모두의 코드
STI (Intel x86/64 assembly instruction)

작성일 : 2020-09-01 이 글은 1616 번 읽혔습니다.

STI

Set Interrupt Flag

참고 사항

아래 표를 해석하는 방법은 x86-64 명령어 레퍼런스 읽는 법 글을 참조하시기 바랍니다.

Opcode

Instruction

Op/
En

64-Bit
Mode

Compat/
Leg Mode

Description

FB

STI

NP

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

NP

NA

NA

NA

NA

Description

If protected-mode virtual interrupts are not enabled, 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.

When protected-mode virtual interrupts are enabled, CPL is 3, and IOPL is less than 3; 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's mode of operation and the CPL/IOPL settings of the running program or procedure.

Operation is the same in all modes.

Table 4-19. Decision Table for STI Results

CR0.PE

EFLAGS.VM

EFLAGS.IOPL

CS.CPL

CR4.PVI

EFLAGS.VIP

CR4.VME

STI Result

0

X

X

X

X

X

X

IF = 1

1

0

>= CPL

X

X

X

X

IF = 1

1

0

< CPL

3

1

X

X

VIF = 1

1

0

< CPL

< 3

X

X

X

GP Fault

1

0

< CPL

X

0

X

X

GP Fault

1

0

< CPL

X

X

1

X

GP Fault

1

1

3

X

X

X

X

IF = 1

1

1

< 3

X

X

0

1

VIF = 1

1

1

< 3

X

X

1

X

GP Fault

1

1

< 3

X

X

X

0

GP Fault

X = This setting has no impact

Operation

IF PE = 0  (* Executing in real-address mode *)
    THEN 
          IF <- 1; (* Set Interrupt Flag *)
    ELSE  (* Executing in protected mode or virtual-8086 mode *)
          IF VM = 0  (* Executing in protected mode*)
                THEN
                      IF IOPL >= CPL
                            THEN
                                  IF <- 1;  (* Set Interrupt Flag *)
                      ELSE
                            IF (IOPL < CPL) and (CPL = 3) and (PVI = 1)
                                  THEN 
                                        VIF <- 1;  (* Set Virtual Interrupt Flag *)
                                  ELSE 
                                        #GP(0);
                            FI;
                      FI;
                ELSE  (* Executing in Virtual-8086 mode *)
                      IF IOPL = 3
                            THEN
                                  IF <- 1;  (* Set Interrupt Flag *)
                      ELSE 
                            IF ((IOPL < 3) and (VIP = 0) and (VME = 1))
                                  THEN
                                        VIF <- 1;  (* Set Virtual Interrupt Flag *)
                            ELSE
                                  #GP(0); (* Trap to virtual-8086 monitor *)
                            FI;)
                      FI;
          FI; 
FI;

Flags Affected

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

Protected Mode Exceptions

#GP(0)

  • If the CPL is greater (has less privilege) than the IOPL of the current program or procedure.

#UD

  • If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD

  • If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

Same exceptions as in protected mode.

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