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80386
31
12
11
10
9
8
7
6 5
4
3 2 1
0
OS
U
R
PAGE
FRAME ADDRESS
31
..
12 RESERVED
0
0
D
A 0 0
-
-
P
S W
Figure 4-21. Page Table Entry (Points
to
Page)
4.5.2.4 PAGE TABLES
Each
Page
Table is 4K bytes and holds
up
to
1024
Page
Table Entries.
Page
Table Entries contain the
starting address of the page frame and statistical
information about the page (see Figure 4-21). Ad-
dress bits A12-A21
are
used
as
an
index to select
one of the 1024
Page
Table Entries. The
20
upper-
bit page frame address
is
concatenated with the
lower 12 bits of the
linear address to form the physi-
cal address.
Page
tables can
be
shared between
tasks
and
swapped to disks.
4.5.2.5 PAGE DIRECTORY/TABLE ENTRIES
The lower 12 bits of the
Page
Table Entries and
Page
Directory Entries contain statistical information
about pages
and
page tables respectively. The P
(Present) bit 0 indicates if a
Page
Directory or
Page
Table entry can
be
used
in
address translation. If
P = 1 the entry can
be
used for address translation
if
P=
0 the entry can not
be
used for translation,
and all of the other bits are available for use by the
software. For example the remaining
31
bits could
be used to indicate where
on
the disk the page is
stored.
The A (Accessed) bit
5,
is
set
by
the 80386 for both
types of entries before a read or write access occurs
to
an
address covered by the entry. The D (Dirty) bit
6 is set to 1 before a write to
an
address covered by
that page table entry occurs. The D bit
is
undefined
for
Page
Directory Entries. When the
P,
A and D bits
are
updated by the 80386, the processor generates
a Read-Modify-Write cycle which locks the bus and
prevents conflicts with other processors or perpheri-
als.
Software which modifies these bits should use
the
LOCK
prefix to ensure the integrity of the page
tables
in
multi-master systems.
The 3 bits marked
OS
Reserved
in
Figure 4-20 and
Figure
4-21
(bits
9-11)
are
software definable. OSs
are
free to
use
these bits for whatever purpose they
wish.
An
example
use
of the
OS
Reserved bits
would
be
to store information about page aging.
By
keeping track of how long a page has been
in
mem-
ory since being accessed,
an
operating system can
implement a page replacement algorithm like Least
Recently Used.
The (User/Supervisor)
U/S
bit 2
and
the (Read/
Write)
R/W
bit 1 are used to provide protection attri-
butes for individual pages.
53
4.5.3 Page Level Protection
(R/W,
U/S
Bits)
The 80386 provides a set of protection attributes for
paging systems. The paging mechanism
distin-
guishes between two levels of protection: User
which corresponds to level 3 of the segmentation
based protection, and supervisor which encompass-
es
all
of the other protection levels
(0,
1,
2).
Pro-
grams executing at Level
0,
1 or 2 bypass the page
protection, although segmentation based protection
is still enforced
by
the hardware.
The
U/S
and
R/W
bits
are
used to provide
Us-
er/Supervisor and Read/Write protection for individ-
ual
pages or for
all
pages covered by a
Page
Table
Directory Entry. The
U/S
and R/W bits
in
the first
level
Page
Directory Table apply to all pages de-
scribed
by
the page table pointed to by that directory
entry. The
U/S
and
R/W
bits
in
the second level
Page
Table Entry apply only to the page described
by
that entry. The
U/S
and
R/W
bits for a given
page
are
obtained by taking the most restrictive of
the
U/S
and
R/W
from the
Page
Directory Table
Entries
and
the
Page
Table Entries and using these
bits to address the page.
Example: If the
U/S
and
R/W
bits for the
Page
Di-
rectory entry were
10
and the U/S
and
R/W
bits for
the
Page
Table Entry were
01,
the access rights for
the page would
be
01,
the numerically smaller of the
two. Table 4-4 shows the affect of the
U/S
and
R/W
bits
on
accessing memory.
Table 4-4. Protection Provided
by
R/W
and
U/S
U/S
R/W
Permitted
Permitted Access
Level 3
Levels
0,
1,
or
2
0 0 None
Read/Write
0 1 None
Read/Write
1
0 Read-Only
Read/Write
1 1 Read/Write
Read/Write
However a given segment can
be
easily made read-
only for level
0,
1,
or 2 via the
use
of segmented
protection mechanisms. (Section 4.4 Protection).