Filter
Top Products
ARCHITECTURE AND INSTRUCTIONS
division. Thus 19/7 is2, whereas
19
MOD7
is
5.
Arithmetic operators may always be applied
to a pair of numeric operands, and the result
will be numeric. The rules for applying
arithmetic operators on memory-addressing
operands are more restrictive: such opera-
tions are valid only
if
the result has a
meaningful physical interpretation.
For
example, the product
of
two memory
addresses has no meaningful interpretation.
What segment would it be in? What offset
would it have? Hence, it
is
a prohibited
operation.
The difference of two memory addresses in
the same segment
is
the numeric distance
between them - the difference in their offsets.
The only other meaningful arithmetic opera-
tion
on
a memory address
is
adding or
subtracting a numeric value. Thus
SUM+2,
CYCLE-5, and
NOLDONE-GO
would all
-be valid expressions in the sample program.
SUM
-CYCLE
would not be a valid expres-
sion because they are
in
different segments.
NOTE: The value of
SUM+2
is
a memory
address two bytes beyond
SUM
in the MY
_ DATA segment; it
is
not
the numeric value
that
is
2 plus-the-contents-of-Iocation-SUM.
Such contents are not known until program
execution, whereas expressions are evaluated
at assembly time.
logical
Operators
The logical operators are bit-by-bit AND,
OR,
XOR
(exclusive-or), and NOT.
The operands of logical operators must be
numeric only - memory-address operands
are
not
allowed - and the result will be
numeric. This
is
shown
by:
1010101
010101010B
AND
11001100110011
OOB
is
1000100010001000B;
1100110011001100B
OR
1111000011110000B
is 1111110011111100B
NOT
1111111111111111 B is
OOOOOOOOOOOOOOOOB
and
1111000011110000B
XOR
SUM
is
invalid.
As
an
example
of
logical operators, consider:
IN AL,PORT_VAL
OUT
PORT_VAL
AND
OFEH,AL
The
IN
instruction gets input from
PORT
_VAL,
wherever that
is.
Execution
of
the
OUT
instruction sends out-
put
to
port
PORT3
AL
AND
OFEH, which
is
either the same port, if PORT_VAL
is
even,
or
the next lower-numbered port, if
PORT
_VAL
is
odd. The actual port value of the
OUT
instruction
is
determined when the
instruction
is
assembled,
not
when it
is
executed.
Observe that AND,
OR, XOR, and
NOT
are
instruction mnemonics as well as
ASM-86
operators.
As
ASM-86 operators, they cause
a value to be computed when the program
is
being assembled.
As
instruction mnemonics,
they perform their roles when the program
is
being executed:
AND OX,PORT_VAL AND
OFEH
will
cause the assembler to compute the value
of PORT_VAL
AND
OFEH and then gener-
ate an AND-immediate instruction contain-
ing
that
value in its data field. When this
instruction
is
later executed, it will cause the
contents
of
the
DX
register to be ANDed
with that value and the result placed in the
DX register.
Relational Operators
2-26
1)
Equal (EQ)
2)
not-equal (NE)
3) less-than (L T)
4)
greater-than (GT)
5)
less-than-or-equal (LE)
6)
greater-than-or-equal (GE)
PORT_VAL
LT
5
is
a relational operator.
The two operands must both be numeric
or
must
both
be memory addresses in the same
segment. The result
is
always a numeric
value.
It
will
be 0
if
the relationship
is
false,