H.
Accessing
the
Full
RAM
Potential
of
the 130XE
The 130XE contains
131
,072 bytes of Random Access Memory
(RAM):
twice
as
much memory as the
ATARI
65XE or the 800XL Computer.
The
extra 65,536 bytes of
RAM
is
transparent to the user
in
most
instances. Software programs may use the extra memory to store
larger databases. And if you use
an
ATARI
1050 Disk Drive with
ATARI
DOS
2.5,
you
can use the extra memory as a
RAM
DISK-a
very fast disk drive. (For more information
on
the 130XE and the
DOS
2.5
RAM
DISK, see the
DOS
2.
5:
ATARI
1050 Disk Drive Owner's
Manual .)
However, you can access and use the extra 65,536 bytes of
RAM
with
ATARI
BASIC through a " bank
switching"
method.
The
two processor
chips inside the
130XE-the
6502 central processor and the ANTIC
video
processor-were
designed to access only 65,536 bytes of
RAM.
When the bank switching method is used, the computer
is
coerced
into using more memory. Bank switching turns off one 16K section
of
RAM
and uses some of another section
in
its place.
In
the 130XE, the
second bank of memory
is
located from memory location 16384 to
32767
in
the memory map
(in
he
xadecimal, the numbers $4000 to
$7FFF).
By
changing the bank switch, the computer can select which
bank of memory
will occupy that 16K area.
The
bank switch
is
located
in
memory location 54017. This location
is
used
as
the port B address of the 6520 Peripheral Interface Adapter
chip, which
controls the computer's input and output. Half of this
memory
location was used by the computer previously.
Bits 4 and 5 select which of the computer's processors can access
the extra bank.
Normally the bits are set to a
1.
(Bit 4 enables the
CPU
banking and Bit 5 enables the Video banking.)
By
changing one bit to a
0,
the 6502 processor gains access to the extra
RAM,
and the other
bit gives access to the
ANTIC video chip. Changing both bits lets both
processors use the extra memory.
Bits 2 and 3
control which section of the extra memory
is
used. There
are four
possible combinations of the two bits, and four different 16K
memory sections from which to choose.
121
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