Atari 800XL - How to Tell Your Computer What to Do; Keyboard Wizardry

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MEMORY
(ROMS AND DRAMS)
O.S. ROM
The SOOXL
operating system
is
resident in one
I6K X 8
ROM, The
ROM address
inputs are
from CPU
address
lines AO through
A! 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 ol
+5 volts.
BASIC ROM
The
Atari 800XL has built-in Atari
BASIC (Rev. B) residing
in an 8K X 8
ROM-
Thli
Is
equivalent
to having an
Atari BASIC cartridge
permanentJy
plugged into
the
console. Any cartridge plugged into the
computer,
takes precedence
over the
buUl-ln
BASIC
DRAMS
The SOOXL Dynamic Random Access
Memories
(DRAMS) consist of
eight
6*fK X 1
DRAMS.
MEMORY
MANAGEMENT UNIT (MMU)
The memory
management unit
(MMU)
address decoding
circuitry
consists
of
a
PLA
(Programmable Logic Array )
LC,
one
3 to
8 decoder
and a
few
gates-
The
input
to the
circuitry
Includes the ad*ess
lines
A8
through A13 as
well as
control
signals, such
as ROM enable (from
PIA) and
DRAM
refresh
from
ANTIC-
Some
of
the
most
imporUnt outputs of
this circuitry
include
select
signals for
the
GT1A»
POKEY, PIA,
OS ROM, DRAMS,
BASIC and
cartridge-
800XL Computer
Field Service
Manual
1-9