Resident Data Memory - Intel MCS48 User Manual

EXPANDED
MCS-48
SYSTEM
PROGRAM MEMORY
SPACE
S355
(2K)
RESIDENT
(1K)
300H
200H
100H
000H
EXTERNAL DATA
MEMORY
SPACE
.
1
8355
I 1
io
I I
8155
F
IO
RESIDENT
DATA
MEMORY
(64)
SECTION ADDRESS DESIGNATION
PROG.
MEM 000— BFF
DATA MEM 100—
IFF
8155
PORTS
300
CMD/STATUS
301
PORTA
302
PORT
B
303
PORTC
304
TIMER
LOW
305
TIMER
HI
8355
PORTS 400
PORTA
401
PORT
B
402
DDR A
403
DDR
B
In this
system
the various locations are ad-
dressed as
follows:
Data
RAM—
Addresses
to
255
when
Port
2
Bit
has been
previously set
=
1
and
Bit
1
set
=
RAM
I/O—
Addresses
to
3
when
Port
2
Bit
=
1
and
Bit
1
=1
ROM
I/O—
Addresses
to
3
when
Port
2
Bit
2
or
Bit
3
=
1
See
the
above
memory
map.
3.5
Memory
Bank
Switching
Certain
systems
may
require
more
than the
4K
words
of
program
memory
which are
directly
addressable by the
program counter
or
more
than the
256
data
memory
and
I/O
locations
directly
addressable by the pointer
registers
R0
and
R1.
These
systems can be
achieved using
"bank
switching" techniques.
Bank
switching
is
merely
the selection of
various blocks or
"banks"
of
memory
using
dedicated output port
lines
from
the processor.
In
the
case
of
the
8048
program
memory
is
selected
in
blocks
of
4K words
at
a time
while
data
memory
and
I/O
are
enabled
256
words
at
a
time.
The most
important consideration
in
imple-
menting two
or
more
banks
is
the software
required
to
cross the
bank
boundaries.
Each
crossing
of
the
boundary
requires that
the
processor
first
write
a
control
bit
to
an
output
port
before accessing
memory
or I/O
in
the
new
bank.
If
program
memory
is
being
switched,
programs
should be organized
to
3-10