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PROTECTION
To
achieve control transfers, a special descriptor type called a gate
is
provided to mediate the change
in privilege level. Control transfer instructions call the gate rather than transfer directly to a code
segment. From the viewpoint
of
the program, a control transfer to a gate
is
the same as to another code
segment.
Gates allow programs to use other programs
at
more privileged levels in the same manner as a program
at
the
same privilege level. Programmers need never distinguish between programs or subroutines
that
are more privileged than the current program and those
that
are not. The system designer may, however,
elect to use gates
only for control transfers
that
cross privilege levels.
7.5.1 Gates
A gate
is
a four-word control descriptor used to redirect a control transfer to a different code segment
in the same or more privileged level or to a different task. There are four types
of
gates: call, trap,
interrupt,
and
task
gates.
The
access rights byte distinguishes a gate from a segment descriptor, and
determines which type
of
gate
is
involved. Figure 7-10 shows the format of a gate descriptor.
A key feature of a gate
is
the re-direction it provides. All four gate types define a new address which
transfers control when invoked. This destination address normally cannot be accessed by a program.
Loading the selector to a call gate into SS, DS, or
ES
will cause a general protection fault with an
error code identifying the invalid selector.
Only the selector portion
of
an address
is
used to invoke a gate. The offset
is
ignored. All
that
a
program need know about the desired function
is
the selector required to invoke the gate.
The
80286
will automatically
start
the execution
at
the correct address stored within the gate.
A
further
advantage of a gate
is
that
it provides a fixed address for any program to invoke another
program.
The
calling program's address remains unaltered even if the entry address of the destination
program changes. Thus, gates provide a fixed set of entry points
that
allow a
task
to access Operating
System functions such as simple subroutines, yet the task
is
prohibited from simply jumping into the
middle
of
the Operating System.
Call gates, as described in the next section, are used for control transfers within a task which must
either be transparently redirected or which require an increase in privilege level. A call gate normally
specifies a subroutine
at
a greater privilege level, and the called routine returns via a return instruction.
Call gates also support delayed binding (resolution of target routine addresses
at
run-time
rather
than
program-generation-time
).
Trap
and interrupt gates handle interrupt operations
that
are to be serviced within the current task.
Interrupt gates cause
interrupTs
io
Ot
ui:sauitd~
trap
gates
do
iict.
Tr:lp
:l!'!d
i!~te!"!"12pt
g~te.s
both require
a return via the interrupt return instruction.
Task
gates are used to control transfers between tasks and to make use of task state segments for task
control
and
status information. Tasks are discussed
in
Chapter
8,
interrupts in
Chapter
9.
In
the
80286 protection model, each privilege level has its own stack. Therefore, a control transfer (call
or
return)
that
changes the privilege level causes a new stack to be invoked,
7-16