M80C286 datasheet(17/60 Pages) INTEL

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M80C286

scriptor 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 segment descriptors for writable data seg-

ments 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

CONTROL TRANSFER

Four types of control transfer can occur when a se-

lector is loaded into CS by a control transfer opera-

tion (see Table 10) Each transfer type can only oc-

cur if the operation which loaded the selector refer-

ences the correct descriptor type Any violation of

these descriptor usage rules (eg JMP through a call

gate or RET to a Task State Segment) will cause

exception 13

The ability to reference a descriptor for control trans-

fer is also subject to rules of privilege A CALL or

JUMP instruction may only reference a code seg-

ment descriptor with DPL equal to the task CPL or a

conforming segment with DPL of equal or greater

privilege than CPL The RPL of the selector used to

reference the code descriptor must have as much

privilege as CPL

RET and IRET instructions may only reference code

segment descriptors with descriptor privilege equal

to or less privileged than the task CPL The selector

loaded into CS is the return address from the stack

After the return the selector RPL is the task’s new

CPL If CPL changes the old stack pointer is popped

after the return address

When a JMP or CALL references a Task State Seg-

ment descriptor the descriptor DPL must be the

same or less privileged than the task’s CPL Refer-

ence to a valid Task State Segment descriptor caus-

es a task switch (see Task Switch Operation) Refer-

ence to a Task State Segment descriptor at a more

privileged level than the task’s CPL generates ex-

ception 13

When an instruction or interrupt references a gate

descriptor the gate DPL must have the same or less

privilege than the task CPL If DPL is at a more privi-

leged level than CPL exeception 13 occurs If the

destination selector contained in the gate refer-

ences a code segment descriptor the code seg-

ment descriptor DPL must be the same or more priv-

ileged than the task CPL If not Exception 13 is is-

sued After the control transfer the code segment

descriptors DPL is the task’s new CPL If the desti-

nation selector in the gate references a task state

segment a task switch is automatically performed

(see Task Switch Operation)

The privilege rules on control transfer require

JMP or CALL direct to a code segment (code

segment descriptor) can only be to a conforming

segment with DPL of equal or greater privilege

than CPL or a non-conforming segment at the

same privilege level

interrupts within the task or calls that may

change privilege levels can only transfer control

through a gate at the same or a less privileged

level than CPL to a code segment at the same or

more privileged level than CPL

return instructions that don’t switch tasks can

only return control to a code segment at the

same or less privileged level

task switch can be performed by a call jump or

interrupt which references either a task gate or

task state segment at the same or less privileged

level

Table 10 Descriptor Types Used for Control Transfer

Control Transfer Types

Operation Types

Descriptor

Referenced

Table

Intersegment within the same privilege level

JMP CALL RET IRET

Code Segment

GDTLDT

Intersegment to the same or higher privilege level Interrupt

CALL

Call Gate

GDTLDT

within task may change CPL

Interrupt Instruction

Trap or

IDT

Exception External

Interrupt

Gate

Intersegment to a lower privilege level (changes task CPL)

RET IRET

Code Segment

GDTLDT

CALL JMP

Task State

GDT

Segment

Task Switch

CALL JMP

Task Gate

GDTLDT

IRET

Interrupt Instruction

Task Gate

IDT

Exception External

Interrupt

NT (Nested Task bit of flag word) e 0

NT (Nested Task bit of flag word) e 1

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