Intel IA-32 Manuale Utente

Vol. 3A 5-17

To return from an exception- or interrupt-handler procedure, the handler must use the IRET (or
IRETD) instruction. The IRET instruction is similar to the RET instruction except that it restores
the saved flags into the EFLAGS register. The IOPL field of the EFLAGS register is restored
only if the CPL is 0. The IF flag is changed only if the CPL is less than or equal to the IOPL.
See Chapter 3, “Instruction Set Reference, A-M,” of the IA-32 Intel® Architecture Software
Developer’s Manual, Volume 2A, for a description of the complete operation performed by the
IRET instruction.

If a stack switch occurred when calling the handler procedure, the IRET instruction switches
back to the interrupted procedure’s stack on the return.

The privilege-level protection for exception- and interrupt-handler procedures is similar to that
used for ordinary procedure calls when called through a call gate (see Section 4.8.4, “Accessing
a Code Segment Through a Call Gate”). The processor does not permit transfer of execution to
an exception- or interrupt-handler procedure in a less privileged code segment (numerically
greater privilege level) than the CPL. 

 CS

Error Code

EFLAGS

CS

 EIP

ESP After
Transfer to Handler

Error Code

ESP Before
Transfer to Handler

 EFLAGS

 EIP

 SS

 ESP

Interrupted Procedure’s 

Interrupted Procedure’s

and Handler’s Stack

Handler’s Stack

ESP After

Transfer to Handler

Transfer to Handler

ESP Before

Stack