80C286 datasheet(21/65 Pages) RENESAS

21 / 65 page

80C286

FN2947 Rev.3.00

Page 21 of 65

January 28, 2008

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 defined 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 control

transfers through gate descriptors to a new code segment

(See Control Transfer). Tasks begin executing at the CPL

value specified by the code segment selector within TSS when

the task is initiated via a task switch operation (See Figure 18).

A task executing at Level 0 can access all data segments

defined in the GDT and the task's LDT and is considered 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

descriptor. Descriptors with DPL = 0 are the most protected.

Only tasks executing at privilege level 0 (CPL = 0) may access

them. Descriptors with DPL = 3 are the least protected (i.e. have

the least restricted access) since tasks can access them when

CPL = 0, 1, 2, or 3). This rule applies to all descriptors, except

LDT descriptors.

Selector Privilege

Selector privilege is specified by the Requested Privilege Level

(RPL) field in the least significant two bits of a selector. Selec-

tor 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 = 0 imposes no additional restriction

on its use while a selector with RPL = 3 can only refer to seg-

ments 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 instruc-

tions).

Descriptor Access and Privilege Validation

Determining the ability of a task to access a segment 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 access

data segments at the same or less privileged levels than the

CPL or RPL (whichever is numerically higher) to prevent a pro-

gram from accessing data it cannot be trusted to use.

An exception to the rule is a readable conforming code seg-

ment. This type of code segment can be read from any privi-

lege level.

If the privilege checks fail (e.g. DPL is numerically less than the

maximum of CPL and RPL) or an incorrect type of descriptor is

referenced (e.g. gate descriptor or execute only code segment)

exception 13 occurs. If the segment is not present, exception

11 is generated.

Instructions that load selectors into SS must refer to data seg-

ment descriptors for writable data segments. The descriptor

privilege (DPL) and RPL must equal CPL. All other descriptor

types or a privilege level violation will cause exception 13. A

not present fault causes exception 12.

TABLE 13. DESCRlPTOR TYPES USED FOR CONTROL TRANSFER

CONTROL TRANSFER TYPES

OPERATION TYPES

DESCRIPTOR

REFERENCED

DESCRIPTOR

TABLE

Intersegment within the same privilege levels

JMP, CALL, RET, lRET (Note 4) Code Segment

GDT/LDT

Intersegment to the same or higher privilege level interrupt

within task may change CPL

CALL

Call Gate

GDT/LDT

Interrupt Instruction, Exception

External Interrupt

Trap or Interrupt Gate lDT

Intersegment to a lower privilege level (changes task CPL) RET, IRET (Note 4)

Code Segment

GDT/LDT

Task Switch

CALL, JMP

Task State Segment

GDT

CALL, JMP

Task Gate

GDT/LDT

lRET (Note 5)

Interrupt Instruction, Exception

External Interrupt

Task Gate

IDT

NOTES:

4. NT (Nested Task bit of flag word) = 0

5. NT (Nested Task bit of flag word) = 1

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