Filter
Top Products
ARCHITECTURE AND INSTRUCTIONS
String Constant
A string constant
is
one or two characters
enclosed with apostrophes. Strings of more
than two characters are permitted in res-
tricted cases, but are
not
discussed here. An
apostrophe itself may be included in a string
constant by writing it as two consecutive
apostrophes. Examples of string constants
are 'A', 'AB', and
"". The last example
is
the
string consisting of the apostrophe character.
The value of a string constant
is
the ASCII
code of the character(s) in the string.
For
example, the value of 'A'
is
41
H and the value
of 'AB'
is
4l42H. Thus, string constants and
whole-number constants can be used inter-
changeably.
COMMENTS
Any sequence
of
characters following a semi-
colon
(;)
up
to
the end
of
the line are com-
ments. They are ignored by the assembler
and
should be used generously in your pro-
gram to document what you are doing. While
comments like
INC CX ;increment CX
convey little information, comments like
INC CX ;increment outer
loop
counter·
make a program more readable.
Expressions
One more building block, namely expres-
sions, must be introduced before
we
can
build statements. Expressions are built up
from some of the tokens just described.
Loosely speaking,
an
expression
is
a sequence
of operands and operators combined to pro-
duce a value
at
program assembly time. How
are operands and operators combined to
produce the value of
an
expression?
OPERANDS
An operand
is
something that has either a
numeric value or a memory address value.
2-25
Operands with numeric values are constants,
or identifiers that represent constants. Some
numeric-valued operands, appearing in
our
sample program are
100
and PORT_VAL.
The permissible range
of
values for such oper-
ands
is
from -65,535 to +65,535.
Note
that
the value of
an
operand may be
negative,
but
a constant
is
never negative. A
minus sign can be written in front
of
a con-
stant,
but
is
never considered a
part
of
the
constant; it
is
an
arithmetic operator.
Memory-address operands are frequently
identifiers, such as
SUM
and CYCLE in the
sample program. The value
of
a memory
address is
not
simply a number; it
is
a set
of
components, each component generally being
a number.
One component
is
the
16
most-
significant bits of the segment starting address
where the memory address
is
contained. The
four least-significant bits of a segment start-
ing address are always zeros.
Another component
is
the offset address
within the segment. These two components
are referred
to
as the segment
and
offset
of
the memory-address operand.
Another operand
is
an
expression itself,
enclosed in parentheses, and used in some
bigger expression,
as
in 3*(PORT_VAL+5).
OPERATORS
An operator takes the value of one
or
more
operands and produces a new value. There
are five kinds of operators in ASM-86
1)
arithmetic operators
2)
logical operators
3)
relational operators
4)
analytic operators
5)
synthetic operators
Arithmetic Operators
Arithmetic operators are the familiar addi-
tion operator (+), subtraction operator
(-),
multiplication operator
(*),
and
division
operator (/). Another arithmetic operator,
MOD,
produces the remainder after doing a