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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
@RO,
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 0 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
assemblel'3,
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.
1-12
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 beonewhich
performs a subtract operation. The
8048
does
not
have
a subtract instruction
as
such but
the operation
can
be
performed easily with
three instructions:
CPLA
ADD
A,
REG
CPLA
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.