RET (Return from Procedure)

Opcodes

Opcode* Instruction Op/En 64-Bit Mode Compat/Leg Mode Description
C3 RET NP Valid Valid Near return to calling procedure.
CB RET NP Valid Valid Far return to calling procedure.
C2 iw RET imm16 I Valid Valid Near return to calling procedure and pop imm16 bytes from stack.
CA iw RET imm16 I Valid Valid Far return to calling procedure and pop imm16 bytes from stack.

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
NP NA NA NA NA
I imm16 NA NA NA

Description

Transfers program control to a return address located on the top of the stack. The address is usually placed on the stack by a CALL instruction, and the return is made to the instruction that follows the CALL instruction.

The optional source operand specifies the number of stack bytes to be released after the return address is popped; the default is none. This operand can be used to release parameters from the stack that were passed to the called procedure and are no longer needed. It must be used when the CALL instruction used to switch to a new procedure uses a call gate with a non-zero word count to access the new procedure. Here, the source operand for the RET instruction must specify the same number of bytes as is specified in the word count field of the call gate.

The RET instruction can be used to execute three different types of returns:

The inter-privilege-level return type can only be executed in protected mode. See the section titled “Calling Proce-dures Using Call and RET” in Chapter 6 of the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, for detailed information on near, far, and inter-privilege-level returns.

When executing a near return, the processor pops the return instruction pointer (offset) from the top of the stack into the EIP register and begins program execution at the new instruction pointer. The CS register is unchanged.

When executing a far return, the processor pops the return instruction pointer from the top of the stack into the EIP register, then pops the segment selector from the top of the stack into the CS register. The processor then begins program execution in the new code segment at the new instruction pointer.

The mechanics of an inter-privilege-level far return are similar to an intersegment return, except that the processor examines the privilege levels and access rights of the code and stack segments being returned to deter-mine if the control transfer is allowed to be made. The DS, ES, FS, and GS segment registers are cleared by the RET instruction during an inter-privilege-level return if they refer to segments that are not allowed to be accessed at the new privilege level. Since a stack switch also occurs on an inter-privilege level return, the ESP and SS registers are loaded from the stack.

If parameters are passed to the called procedure during an inter-privilege level call, the optional source operand must be used with the RET instruction to release the parameters on the return. Here, the parameters are released both from the called procedure’s stack and the calling procedure’s stack (that is, the stack being returned to).

In 64-bit mode, the default operation size of this instruction is the stack-address size, i.e. 64 bits. This applies to near returns, not far returns; the default operation size of far returns is 32 bits.

