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SYSTEM
ARCHITECTURE
infrequently, a virtual memory system
will
per-
form nearly as
well
as one with far more memory
at a fraction
of
the cost. The
key
architectural
features needed to efficiently support virtual
memory are:
o A bit for each segment or page that tells the
processor (or memory management unit) if the
segment
or
page
is
"present" in memory or
needs to be swapped in from disk.
o A trap or exception mechanism by which the
processor can notify the operating system to
swap in a not-present segment
or
page.
o Restartable instructions that enable the proces-
sor to retry
an
instruction after the operating
system has loaded the formerly not-present
page into memory and marked it present.
The 80386 has all of these necessary facilities,
plus others that improve the efficiency of virtual
memory management. Both descriptors and
page table entries have a Present bit, and
therefore can be used as the basis
of
a virtual
memory design. Swapping segments between
memory and disk
is
a reasonable approach when
ATTRIBUTES
the segments are relatively small as they are in
l6-bit architectures. When segments can be very
large,
as
they can on the
80386,
swapping pages
is
usually a more effective approach, due to the
fixed
size
of
pages. In a page-based system, the
operating system allocates and frees memory in
page-sized units called page frames; a page
swapped
in
from disk will
fit
into any available
frame. Because most 32-bit virtual memory
systems are page-based, the remainder of this
section describes the
80386's page-based virtual
memory support.
In general, a page-based virtual memory operat-
ing system transfers not-present pages from disk
to page frames on demand, that
is,
when notified
by
the processor that an instruction refers to a
not-present page. When the number offree page
frames runs
low,
the operating system also
transfers pages from page frames to disk, at-
tempting to remove the pages that are least likely
to
be referenced in the near future.
By
trans-
parently swapping pages between page frames
and disk, the operating system gives application
software the illusion of a physical memory that
is
as large as the swap area on the disk. The details
of these operations are described
below.
PRESENT
----------------------,
RIGHTS--------------------,
PRIVILEGE------------------,
ACCESSED--------------,
I
:::.-D-EF-IN-E-D-)--------l--~l
I
PAGE
ADDRESS
I I I I I I
Figure
3-7.
Principal Page Table Entry Fields
3-9