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Vol. 2A 3-491
INSTRUCTION SET REFERENCE, A-M
IRET/IRETD—Interrupt Return
As with a real-address mode interrupt return, the IRET instruction pops the return
instruction pointer, return code segment selector, and EFLAGS image from the stack
to the EIP, CS, and EFLAGS registers, respectively, and then resumes execution of
the interrupted program or procedure. If the return is to another privilege level, the
IRET instruction also pops the stack pointer and SS from the stack, before resuming
program execution. If the return is to virtual-8086 mode, the processor also pops the
data segment registers from the stack.
If the NT flag is set, the IRET instruction performs a task switch (return) from a
nested task (a task called with a CALL instruction, an interrupt, or an exception) back
to the calling or interrupted task. The updated state of the task executing the IRET
instruction is saved in its TSS. If the task is re-entered later, the code that follows the
IRET instruction is executed.
If the NT flag is set and the processor is in IA-32e mode, the IRET instruction causes
a general protection exception.
In 64-bit mode, the instruction’s default operation size is 32 bits. Use of the REX.W
prefix promotes operation to 64 bits (IRETQ). See the summary chart at the begin-
ning of this section for encoding data and limits.
See “Changes to Instruction Behavior in VMX Non-Root Operation” in Chapter 21 of
the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3B, for
more information about the behavior of this instruction in VMX non-root operation.
Operation
IF PE = 0
THEN
GOTO REAL-ADDRESS-MODE;
ELSE
IF (IA32_EFER.LMA
= 0)
THEN (* Protected mode *)
GOTO PROTECTED-MODE;
ELSE (* IA-32e mode *)
GOTO IA-32e-MODE;
FI;
FI;
REAL-ADDRESS-MODE;
IF OperandSize = 32
THEN
IF top 12 bytes of stack not within stack limits
THEN #SS; FI;
tempEIP ← 4 bytes at end of stack
IF tempEIP[31:16] is not zero THEN #GP(0); FI;
EIP ← Pop();
CS
← Pop(); (* 32-bit pop, high-order 16 bits discarded *)
tempEFLAGS
← Pop();