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SYSTEM ARCHITECTURE
Memory-mapped devices
can
be
protected by
the
standard 80386 segment
and
page protection
mechanisms.
In
addition
to
its memory address space, the
80386 has a 64 kilobyte
I/O
address space.
Devices
mapped
into this space
are
manipulated
with the Input,
Output,
Input
String,
and
Output
String instructions.
The
first two instructions
transfer a byte, word,
or
dword
to
or
from the
EAX
register.
The
latter two instructions transfer
a string
of
bytes, words,
or
dwords to
or
from
memory.
The
80386
I/O
instructions
are
privilege level
sensitive.
In
the Flags register
is
a field called
I/O
Privilege Level
(lOPL),
which defines the
minimum privilege level
at
which the running
task
can
execute
I/O
instructions.
(IOPL
is
loaded from the
TSS
so tasks
can
have different
IOPLs).
For
example, if a task's
10PL
is
I, then
the task
cannot
issue
I/O
instructions except
when
it is
running
at
privilege level I
or
O.
The
.
10PL
mechanism supports multilevel protected
operating systems in which, for example, critical
and
stable kernel procedures
run
at
privilege
level
0,
and
more
volatile
I/O
procedures
run
at
privilege level
I;
in
this case the operating system
has only
to
set
IOPL
to
I when it creates a task.
Because
IOPL
is task-specific, trusted tasks
can
be allowed execute
I/O
instructions while running
application code, allowing them, for example,
to
directly manipulate special devices,
for
which
no
operating system driver
is
available.
3-17
To
perform
direct memory access
(DMA)
I/O,
an
80386 operating system passes a physical
address to the
DMA
controller
and
must
guar-
antee
that
the
target segment(s)
and/
or
page(s)
do
not
move during the transfer.
One
way
to
mark
pages "locked for
I/O"
is
to
use one
of
the
three user-defined page table bits.