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INTRODUCTION
FOUR-COMPONENT
ADDRESSING
An example of four-component addressing
(three-component logical address plus
seg-
ment base)
is
shown in Figure
1-15,
and
is
described as follows:
Suppose you're writing a program to com-
pute the payroll for a large corporation. This
corporation has several groups of employees.
Within each group there are multiple em-
ployees, and for each employee certain data
is
kept in a record of information. Included
in this data are the employee's address, social
security number, and a wage code indicating
how much that employee
is
being paid.
The task
at
hand
is
to select the wage code for
a particular employee from the entire com-
plex array of employee data. The
8088
can do
it with a single instruction
after the registers
are set llP. Here's how: First, set the data
segment register to the base of the employee
data, set a base register such
as
BX to contain
the offset number of bytes between the
employee data base address and the start of
the data that applies only to the desired
group of employees. Next
we
set
an
index
register such as SI to index to the desired
employee's information within the given
group of employees. Finally,
we
use an abso-
<
PAYROLL
<
DATA
<
<
lute displacement value to point to the given
employee's wage code within the employee's
data record.
The
single
instruction MOV
AX,
[BX + SI +
12]
then,
will
select the appropriate employee's
wage code. To implement the same function
with any other 8-bit microprocessor would
require multiple instructions to build the
address.
Symmetric Use of Memory
Another
way these powerful addressing
modes work
is
that memory locations can be
used as either source
or
destination operand
of most instructions. A single
8088
instruc-
tion can perform a logical AND between the
contents of a given memory address and an
immediate data value, and store the results
back in the same memory address. The equi-
valent
function
would
take
multiple
instructions on
an
8-bit processor such as
an
8080.
It
is
as
though you can treat any
memory location as a
CPU register for sim-
ple arithmetic and logic operations. Follow-
ing are several operations which can be
performed directly on memory locations.
AND [memory address], 7FH
OR
[BX + SI +
12],
lF80H
ADD
[memory address],
2500
I DISPLACEMENT =
12
(WAGE CODE)
I INDEX
=
SI
(EMPLOYEE #N)
I
BASE
=
BX
(EMPLOYEE GROUP)
ISEGMENT
=
OS
(PAYROLL SEGMENT)
MOVAX,[BX+SI+12];GETWAGE
CODE
Figure 1-15. Four-Component Addressing
Example
1-9