Filter
Top Products
inter
PROTECTION
If
the
IP
value
is
not within the limit of the code segment, a general protection fault occurs with an
error code of
O.
If
a CALL instruction
is
used, the return address
is
saved in the normal manner. The
only effect of the call gate
is
to place a different address into CS:IP than that specified in the desti-
nation address of the
JMP
or CALL instruction. This feature
is
useful for systems which require
that
a fixed address be provided
to
programs, even though the entry address for the routine may change
due to different functions, software changes, or segment relocation.
7.5.1.3
INTER-LEVEL CONTROL TRANSFER VIA CALL GATES
If
the destination code segment of the call gate
is
at a different privilege level than the CPL, an inter-
level transfer
is
being requested. However, if the destination code segment DPL > CPL, then a general
protection fault occurs with an error code identifying the destination code segment.
The gate guarantees that all transitions
to
a more privileged level will
go
to a valid entry point rather
than possibly into the middle of a procedure (or worse, into the middle of an instruction).
See
figure
7-11.
Calls to more privileged levels may
be
performed only through call gates. A
JMP
instruction can never
cause a privilege change. Any attempt
to
use a call gate in this manner
will
cause a general protection
fault with an error code identifying the gate. Returns to more privileged levels are also prohibited.
Inter-level transitions due to interrupts use a different gate,
as
discussed in Chapter
9.
The RPL field of the CS selector saved
as
part of the return address
will
always identify the caller's
CPL. This information
is
necessary to correctly return to the caller's privilege level during the return
instruction.
Since the CALL instruction places the CS value
on
the more privileged stack, and
JMP
instructions cannot change privilege levels, it
is
not possible for a program to maliciously place an
invalid return address
on
the caller's stack.
J
COOE
SEG.
DESCR.
J
t
J
J
CALL
OFFSET SELECTOR
OPCOOE
! c
J J
CALL
GATE
_,
c
c
J J
J
! OFFSE
J
ENTER
c
Figure 7-11. Call Gate
7-19
T
INSTRUCTION
DESCRIPTOR
TABLES
TARGET
CODE
SEGMENT
G3010B