M80C286 datasheet(16/60 Pages) INTEL

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M80C286

271103 – 17

Figure 19 Privilege Levels

Level 0 is the most privileged level Privilege levels

provide protection within a task (Tasks are isolated

by providing private LDT’s for each task) Operating

system routines interrupt handlers and other sys-

tem software can be included and protected within

the virtual address space of each task using the four

levels of privilege Each task in the system has a

separate stack for each of its privilege levels

Tasks descriptors and selectors have a privilege

level attribute that determines whether the descrip-

tor may be used Task privilege effects the use of

instructions and descriptors Descriptor and selector

privilege only effect access to the descriptor

TASK PRIVILEGE

A task always executes at one of the four privilege

levels The task privilege level at any specific instant

is called the Current Privilege Level (CPL) and is de-

fined by the lower two bits of the CS register CPL

cannot change during execution in a single code

segment A task’s CPL may only be changed by con-

trol transfers through gate descriptors to a new code

segment (See Control Transfer) Tasks begin exe-

cuting at the CPL value specified by the code seg-

ment selector within TSS when the task is initiated

via a task switch operation (See Figure 20) A task

executing at Level 0 can access all data segments

defined in the GDT and the task’s LDT and is con-

sidered the most trusted level A task executing a

Level 3 has the most restricted access to data and is

considered the least trusted level

DESCRIPTOR PRIVILEGE

Descriptor privilege is specified by the Descriptor

Privilege Level (DPL) field of the descriptor access

byte DPL specifies the least trusted task privilege

level (CPL) at which a task may access the descrip-

tor Descriptors with DPL e 0 are the most protect-

Only tasks executing at privilege level 0

(CPL e 0) may access them Descriptors with DPL

3 are the least protected (ie have the least re-

stricted access) since tasks can access them when

CPL e 0 1 2 or 3 This rule applies to all descrip-

tors except LDT descriptors

SELECTOR PRIVILEGE

Selector privilege is specified by the Requested Priv-

ilege Level (RPL) field in the least significant two bits

of a selector Selector RPL may establish a less

trusted privilege level than the current privilege level

for the use of a selector This level is called the

task’s effective privilege level (EPL) RPL can only

reduce the scope of a task’s access to data with this

selector A task’s effective privilege is the numeric

maximum of RPL and CPL A selector with RPL e 0

imposes no additional restriction on its use while a

selector with RPL e 3 can only refer to segments at

privilege Level 3 regardless of the task’s CPL RPL

is generally used to verify that pointer parameters

passed to a more trusted procedure are not allowed

to use data at a more privileged level than the caller

(refer to pointer testing instructions)

Descriptor Access and Privilege

Validation

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 The two basic types of segment

accesses are control transfer (selectors loaded into

CS) and data (selectors loaded into DS ES or SS)

DATA SEGMENT ACCESS

Instructions that load selectors into DS and ES must

refer to a data segment descriptor or readable code

segment descriptor The CPL of the task and the

RPL of the selector must be the same as or more

privileged (numerically equal to or lower than) than

the descriptor DPL In general a task can only ac-

cess data segments at the same or less privileged

levels than the CPL or RPL (whichever is numerically

higher) to prevent a program from accessing data it

cannot be trusted to use

An exception to the rule is a readable conforming

code segment This type of code segment can be

read from any privilege level

If the privilege checks fail (eg DPL is numerically

less than the maximum of CPL and RPL) or an incor-

rect type of descriptor is referenced (eg gate de-

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