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INTRODUCTION
Decimal
Hex
Binary
0
0 0000
1 1
0001
2 2
0010
3 3
0011
4 4 0100
5 5
0101
6 6 0110
7 7
0111
8 8 1000
9 9 1001
10
A 1010
11
B
1011
12 C 1100
13 0
1101
14 E 1110
15 F
1111
Our
machine language program then
becomes:
Step Hex
Code
o B8
1 20
2 BA
3 05
4 09
5
FO
6 18
7 EA
8 04
This coding
is
now quite efficient to write
and read and coding errors are much easier
to
detect. Hex coding
is
usually very
efficient for
small programs
(a
few hundred
lines
of
code). However, it does have two
major
limitations
in
larger programs:
1.
Hex coding
is
not self-documenting, that
is,
the code itself does not give any indication
in
human terms of the operation to
be
performed. The user must learn each code or
constantly use a Program Reference Card to
convert.
2.
Hex coding
is
absolute, that
is,
the
program
will work only when stored
in
a
specific
location
in
program memory. This
is
because the branch or jump instructions in
the program reference specific addresses
elsewhere in the program.
In
the example
above steps 7
and
8 reference step (or
address)
4.
If the program were to
be
moved,
1·11
step 8 would
have
to
be
changed to refer to
the new address of step
4.
1.2.2 Assembly Language Programming
Assembly language overcomes the dis-
advantages of machine
language by allowing
the
use
of alphanumeric symbols to repre-
sent machine operation codes, branch
addresses, and other operands. For
example,
the instruction to increment the contents of
register
0 becomes INC
RO
instead of the hex
18,
giving the user at a glance the meaning
of
the instruction. Our example program can
be
written
in
assembly language
as
follows:
Step No. Hex
Code
o
88
1 20
2
8A
3 05
4 09 INP:
5
AO
6 18
7 EA
8 04
Assembly
Code
MOV
RO,
#32
MOV
R2,
#05
IN
A,
P1
MOV
@RO,
A
INC
RO
DJNZ
R2,
INP
The first statement
can
be
verbalized
as
follows: Move to Register 0 the decimal
number
32.
Move instructions are always
structured such that the destination
is
first
and
the source
is
second. The pound sign "#"
indicates that the source
is
"immediate" data
(data contained in the
following byte of
program memory).
In
this case data
was
specified
as
a decimal
32,
however, this could
have
been
written
as
a hex
20H
or a binary
0010
00008 since the assembler will accept
either form. Notice
also that in this instance
two
lines of hex code
are
represented by one
line of assemlJly code.
The input instruction
IN
A,
P1
has
the same
form
as
a MOV instruction indicating that the
contents
of
Port 1 are to
be
transferred to the
accumulator.
In
front
of
the input instruction
is
an
address label which
is
delineated by a
colon. This label allows the program to
be
written in a form independent
of
its final
location
in
program memory since
the
branch instruction at the end of the program
can
refer to this label rather than a specific
address. This
is
a very important advantage
of
assembly language programs since it