Intel MCS48 User Manual page 28

INTRODUCTION
1.2
Programming
a
Microcomputer
1.2.1
Machine Language Programming
A
microprocessor
is
instructed
what
to
do by
programming
it
with a
series of instructions
stored
in
Program Memory. The
processor
fetches these
instructions
one
at
a time
and
performs
the operation
indicated.
These
instructions
must
be
stored
in
a
form
that
the
processor can understand. This format
is
referred to as
Machine
Language. For most
microprocessors
this
instruction
is
a
group
of
8 binary
bits (1's
and
O's)
called
a
word
(also called
a byte
if
the
word
is
8-bits).
Some
instructions require
more
than
one
location
in
Program Memory. To
execute a
multi-byte
instruction,
the processor
must execute
multiple fetches of
program
memory
before
performing
the
instruction.
Because
multi-
byte
instructions
take
more Program
Mem-
ory
and
take longer
to
execute than single
byte
instructions
their
use
is
usually kept to
a
minimum.
A
processor
may
be
programmed
by
writing a
sequence
of instructions
in
the
binary
code
(ones
and
zeros)
which
the
machine can
interpret
directly.
This
is
machine language programming and
it
is
very useful
where
the
program
to
be
written
is
small
and
the application requires that
the designer
have an
intimate
knowledge
of
the microprocessor.
Machine
language pro-
gramming
allows the
user,
because
of his
detailed
knowledge,
to
use
many
program-
ming
"tricks" to
produce
the
most compact
and
efficient
code
possible.
The
following
is
an
example
of
a
machine
language program:
This
program
reads 5
sequential
8-bit
words
in
from an I/O port
and
stores
them
sequentially
in
data
memory. The program
starts
by
initializing
two
registers,
one which determines
where
the data
is
to
be stored
and another which
counts
the
number
of
words
to
be
stored.
When
finished the processor continues
on
to
the next
instructions.
Step Machine
Number
Code
Explanation
1011
1000 Load decimal 32
in
1
0010 0000
register
R0
2 1011 1010
Load
decimal 5
in
3
0000 0101
register
R2
4
0000 1001
Load
Port
1
to
accu-
mulator
5
1010 0000 Transfer contents
of
accumulator
to reg-
ister
addressed by
register
6 0001 1000 Increment
R0
by
1
7 1110 1010
Decrement
register 2
8
0000 0100 by
1,
if
result
is
zero
continue
to
step
9, if
not
go
to step 4
9
10
As you
can
see, writing
machine
instruc-
tions
in
ones and
zeros
can be
very
laborious
and
subject to
error.
It
is
almost
always
more
efficient to
represent
each
8-bits of
machine
language
code
in
a
shorthand format
called
Hexadecimal.
The
term hexadecimal
results
from
the
character
set
used
in
hexadecimal
notation.
Hexadecimal
is
merely an extension
of
the
normal decimal
numbers
by the addition of
the
first
six letters of
the alphabet. This
gives a
total
of
1
6 different
characters.
Each
hexadecimal
"digit"
can represent 16
values or the equivalent
of
four binary
bits;
therefore,
each
8-bit
machine language
word can be represented by
2
hexadecimal
(hex
for short)
digits.
The correspondence
among
the
decimal,
binary,
and hex
number
systems
is
given below:
1-10