376 datasheet(28/95 Pages) INTEL

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376 EMBEDDED PROCESSOR

by control transfers through gate descriptors to a

code segment with a different privilege level Thus

an application program running at PLe3 may call an

operating system routine at PLe1 (via a gate) which

would cause the task’s CPL to be set to 1 until the

operating system routine was finished

Selector Privilege (RPL)

The privilege level of a selector is specified by the

RPL field The selector’s RPL is only used to estab-

lish a less trusted privilege level than the current

privilege level of the task for the use of a segment

This level is called the task’s effective privilege level

(EPL) The EPL is defined as being the least privi-

leged (numerically larger) level of a task’s CPL and a

selector’s RPL The RPL is most commonly used to

verify that pointers passed to an operating system

procedure do not access data that is of higher privi-

lege than the procedure that originated the pointer

Since the originator of a selector can specify any

RPL value the Adjust RPL (ARPL) instruction is pro-

vided to force the RPL bits to the originator’s CPL

IO Privilege

The IO privilege level (IOPL) lets the operating sys-

tem code executing at CPLe0 define the least privi-

leged level at which IO instructions can be used An

exception 13 (General Protection Violation) is gener-

ated if an IO instruction is attempted when the CPL

of the task is less privileged than 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 and LOCK prefix

Descriptor Access

There are basically two types of segment acces-

sess those involving code segments such as con-

trol transfers and those involving data accesses

Determining the ability of a task to access a seg-

ment involves the type of segment to be accessed

the instruction 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 80376 makes protection

validation checks Selectors loaded 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 32

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

CALL GATES

Gates provide protected indirect CALLs One of the

major uses of gates is to provide a secure method of

privilege transfers within a task Since the operating

system defines all of the gates in a system it can

ensure that all gates only allow entry into a few trust-

ed procedures

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