Intel MCS48 User Manual page 37

THE
SINGLE
COMPONENT
MCS-48™
SYSTEM
2.0
Summary
Sections
2.1
through
2.4
describe
in detail
the functional characteristics
of
the
8748
EPROM,
8048/8049
ROM
and
8035/8039
single
component
microcomputers. Unless
otherwise
noted,
details within these sec-
tions
apply
to
all
versions. Sections
2.5
through
2.11
describe the operation
of
the
8021, while Sections
2.12 through
2.21
de-
scribe the 8022.
This chapter
is
limited to
those functions
useful
in
single-chip imple-
mentations
of
the
MCS-48. Chapter
3
dis-
cusses
functions
which
allow expansion
of
program memory,
data
memory, and
input-
output
capability.
2.1
Architecture
The
following sections break
the
8048
into
functional blocks
and
describe
each
in detail.
2.1.1
Arithmetic Section
The
arithmetic section
of
the processor con-
tains
the basic data manipulation
functions
of
the
8048 and
can be
divided
into
the
following
blocks:
Arithmetic Logic Unit
(ALU)
Accumulator
Carry Flag
Instruction
Decoder
In
a
typical operation data stored
in
the
accumulator
is
combined
in
the
ALU
with data
from another source
on
the
internal
bus
(such
as
a
register or I/O
port)
and
the
result
is
stored
in
the
accumulator
or another
register.
The
following
is
a
more
detailed description
of
the function
of
each
block:
Instruction
Decoder
The
operation
code
(op code)
portion of each
program
instruction
is
stored
in
the
Instruction
Decoder and
converted
to
outputs which
control the function
of
each
of
the blocks
of
the Arithmetic Section.
These
lines
control
the
source
of
data and
the destination
register
as
well as
the function performed
in
the
ALU.
Arithmetic Logic Unit
The
ALU
accepts
8-bit
data
words
from one
or
two sources and
generates an
8-bit result
under
control
of the
Instruction Decoder.
The
ALU
can perform
the following
functions:
Add
With
or Without Carry
AND,
OR,
Exclusive
OR
Increment/Decrement
Bit
Complement
Rotate
Left,
Right
Swap
Nibbles
BCD
Decimal
Adjust
If
the operation performed by
the
ALU
results
in
a
value represented by more
than 8
bits
(overflow
of
most
significant
bit)
a Carry
Flag
is
set
in
the
Program
Status Word.
Accumulator
The
accumulator
is
the single
most
impor-
tant
data
register
in
the processor,
being
one
of the sources
of input to
the
ALU
and
often the destination of the
result of opera-
tions
performed
in
the
ALU.
Data
to
and
from
I/O ports
and
memory
also normally
passes through
the accumulator.
2.1.2
Program Memory
Resident
program memory
consists
of
1024
or
2048
words
eight
bits
wide which
are
addressed by
the
program
counter.
In
the
8748
this
memory
is
user
programmable
and
erasable
EPROM;
in
the 8048/8049
the
memory
is
ROM
which
is
mask
program-
mable
at
the
factory.
The
8035/8039 has no
internal
program
memory
and
is
used
with
external devices.
Program code
is
com-
pletely
interchangeable
among
the various
versions.
See
Section
2.3 for
EPROM
pro-
gramming
techniques.
2-1