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INTRODUCTION
as the base, the logical address can be made
up of many different types of values: it can be
just the immediate data value contained in
the instruction, or, it can be the sum of
an
immediate
data
value, plus a base register,
plus
an
index register.
F
or
the sum
of
the addition to be 20-bits
wide, the segment register value
is
automati-
cally shifted left by four binary bits before it
is
added to the 16-bit logical address. The
result
is
always 20-bits
of
physical address.
Note that since logical addresses are always
16-bits wide, you can address up to 64K bytes
in a given segment without changing the value
ofthe segment base register. In systems that do
not have more than 64K bytes of program plus
64K
bytes of stack, plus 64K bytes
in
each of
two different data areas, it is possible to set the
segment registers at the beginning of the pro-
gram and then forget them. In a system where
the
total amount of memory is 64K bytes or
less, it
is
possible to set all segment registers
equal and have fully overlapping segments.
On the other hand, segment registers are very
useful when you have a large programming
task and you want isolation between your
program code and the
data
area or isolation
between module data and the stack informa-
tion, etc. Segmentation also makes it easy to
build relocatable
and/
or
reentrant programs.
RELOCATABLE AND REENTRANT
PROGRAMS
In many cases, the task
of
relocating
an
8088
program (relocation means having the ability
to run the same program in several different
areas
of
memory without changing the pro-
gram itself) simply requires moving the
program code
and
then adjusting of the code
segment register to point to the base of the
new code area. Since programs can be writ-
ten for the
8088
where branches
or
jumps in
program flow may occur using new locations
1-7
relative only to the instruction pointer, the
program does not care what value
is
kept in
the code segment register. Figure
1-
13
shows
how
an
entire process, consisting of code,
stack and data areas, can
be
relocated.
Likewise in a
reentrant program, a single
program uses
mUltiple data areas. Before the
reentrant code
is
entered the second time, the
data
segment register value
is
changed so that
a different
data
area
is
made available to the
program.
ADDRESSING MODES
Now, let's continue
our
discussion of address-
ing modes, providing more detail about how
addresses are formed.
The
8088
has
24
different addressing modes
to generate logical addresses. Figure
1-14
shows the different logical address combina-
tions, from the simplest immediate
data
mode to the register addressing mode, where
a selected register contains the
data
being
used by the instruction. In the direct address-
ing mode, the instruction itself contains the
address
of
the data. In the register indirect
mode, the instruction points to a register con-
taining the memory address
of
the desired
data. There are both indexed and based
addressing modes where the contents of
an
index
or
based register
is
added to
an
imme-
diate data value contained in the instruction
to
form the memory address.
Exactly how the
8088
selects
an
addressing
mode for a given instruction
is
encoded
within the bits
of
the instruction code. This
is
discussed in more detail in Chapter
2.
If
we
examine the most complex and power-
ful of the addressing modes, which includes
base register, index register, and displace-
ment in the logical address,it can be seen that
some fairly complex
data
structures can be
easily addressed in a single instruction by the
8088.