User's Manual

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