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SYSTEM ARCHITECTURE
A selector
is
an
index into a segment descriptor
table; that
is,
it
is
a segment number. Each entry
in a segment descriptor table contains the base
address
of
a segment. The processor adds the
offset to the segment's base address to produce a
32-bit
linear address. If paging
is
not enabled, the
processor considers the linear address to be the
physical address and emits it on the address pins.
If paging
is
enabled, the 80386 translates the
linear address into a physical address.
It
does this
with the aid
of
page tables. A page table
is
conceptually similar to a descriptor table except
that each page table entry contains the physical
base address
of
a 4 kilobyte page.
Because it embraces both traditional address
space structuring units (segments and, optionally,
pages), and because segments can
be
very large
(up to 4 gigabytes), the 80386's addressing
technique
is
very flexible. An operating system
can provide a task with a single flat address
space, a flat address space that
is
paged, a
LOGICAL ADDRESS
OFFSET
31
LINEAR ADDRESS
DESCRIPTOR
TABLES
PAGE
TABLES
segmented address space,
or
a segmented address
space that
is
paged.
With all its flexibility, the
80386's multistage
address translation facility
is
nevertheless quite
fast. The
80386 typically computes an offset and
translates the resulting logical address to a
physical address
in
1.5
clocks. Moreover, address
translation time
is
not visible to the application
because the 80386's on-chip
MMU
translates
addresses
in
parallel with other processor activ-
ities (except when a
Jump
or
Call instruction
temporarily interrupts pipelining).
3.3.2 Segments
The segment
is
the unit the 80386 provides for
defining a task's logical address space; that
is,
a
task's logical address space consists of one
or
more segments. Operating systems differ sub-
stantially
in
the way in which they define a task's
logical address space.
For
example,
an
embedded
real-time system may define a task's logical
PAGING ENABLED
PHYSICAL ADDRESS
Figure 3-3.
Address
Translation
Overview
3-4