Operation

(* Near return *)
IF instruction = near return
    THEN;
         IF OperandSize = 32
              THEN
                    IF top 4 bytes of stack not within stack limits
                         THEN #SS(0); FI;
                    EIP ← Pop();
              ELSE
                    IF OperandSize = 64
                         THEN
                              IF top 8 bytes of stack not within stack limits
                                    THEN #SS(0); FI;
                              RIP ← Pop();
                         ELSE (* OperandSize = 16 *)
                              IF top 2 bytes of stack not within stack limits
                                    THEN #SS(0); FI;
                              tempEIP ← Pop();
                              tempEIP ← tempEIP AND 0000FFFFH;
                              IF tempEIP not within code segment limits
                                    THEN #GP(0); FI;
                              EIP ← tempEIP;
                    FI;
         FI;
    IF instruction has immediate operand
         THEN (* Release parameters from stack *)
              IF StackAddressSize = 32
                    THEN
                         ESP ← ESP + SRC;
                    ELSE
                         IF StackAddressSize = 64
                              THEN
                                    RSP ← RSP + SRC;
                              ELSE (* StackAddressSize = 16 *)
                                    SP ← SP + SRC;
                         FI;
              FI;
    FI;
FI;
(* Real-address mode or virtual-8086 mode *)
IF ((PE = 0) or (PE = 1 AND VM = 1)) and instruction = far return
    THEN
         IF OperandSize = 32
              THEN
                    IF top 8 bytes of stack not within stack limits
                         THEN #SS(0); FI;
                    EIP ← Pop();
                    CS ← Pop(); (* 32-bit pop, high-order 16 bits discarded *)
              ELSE (* OperandSize = 16 *)
                    IF top 4 bytes of stack not within stack limits
                         THEN #SS(0); FI;
                    tempEIP ← Pop();
                    tempEIP ← tempEIP AND 0000FFFFH;
                    IF tempEIP not within code segment limits
                         THEN #GP(0); FI;
                    EIP ← tempEIP;
                    CS ← Pop(); (* 16-bit pop *)
         FI;
    IF instruction has immediate operand
         THEN (* Release parameters from stack *)
              SP ← SP + (SRC AND FFFFH);
    FI;
FI;
(* Protected mode, not virtual-8086 mode *)
IF (PE = 1 and VM = 0 and IA32_EFER.LMA = 0) and instruction = far return
    THEN
         IF OperandSize = 32
              THEN
                    IF second doubleword on stack is not within stack limits
                         THEN #SS(0); FI;
              ELSE (* OperandSize = 16 *)
                    IF second word on stack is not within stack limits
                         THEN #SS(0); FI;
         FI;
    IF return code segment selector is NULL
         THEN #GP(0); FI;
    IF return code segment selector addresses descriptor beyond descriptor table limit
         THEN #GP(selector); FI;
    Obtain descriptor to which return code segment selector points from descriptor table;
    IF return code segment descriptor is not a code segment
         THEN #GP(selector); FI;
    IF return code segment selector RPL 
 CPL
         THEN #GP(selector); FI;
    IF return code segment descriptor is conforming
    and return code segment DPL 
 return code segment selector RPL
         THEN #GP(selector); FI;
    IF return code segment descriptor is non-conforming and return code segment DPL ≠ return code segment selector RPL
         THEN #GP(selector); FI;
    IF return code segment descriptor is not present
         THEN #NP(selector); FI:
    IF return code segment selector RPL 
 CPL
         THEN GOTO RETURN-OUTER-PRIVILEGE-LEVEL;
         ELSE GOTO RETURN-TO-SAME-PRIVILEGE-LEVEL;
    FI;
FI;
RETURN-SAME-PRIVILEGE-LEVEL:
    IF the return instruction pointer is not within the return code segment limit
         THEN #GP(0); FI;
    IF OperandSize = 32
         THEN
              EIP ← Pop();
              CS ← Pop(); (* 32-bit pop, high-order 16 bits discarded *)
         ELSE (* OperandSize = 16 *)
              EIP ← Pop();
              EIP ← EIP AND 0000FFFFH;
              CS ← Pop(); (* 16-bit pop *)
    FI;
    IF instruction has immediate operand
         THEN (* Release parameters from stack *)
              IF StackAddressSize = 32
                    THEN
                         ESP ← ESP + SRC;
                    ELSE (* StackAddressSize = 16 *)
                         SP ← SP + SRC;
              FI;
    FI;
RETURN-OUTER-PRIVILEGE-LEVEL:
    IF top (16 + SRC) bytes of stack are not within stack limits (OperandSize = 32)
    or top (8 + SRC) bytes of stack are not within stack limits (OperandSize = 16)
              THEN #SS(0); FI;
    Read return segment selector;
    IF stack segment selector is NULL
         THEN #GP(0); FI;
    IF return stack segment selector index is not within its descriptor table limits
         THEN #GP(selector); FI;
    Read segment descriptor pointed to by return segment selector;
    IF stack segment selector RPL ≠ RPL of the return code segment selector
    or stack segment is not a writable data segment
    or stack segment descriptor DPL ≠ RPL of the return code segment selector
              THEN #GP(selector); FI;
    IF stack segment not present
         THEN #SS(StackSegmentSelector); FI;
