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Henk-Jan Lebbink edited this page Jan 8, 2018 · 13 revisions

STI — Set Interrupt Flag

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

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
ZO NA NA NA NA

Description

In most cases, STI sets the interrupt flag (IF) in the EFLAGS register. After the IF flag is set, the processor begins responding to external, maskable interrupts after the next instruction is executed. 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 recognized1. If the STI instruction is followed by a CLI instruction (which clears the IF flag), the effect of the STI instruction is negated.

The IF flag and the STI and CLI instructions do not prohibit the generation of exceptions and NMI interrupts. NMI interrupts (and SMIs) may be blocked for one macroinstruction following an STI.

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 4-19 indicates the action of the STI instruction depending on the processor operating mode, IOPL, CPL, and EFLAGS.VIP.

Table 4-19. Decision Table for STI Results

Mode IOPL EFLAGS.VIP STI Result
Real-address X1 X IF = 1
Protected, not PVI2 ≥ CPL X IF = 1
< CPL X #GP fault
Protected, PVI3 3 X IF = 1
0–2 0 VIF = 1
1 #GP fault
Virtual-8086, not VME3 3 X IF = 1
0–2 X #GP fault
  1. The STI instruction delays recognition of interrupts only if it is executed with EFLAGS.IF = 0. In a sequence of STI instructions, only the first instruction in the sequence is guaranteed to delay interrupts. In the following instruction sequence, interrupts may be recognized before RET executes: STI STI RET Table 4-19. Decision Table for STI Results
Mode IOPL EFLAGS.VIP STI Result
Virtual-8086, VME3 3 X IF = 1
0–2 0 VIF = 1
1 #GP fault

NOTES:

  1. X = This setting has no effect on instruction operation.
  2. For this table, “protected mode” applies whenever CR0.PE = 1 and EFLAGS.VM = 0; it includes compatibility mode and 64-bit mode.
  3. PVI mode and virtual-8086 mode each imply CPL = 3.

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.

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.

Source: Intel® Architecture Software Developer's Manual (DECEMBER 2017)
Generated: 8-1-2018

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