ARCHITECTURE AND INSTRUCTIONS
and OFFFFH
(16
bits of l's) if the relation-
ship
is
true.
Using a relational operator:
MOV BX,PORT_VAL
LT
5
The assembler will assemble
MOV
BX,OFFFFH
if
the value of PORT_VAL
is
<
5;
otherwise the assembler
will
assemble
MOV
BX,O
At first it may appear that relational opera-
tors are not useful. It's not often that you
want to generate
an
instruction with a field
that
contains either 0
or
OFFFFH, and no
other choices. However, by combining rela-
tional operators with logical operators, the
two relational results
of
0 and OFFFFH can
be molded into any numeric values you
desire:
MOV BX,((PORT_VAL
LT
5)AND
20)
&
OR
«PORT_VAL
GE5)
AND
30)
will
assemble
MOV BX,20
if PORT_VAL
is
less than
5,
and
MOV BX,30
otherwise.
Note the generous use of parentheses to force
the order that operators are applied.
If
you
always use parentheses to make the ordering
explicit, you won't have to memorize the
rules about which operators get evaluated
first.
Analytic and Synthetic Operators
The analytic operators decompose memory-
address operands into their components,
while synthetic operators build memory-
address operands from their components. A
discussion of these operators
is
presented
after
we
learn more about memory-address
operands.
(see
page
2-30)
2-27
Statements
There are two kinds. of ASM-86 program
statements: instruction statements
(MOV,
ADD,
JMP,
etc.) and directive statements
(DB, SEGMENT, EQU, etc.)
Each instruction statement causes the assem-
bler to generate
an
instruction in the object
code. Directive statements tell the assembler
what kind
of
code to generate for succeed-
ing instruction statements.
The
directive
statement
DB ?
tells the assembler that MY_
PLACE
IS
defined as a byte. The assembler allocates a
memory address for
MY_PLACE. Later,
when the assembler encounters the instruc-
tion statement
INC MY_PLACE
it will generate an object code instruction to
increment the contents of
MY_ PLACE.
Because
of
the previously-encountered direc-
tive statement, the assembler will know to
place a
'0' (to indicate a byte) in the w field
of
the increment instruction.
The formats
of
the two kinds of statement are
similar. The instruction statements are
of
the
form
label:
mnemonic argument,,,.,argument ;comment
The directive statements are of the form
name directive argument,
...
,argument ;comment
The label in
an
instruction statement
is
fol-
lowed by a colon, whereas the name in a
directive statement
is
not. This highlights
the difference between the two kinds
of
statements.
A label associates a symbolic name with the
location of
an
instruction. A label can
be
used as
an
operand in a
jump
or
call
instruction.
The name in a directive statement has no
relation to
an
instruction location and can
never
be
jumped to.
