Atari 800XL Service Manual

MEMORY (ROMS AND DRAMS)
BASIC
RQM
The
Atari
800XL
has
built-in
Atari
BASIC
(Rev.
B)
residing
in
an
8K X
8 ROM.
This
is
equivalent
to
having
an
Atari
BASIC
cartridge
permanently
plugged
into
the
console.
Any
cartridge
plugged
into
the
computer
takes
precedence
over
the
built-in
BASIC.
DRAMS
The
800XL
Dynamic
Random
Access
Memories
( DRAMS)
consist
of
eight
64K X I
DRAMS.
MEMORY
MANAGEMENT
UNIT (MMUl
The
memory
management
unit
(MMU)
address
decoding
circuitry
consists
of
a
PLA
(
Programmable
Logic Array)
I.C.
,
one
3 to
8
decoder
and
a few
gates. The
input
to the
circuitry
includes
the
address
lines
A8
through
A
1 5 as
well
ss control
signals,
such
as ROM
enable
(
from
PIA)
and
DRAM
refresh
from
ANTIC.
Some of
the
most
important
outputs
of
this
circuitry
include
select
signals
for
the 0TIA,
POKEY,
PIA,
OS
ROM
,
DRAMS,
BASIC
and
cartridge.
Q.5.
RQM
The
800XL
operating
system
is
resident in
one
1 6K X
8
ROM.
The
ROM
address
inputs
are
from
CPU
address
lines
AO
through
A1
3.
The
chip
selects
from
the
address
decoding
circuitry
and
generates
data
on CPU
data
lines
DO
through
D7. The
ROM
requires
a power
input
of
+5
volts.
RF MODULATOR
The
RF
modulator
accepts
the
composite
video
from
the
video
summation
circuitry
and
the
mono-aural
audio
signals
and
produces
a
modulated
signal
suitable
for
the
television.
With
a
75
Ohm
termination,
the
modulated
signal
has
the
following
characteristics:
Maximum
Voltage:
2mV
Minimum
Voltage:
ImV
Audio
Sound
Carrier
Frequency:
4.5
MH2
15

Atari 800XL Specifications

General

Manufacturer	Atari
Type	Home computer
Release year	1983
CPU	MOS Technology 6502C
RAM	64 KB
ROM	24 KB
Operating System	Atari OS
CPU Speed	1.79 MHz (NTSC) / 1.77 MHz (PAL)
Graphics	ANTIC and GTIA chips
Display resolution	160×192 to 320×192 pixels
Sound	POKEY chip
Storage	Cassette tape, floppy disk
Ports	Serial, parallel, monitor, joystick ports