Intel MCS48 User Manual page 30

INTRODUCTION
allows
instructions to
be added
or deleted
throughout
the
program
during
debugging
without
requiring that
any jump
addresses be
changed.
The
next
instruction
MOV
@R0,
A
can be
verbalized
as,
Move
to
the data
memory
location
addressed by
RO, the contents
of
the
accumulator.
The
@
sign indicates
an
indirect
operation
whereby
the contents
of
either register
or
register
1
acts as a pointer
to
the data
memory
location to
be operated
on.
The
last
instruction
is
a
Decrement and
Jump
if
Not
Zero
instruction
which
acts
in
combination
with the specified
register
as a
loop
counter.
In this
case
register 2
is
loaded
with
5
initially
and
then
decremented by one
each
time the loop
is
executed.
If
the result of
the
decrement
is
not
zero,
the
program jumps
to
INP and
executes another
input operation.
The
fifth
time
thru
the loop the
result
is
zero
and
execution
falls
through
to
whatever
routine follows the
DJNZ
instruction.
In
addition
to
the
normal
features
provided
by assemblers,
more
advanced assemblers
such
as that for the
MCS-48
offer
such
things
as evaluation
of
expressions
at
assembly
time, conditional
assembly,
and
macro
capability.
1.
Evaluation
of
Expressions
-
Certain
assemblers
allow the
use
of
arithmetic
expressions
and
multiple
symbols
in
the
operand
portion
of instructions.
For instance
the
MCS-48
assembler accepts
instructions
such
as:
ADD
A,
#
ALFA*BETA/2
ALFA
and
BETA
are
two
previously defined
symbols. At assembly time the expression
ALFA*BETA/2
will
be evaluated
and
the
resulting
number
(which
is
the
average
of
ALFA
and
BETA)
will
be
treated as
immediate
data
and designated
as
the
second
byte
of
the
ADD
immediate
instruc-
tion.
This expression
has allowed the
immediate
data
of this
instruction to
be
defined
in
a single
statement
and
eliminated
the
need
for
a third
symbol equal
to
ALFA*BETA/2.
2.
Conditional
Assembly
-
Conditional
as-
sembly
allows the
programmer
to select only
certain portions
of his
assembly language
(source)
program
for
conversion
to
machine
(object)
code
at
assembly
time.
This allows
for instance,
the
inclusion of various
"debug"
routines to be included
in
the
program
during
development. Using
conditional
assembly,
they can
then be
left
out
when
the
final
assembly
is
done.
Conditional assembly
also allows several
versions
of
one
basic
program
to
be
generated by
selecting various portions
of
a
larger
program
at
assembly
time.
3.
Macro's
-
A
macro
instruction
is
essen-
tially
a
symbol which
is
recognized by the
assembler
to
represent a
specific
sequence
of several
standard
instructions.
A
macro
is
a
shorthand
way
of
generating
the
same
sequence
of instructions
at
several locations
in
a
program
without having
to rewrite
the
sequence each
time
it
is
used.
For example, a
typical
macro
instruction
might be
one which
performs a
subtract operation.
The
8048 does
not
have
a subtract
instruction
as
such
but
the operation
can be performed
easily
with
three
instructions:
CPL A
ADD
A,
REG
CPL A
This routine subtracts a
register
from the
accumulator and
leaves the result
in
the
accumulator. This
sequence can be defined
as a
macro
with the
name
SUB
and an
operand which can be
RO
to R7.
To
subtract
R7
from
the
accumulator
then, the
program-
mer
merely has
to write:
SUB
R7
and
the
assembler
will
automatically insert
the three
instructions
above
with
R7
substi-
tuted
for
REG.
Once
the
assembly language source code
is
written
it
can be
converted
to
machine
executable object
code
by passing
it
through
an assembler program.
The MCS-48 assem-
bler
is
a
program which
runs
on
the
8080-
based
Intellec
MDS
system explained
in
the
next
section.
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