KU386 Datasheet, PDF (30/102 Pages) Intel Corporation – SX MICROPROCESSOR

Intel386^TM SX MICROPROCESSOR
Table 4 2 Descriptor Types Used for Control Transfer
Control Transfer Types
Operation Types
Descriptor
Referenced
Intersegment within the same privilege level
JMP CALL RET IRET Code Segment
Intersegment to the same or higher privilege level
Interrupt within task may change CPL
CALL
Interrupt instruction
Exception External
Interrupt
Call Gate
Trap or
Interrupt
Gate
Intersegment to a lower privilege level
(changes task CPL)
RET IRET
Code Segment
CALL JMP
Task State
Segment
Task Switch
CALL JMP
Task Gate
NT (Nested Task bit of flag register) e 0
NT (Nested Task bit of flag register) e 1
IRET
Interrupt instruction
Exception External
Interrupt
Task Gate
Descriptor
Table
GDT LDT
GDT LDT
IDT
GDT LDT
GDT
GDT LDT
IDT
I O Privilege
The I O privilege level (IOPL) lets the operating sys-
tem code executing at CPL^e0 define the least privi-
leged level at which I O instructions can be used An
exception 13 (General Protection Violation) is gener-
ated if an I O instruction is attempted when the CPL
of the task is less privileged then the IOPL The
IOPL is stored in bits 13 and 14 of the EFLAGS reg-
ister The following instructions cause an exception
13 if the CPL is greater than IOPL IN INS OUT
OUTS STI CLI LOCK prefix
Descriptor Access
There are basically two types of segment accesses
those involving code segments such as control
transfers and those involving data accesses Deter-
mining the ability of a task to access a segment in-
volves the type of segment to be accessed the in-
struction used the type of descriptor used and CPL
RPL and DPL as described above
Any time an instruction loads a data segment regis-
ter (DS ES FS GS) the Intel386 SX Microprocessor
makes protection validation checks Selectors load-
ed in the DS ES FS GS registers must refer only to
data segment or readable code segments
Finally the privilege validation checks are performed
The CPL is compared to the EPL and if the EPL is
more privileged than the CPL an exception 13 (gen-
eral protection fault) is generated
The rules regarding the stack segment are slightly
different than those involving data segments In-
structions that load selectors into SS must refer to
data segment descriptors for writeable data seg-
ments The DPL and RPL must equal the CPL of all
other descriptor types or a privilege level violation
will cause an exception 13 A stack not present fault
causes an exception 12
PRIVILEGE LEVEL TRANSFERS
Inter-segment control transfers occur when a selec-
tor is loaded in the CS register For a typical system
most of these transfers are simply the result of a call
or a jump to another routine There are five types of
control transfers which are summarized in Table 4 2
Many of these transfers result in a privilege level
transfer Changing privilege levels is done only by
control transfers using gates task switches and in-
terrupt or trap gates
Control transfers can only occur if the operation
which loaded the selector references the correct de-
scriptor type Any violation of these descriptor usage
rules will cause an exception 13
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