Atari 400

ROM Personality
Board
The ROM
Personality
printed circuit board
plugs into connector
Jack 3107
and
occupies
memory
locations D800 thru FFFF. The
Personality
board contains the
operating system on three
ROM
chips, two 4K
chips and
one 2K
chip
(see Figures
1-13
and 1-U). The
two *K ROM chips (A^Ol
and
A403)
contain
the I/O subsystem,
interrupt
processing,
initialization
Power-Up
and
Reset.
The 2K ROM
chip (AW2)
contains the
floating point arithmetic
package.
•
I/O
Subsystem
The I/O
subsystem
contained in the ROM
Operating System
provides
a
high-level
interface
between the
user programs
and the Console
and
peripheral hardware. All
peripheral
devices
capable of
dealing with
character data have
symbolic names
(such as K,
D,
P,
E)
and
may
be
accessed
using
a
Central I/O (CIO)
routine.
•
Interrupt
Processing
All
hardware
interrupts
are handled
by the interrupt
subsystem in the
Operating System.
Vectored
addresses
contained
in
RAM
memory
point
to
subroutines
in ROM
to handle
each type of
interrupt.
• Initialization
There
are two levels
of initialization
provided
by
the system, Power-Up
and Reset.
Power-Up
initialization
is
performed
each time
the system
power
is turned
on, and Reset
initialization
is
performed
each time the
Reset
key is
pressed.
Whenever
the
system power
is turned
on, the Operating
System
examines
and notes
the
configuration
of the
unit. The
Operating
System
performs
the
following
actions
at
Power-Up:
Determines
the highest
available RAM
address
Clears
all RAM
to zeroes
Establishes
all RAM
interrupt
vectors
Initializes
the ROM
Cartridges
Sets-up
the
video
screen
(2^ x
i4-0
text mode)
Boots
the
cassette
if desired
Checks ROM
Cartridge
for
disk boot
instructions
Boots
the disk
if desired
and a disk drive
is attached
Transfers
control
to
the ROM
cartridge,
and booted
program
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