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Atari 400 Hardware Manual

Atari 400
113 pages
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Page #15 background image
Bit
7
of
a
display
list
instruction
can
be
set
to
create
a
display
list
interrupt
if
bit
7
of
NMIEN
is
set.
The
display
list
interrupt
code
can
change
the
colors
or
graphics
during
the
middle
of
the
TV
disnlay.
The
type
of
interrupt
is
determined
by
checking
NMTST.
NMIRES
clears
NMIST.
The
current
OS
will
vector
through
VDSLST
(Hex 200 and 201)
to
the
user's
display
list
interrupt
routine.
See
the
JS
manual
for
program-
ming
details.
Bits
5
and
4
of
a
display
type
of
display
list
instruc\
.
on~
~
re
used
to
enable
vertical
and
horizontal
scrolling.
The
amou11t
of
scrolling
depends
on
the
values
in
the
VSCROL
and
HSCROL
registers
(to
be
described
later).
Memory
Scan
Counter:
This
counter
is
not
directly
accessible
by
the
programmer.
It
is
loaded
with
the
value
in
the
last
2
bytes
of
a 3
byte
(non-Jump)
instruction.
This
counter
points
to
the
location
(address)
in
memory
of
data
to
be
directly
displayed
(memory
map
display)
or
to
the
location
of
character
name
strings
to
be
indirectly
displayed
(character
display).
A
single
byte
instruction
does
not
reload
this
counter.
This
implies
a
continuation
in
memory
of
data
to
be
displayed
from
that
displayed
by
the
previous
instruction.
Since
this
counter
really
consists
of
4
bits
of
register
and 12
of
actual
counter,
a
continuous
memory
block
cannot
cross
~
4K
byte
memory
boundaries,
unless
the
counter
is
repositioned
with
a 3
byte
Load Memory
Scan
Counter
instruction.
MSB
third
byte
of
3
byte
byte
instruction
7 6 5 4 3
2 1
\
0
7
Counter
LSB
Second
byte
of
3
byte
instruction
6 5
4
3
2
/
1
0
Memory
Map
Display
Instructions:
Data
in
memory
(addressed
by
the
Memory
Scan
Counter)
is
displayed
directly
when
executing
a memory
(bit)
map
display
instruction.
As
data
is
being
displayed
it
is
also
stored
in
a
shift
register
so
that
it
can
be
redisplayed
for
as
many
TV
lines
as
required
by
the
instruction.
rr.10

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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