtari
Model400
TypeHome Computer
Release Year1979
CPUMOS Technology 6502
CPU Speed1.79 MHz
ROM10 KB
Operating SystemAtari OS
KeyboardMembrane keyboard
RAM8 KB (expandable to 48 KB)
GraphicsANTIC and GTIA chips
Sound4 channels
DisplayRF output for connection to TV
StorageOptional cassette tape drive or floppy disk drive
PortsCartridge
Display Resolution320x192 (16 colors)

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