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APPLICATION ARCHITECTURE
REGISTER STACK
79 78
64
63
± EXPONENT SIGNIFICAND
CONTROL
REGISTER
STATUS REGISTER
Figure 2-3. Numeric Coprocessor Registers
2.2
Memory and
logical
Addressing
80386
application programs
use
logical addresses
to specify the locations of operands in a 4-
gigabyte physical address space. The processor
automatically translates these logical addresses
to the physical addresses that it emits on the
system bus.
As
discussed more fully in Chapter
3,
an 80386 operating system can tailor
an
appli-
cation program's view of its logical address
space.
For
example,
an
operating system can
define the logical address space as it
is
defined by
many architectures, as a simple array of 2
32
bytes.
Alternatively,
an
80386 operating system can
organize the logical address space as a collection
of variable-length segments. An operating system
can define many segments or just a
few,
as
appropriate to its view of logical memory; the
80386 does not dictate the
use
of segments,
but
rather allows them to be used as they support
application needs. When reading the following
sections, bear in mind that the extent to which
an
2-3
application program actively uses segments de-
pends on the framework established by the
operating system.
2.2.1
Segments
As
just mentioned,
an
operating system can
define the
80386 logical address space as one
or
more segments. Segments are logical units that
map well to programming structures, which are
inherently variable in length.
For
example, a
l5l6-byte procedure fits exactly into a
1516-byte
segment, as
an
8-megabyte array (for example, a
1028x1028x8
display buffer) fits exactly into a
segment of the same size.
By
providing architec-
tural support for segments (for example, segments
can be individually protected, and can be shared
selectively between tasks), the
80386 improves
the performance of systems that choose segments
as a structuring mechanism. (Pages, which are
described in Chapter
3,
are fixed-size; they
do
not map well to programming constructs, but,
on the other hand, are better-suited to operating
system functions such
as
swapping.)
An
80386 segment can be any
size
from 1 byte to
4 gigabytes.
For
every segment, the operating
system maintains an architecture-defined descrip-
tor
that specifies the attributes of the segment.
Segment attributes include a 32-bit base address
and limit (length), and protection information
that can guard a segment against incorrect use.
Because descriptors are maintained by operating
systems, fuller coverage
of
them
is
deferred to
Chapter
3.
Application programs deal only
indirectly with descriptors, referring to segments
by means of logical addresses.
2.2.2 Logical Addresses
Because a program may potentially refer to
multiple segments, an
80386
logical address must
identify a segment. Therefore,
an
80386 logical
address consists of two parts, a 16-bit segment
selector and a 32-bit offset into the selected
segment (see Figure
2-4).
The selector part of a