sti

Set Interrupt Flag

STI

Enables maskable hardware interrupts.

Details

Sets the Interrupt Flag (IF) in EFLAGS, enabling maskable hardware interrupts at the next instruction boundary. This is a privileged instruction that requires CPL=0 (kernel mode). The instruction takes effect after the next instruction executes, allowing one more instruction to complete before interrupts are actually serviced.

Pseudocode Operation

IF (CPL == 0) THEN IF ← 1; ELSE #GP(0); FI;

Example

STI

Encoding

Binary Layout

Format Legacy

Opcode FB

Extension Base

Operands

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	CPUID	Description
FB	STI	ZO	Valid Valid		Set interrupt flag; external, maskable interrupts enabled at the end of the next instruction.

Description

In most cases, STI sets the interrupt flag (IF) in the EFLAGS register. This allows the processor to respond to maskable hardware interrupts. If IF = 0, maskable hardware interrupts remain inhibited on the instruction boundary following an execution of STI. (The delayed effect of this instruction is provided to allow interrupts to be enabled just before returning from a procedure or subroutine. For instance, if an STI instruction is followed by an RET instruction, the RET instruction is allowed to execute before external interrupts are recognized. No interrupts can be recognized if an execution of CLI immediately follow such an execution of STI.) The inhibition ends after delivery of another event (e.g., exception) or the execution of the next instruction. The IF flag and the STI and CLI instructions do not prohibit the generation of exceptions and nonmaskable interrupts (NMIs). However, NMIs (and system-management interrupts) may be inhibited on the instruction boundary following an execution of STI that begins with IF = 0. Operation is different in two modes defined as follows: • PVI mode (protected-mode virtual interrupts): CR0.PE = 1, EFLAGS.VM = 0, CPL = 3, and CR4.PVI = 1; • VME mode (virtual-8086 mode extensions): CR0.PE = 1, EFLAGS.VM = 1, and CR4.VME = 1. If IOPL < 3, EFLAGS.VIP = 1, and either VME mode or PVI mode is active, STI sets the VIF flag in the EFLAGS register, leaving IF unaffected. Table 4-22 indicates the action of the STI instruction depending on the processor operating mode, IOPL, CPL, and EFLAGS.VIP. Table 4-22. Decision Table for STI Results Mode IOPL EFLAGS.VIP STI Result Real-address X1 X IF = 1 ≥ CPL X IF = 1 Protected, not PVI2 < CPL X #GP fault 3 X IF = 1 Protected, PVI3 0 VIF = 1 0–2 1 #GP fault 3 X IF = 1 Virtual-8086, not VME3 0–2 X #GP fault 3 X IF = 1 Virtual-8086, VME3 0 VIF = 1 0–2 1 #GP fault

Operation

IF CR0.PE = 0 (* Executing in real-address mode *)
THEN IF := 1; (* Set Interrupt Flag *)
ELSE
IF IOPL ≥ CPL (* CPL = 3 if EFLAGS.VM = 1 *)
THEN IF := 1; (* Set Interrupt Flag *)
ELSE
IF VME mode OR PVI mode
THEN
IF EFLAGS.VIP = 0
THEN VIF := 1; (* Set Virtual Interrupt Flag *)
ELSE #GP(0);
FI;
ELSE #GP(0);
FI;
FI;
FI;

Flags Affected

Either the IF flag or the VIF flag is set to 1. Other flags are unaffected.

Exceptions

Protected Mode Exceptions

#GP(0) If CPL is greater than IOPL and PVI mode is not active. If CPL is greater than IOPL and EFLAGS.VIP = 1. #UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#GP(0) If IOPL is less than 3 and VME mode is not active. If IOPL is less than 3 and EFLAGS.VIP = 1. #UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

Same exceptions as in protected mode. STI—Set Interrupt Flag Vol. 2B 4-674