    IF the return instruction pointer is not within the return code segment limit
         THEN #GP(0); FI;
    CPL ← ReturnCodeSegmentSelector(RPL);
    IF OperandSize = 32
         THEN
              EIP ← Pop();
              CS ← Pop(); (* 32-bit pop, high-order 16 bits discarded; segment descriptor loaded *)
              CS(RPL) ← CPL;
              IF instruction has immediate operand
                    THEN (* Release parameters from called procedure’s stack *)
                         IF StackAddressSize = 32
                              THEN
                                    ESP ← ESP + SRC;
                              ELSE (* StackAddressSize = 16 *)
                                    SP ← SP + SRC;
                         FI;
              FI;
              tempESP ← Pop();
              tempSS ← Pop(); (* 32-bit pop, high-order 16 bits discarded; seg. descriptor loaded *)
              ESP ← tempESP;
              SS ← tempSS;
         ELSE (* OperandSize = 16 *)
              EIP ← Pop();
              EIP ← EIP AND 0000FFFFH;
              CS ← Pop(); (* 16-bit pop; segment descriptor loaded *)
              CS(RPL) ← CPL;
              IF instruction has immediate operand
                    THEN (* Release parameters from called procedure’s stack *)
                         IF StackAddressSize = 32
                              THEN
                                    ESP ← ESP + SRC;
                              ELSE (* StackAddressSize = 16 *)
                                    SP ← SP + SRC;
                         FI;
              FI;
              tempESP ← Pop();
              tempSS ← Pop(); (* 16-bit pop; segment descriptor loaded *)
              ESP ← tempESP;
              SS ← tempSS;
    FI;
    FOR each of segment register (ES, FS, GS, and DS)
         DO
              IF segment register points to data or non-conforming code segment
              and CPL 
 segment descriptor DPL (* DPL in hidden part of segment register *)
                    THEN SegmentSelector ← 0; (* Segment selector invalid *)
              FI;
         OD;
    IF instruction has immediate operand
         THEN (* Release parameters from calling procedure’s stack *)
              IF StackAddressSize = 32
                    THEN
                         ESP ← ESP + SRC;
                    ELSE (* StackAddressSize = 16 *)
                         SP ← SP + SRC;
              FI;
    FI;
(* IA-32e Mode *)
    IF (PE = 1 and VM = 0 and IA32_EFER.LMA = 1) and instruction = far return
         THEN
              IF OperandSize = 32
                    THEN
                         IF second doubleword on stack is not within stack limits
                              THEN #SS(0); FI;
                         IF first or second doubleword on stack is not in canonical space
                              THEN #SS(0); FI;
                    ELSE
                         IF OperandSize = 16
                              THEN
                                    IF second word on stack is not within stack limits
                                         THEN #SS(0); FI;
                                    IF first or second word on stack is not in canonical space
                                         THEN #SS(0); FI;
                              ELSE (* OperandSize = 64 *)
                                    IF first or second quadword on stack is not in canonical space
                                         THEN #SS(0); FI;
                         FI
              FI;
         IF return code segment selector is NULL
              THEN GP(0); FI;
         IF return code segment selector addresses descriptor beyond descriptor table limit
              THEN GP(selector); FI;
         IF return code segment selector addresses descriptor in non-canonical space
              THEN GP(selector); FI;
         Obtain descriptor to which return code segment selector points from descriptor table;
         IF return code segment descriptor is not a code segment
              THEN #GP(selector); FI;
         IF return code segment descriptor has L-bit = 1 and D-bit = 1
              THEN #GP(selector); FI;
         IF return code segment selector RPL 
 CPL
              THEN #GP(selector); FI;
         IF return code segment descriptor is conforming
         and return code segment DPL 
 return code segment selector RPL
              THEN #GP(selector); FI;
         IF return code segment descriptor is non-conforming
         and return code segment DPL ≠ return code segment selector RPL
              THEN #GP(selector); FI;
         IF return code segment descriptor is not present
              THEN #NP(selector); FI:
         IF return code segment selector RPL 
 CPL
              THEN GOTO IA-32E-MODE-RETURN-OUTER-PRIVILEGE-LEVEL;
              ELSE GOTO IA-32E-MODE-RETURN-SAME-PRIVILEGE-LEVEL;
         FI;
    FI;
IA-32E-MODE-RETURN-SAME-PRIVILEGE-LEVEL:
IF the return instruction pointer is not within the return code segment limit
    THEN #GP(0); FI;
IF the return instruction pointer is not within canonical address space
    THEN #GP(0); FI;
IF OperandSize = 32
    THEN
         EIP ← Pop();
         CS ← Pop(); (* 32-bit pop, high-order 16 bits discarded *)
    ELSE
         IF OperandSize = 16
              THEN
                    EIP ← Pop();
                    EIP ← EIP AND 0000FFFFH;
                    CS ← Pop(); (* 16-bit pop *)
              ELSE (* OperandSize = 64 *)
                    RIP ← Pop();
                    CS ← Pop(); (* 64-bit pop, high-order 48 bits discarded *)
         FI;
FI;
IF instruction has immediate operand
