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Processor Programming (Continued)

3.9.3

Privilege Level Transfers

Atask’s CPL can be changed only through intersegment

control transfers using gates or task switches to a code seg-

ment with a different privilege level. Control transfers result

from exception and interrupt servicing and from execution

of the CALL, JMP, INT, IRET and RET instructions.

There are five types of control transfers that are summa-

rized in Table 3-37. Control transfers can be made only

when the operation causing the control transfer references

the correct descriptor type. Any violation of these descriptor

usage rules causes a general protection fault.

Any control transfer that changes the CPL within a task

results in a change of stack. The initial values for the stack

segment (SS) and stack pointer (ESP) for privilege levels 0,

1, and 2 are stored in the TSS. During a JMP or CALL con-

trol transfer, the SS and ESP are loaded with the new stack

pointer and the previous stack pointer is saved on the new

stack. When returning to the original privilege level, the

RET or IRET instruction restores the SS and ESP of the

less-privileged stack.

3.9.3.1

Gates

Gate descriptors described in Section “Gate Descriptors”

on page 74, provide protection for privilege transfers among

executable segments. Gates are used to transition to rou-

tines of the same or a more privileged level. Call gates,

interrupt gates and trap gates are used for privilege transfers

within a task. Task gates are used to transfer between

tasks.

Gates conform to the standard rules of privilege. In other

words, gates can be accessed by a task if the effective priv-

ilege level (EPL) is the same or more privileged than the

gate descriptor’s privilege level (DPL).

Table 3-37. Descriptor Types Used for Control Transfer

Type of Control Transfer

Operation Types

Descriptor

Referenced

Descriptor

Table

Intersegment within the same privilege

level.

JMP, CALL, RET, IRET1

Code Segment

GDT or LDT

Intersegment to the same or a more

privileged level. Interrupt within task

(could change CPL level).

CALL

Gate Call

GDT or LDT

Interrupt Instruction, Exception,

External Interrupt

Trap or Interrupt Gate

IDT

Intersegment to a less privileged level

(changes task CPL).

RET, IRET1

Code Segment

GDT or LDT

Task Switch via TSS

CALL, JMP

Task State Segment

GDT

Task Switch via Task Gate

CALL, JMP

Task Gate

GDT or LDT

IRET2, Interrupt Instruction,

Exception, External Interrupt

Task Gate

IDT

NT = 0 (Nested Task bit in EFLAGS, bit 14)

NT =1 (Nested Task bit in EFLAGS, bit 14)

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