Atari 400

Central Processing
Unit
The
Centrai Processing
Unit (CPU)
PCB
is the controller
of the entire
Console system.
The CPU PCB
contains the
6502 microprocessor,
and the ANTIC
and CTIA
(or GTIA)
chips. The CPU PCB
controls the
Console system
and its
peripheral
devices
through
address lines
(to select which
device it
needs to
communicate with)
and
data lines (to
transmit
and/or receive
data from
a selected
device) common
to the
entire system.
Operating
instructions
for
the microprocessor
come
from the ROM
Operating
System
on the Personality
PCB,
Additional
functions
of the
CPU
PCB
are:
•
deceives
the master
clock from
the motherboard
and
generates
Phase 1 (/I
or
Ph
1)
and Phase
2
(/2
or
Ph
2) clocks used to
synchronize
the
entire
system.
•
Transmits
a
REFRESH
signal
at
least
every
2 miliseconds
to
refresh the
dynamic RAM
chips
on the RAM
PCB(s).
•
Receives
the
four TRIGGER
lines from
the fire
button
on each
of the hand
controller
accessories.
•
Receives
the
lines from
the four
control
switches
located
to the right
of
the keyboard-
•
Generates
video
signals
to be
processed
by the
motherboard
before
they
are
sent to the RF
module
on the
Power Supply
PCB.
ROM
Personality
PCB
The ROM
Personality
PCB
contains
information
in
Read-Only
Memory
(ROM),
the
program
of operating
instructions
for
the
microprocessor.
Two
*K ROMs
contain
the
Operating
System,
and
one
2K ROM
contains
the
arithmatic
functions
used
for BASIC
programming.
Information
is
retrieved
from
the ROMs
by
addressing
a particular
location
on the ROM
using
the
Address Bus.
The
data
contained
at that
location
is
placed
on the
Data Bus
to be read
by the
microprocessor.
The
ROM
Personality
PCB
also provides
the CHIP
SELECT
signals
used to
select LSI
chips
throughout
the
Console
system
and for
the
bi-directional
data buffers
on the
motherboard.
RAM
PCB
The
Random
Access
Memory
(RAM)
PCB
performs
the
function
of
temporary
data
storage
for the
system.
The RAM
is
dynamic,
requiring
REFRESH,
and is available
in
SK
or 16K
versions.
Each
RAM
chip
on
the RAM PCB
has
only seven
address lines.
To
address
16K
locations
requires
U address
lines. To
accomplish
this,
a I*-bit
address
is
ATARI
Home
Computers
1-5