    THEN (* Release parameters from stack *)
         IF StackAddressSize = 32
              THEN
                    ESP ← ESP + SRC;
              ELSE
                    IF StackAddressSize = 16
                         THEN
                              SP ← SP + SRC;
                         ELSE (* StackAddressSize = 64 *)
                              RSP ← RSP + SRC;
                    FI;
         FI;
FI;
IA-32E-MODE-RETURN-OUTER-PRIVILEGE-LEVEL:
IF top (16 + SRC) bytes of stack are not within stack limits (OperandSize = 32)
or top (8 + SRC) bytes of stack are not within stack limits (OperandSize = 16)
    THEN #SS(0); FI;
IF top (16 + SRC) bytes of stack are not in canonical address space (OperandSize = 32)
or top (8 + SRC) bytes of stack are not in canonical address space (OperandSize = 16)
or top (32 + SRC) bytes of stack are not in canonical address space (OperandSize = 64)
    THEN #SS(0); FI;
Read return stack segment selector;
IF stack segment selector is NULL
    THEN
         IF new CS descriptor L-bit = 0
              THEN #GP(selector);
         IF stack segment selector RPL = 3
              THEN #GP(selector);
FI;
IF return stack segment descriptor is not within descriptor table limits
         THEN #GP(selector); FI;
IF return stack segment descriptor is in non-canonical address space
         THEN #GP(selector); FI;
Read segment descriptor pointed to by return segment selector;
IF stack segment selector RPL ≠ RPL of the return code segment selector
or stack segment is not a writable data segment
or stack segment descriptor DPL ≠ RPL of the return code segment selector
    THEN #GP(selector); FI;
IF stack segment not present
    THEN #SS(StackSegmentSelector); FI;
IF the return instruction pointer is not within the return code segment limit
    THEN #GP(0); FI:
IF the return instruction pointer is not within canonical address space
    THEN #GP(0); FI;
CPL ← ReturnCodeSegmentSelector(RPL);
IF OperandSize = 32
    THEN
         EIP ← Pop();
         CS ← Pop(); (* 32-bit pop, high-order 16 bits discarded, segment descriptor loaded *)
         CS(RPL) ← CPL;
         IF instruction has immediate operand
              THEN (* Release parameters from called procedure’s stack *)
                    IF StackAddressSize = 32
                         THEN
                              ESP ← ESP + SRC;
                         ELSE
                              IF StackAddressSize = 16
                                    THEN
                                         SP ← SP + SRC;
                                    ELSE (* StackAddressSize = 64 *)
                                         RSP ← RSP + SRC;
                              FI;
                    FI;
         FI;
         tempESP ← Pop();
         tempSS ← Pop(); (* 32-bit pop, high-order 16 bits discarded, segment descriptor loaded *)
         ESP ← tempESP;
         SS ← tempSS;
    ELSE
         IF OperandSize = 16
              THEN
                    EIP ← Pop();
                    EIP ← EIP AND 0000FFFFH;
                    CS ← Pop(); (* 16-bit pop; segment descriptor loaded *)
                    CS(RPL) ← CPL;
                    IF instruction has immediate operand
                         THEN (* Release parameters from called procedure’s stack *)
                              IF StackAddressSize = 32
                                    THEN
                                         ESP ← ESP + SRC;
                                    ELSE
                                         IF StackAddressSize = 16
                                              THEN
                                                    SP ← SP + SRC;
                                              ELSE (* StackAddressSize = 64 *)
                                                    RSP ← RSP + SRC;
                                         FI;
                              FI;
                    FI;
                    tempESP ← Pop();
                    tempSS ← Pop(); (* 16-bit pop; segment descriptor loaded *)
                    ESP ← tempESP;
                    SS ← tempSS;
              ELSE (* OperandSize = 64 *)
                    RIP ← Pop();
                    CS ← Pop(); (* 64-bit pop; high-order 48 bits discarded; seg. descriptor loaded *)
                    CS(RPL) ← CPL;
                    IF instruction has immediate operand
                         THEN (* Release parameters from called procedure’s stack *)
                              RSP ← RSP + SRC;
                    FI;
                    tempESP ← Pop();
                    tempSS ← Pop(); (* 64-bit pop; high-order 48 bits discarded; seg. desc. loaded *)
                    ESP ← tempESP;
                    SS ← tempSS;
         FI;
FI;
FOR each of segment register (ES, FS, GS, and DS)
    DO
         IF segment register points to data or non-conforming code segment
         and CPL 
 segment descriptor DPL; (* DPL in hidden part of segment register *)
              THEN SegmentSelector ← 0; (* SegmentSelector invalid *)
         FI;
    OD;
IF instruction has immediate operand
    THEN (* Release parameters from calling procedure’s stack *)
         IF StackAddressSize = 32
              THEN
                    ESP ← ESP + SRC;
              ELSE
                    IF StackAddressSize = 16
                         THEN
                              SP ← SP + SRC;
                         ELSE (* StackAddressSize = 64 *)
                              RSP ← RSP + SRC;
                    FI;
         FI;
FI;

Flags Affected

None.

Protected Mode Exceptions

#GP(0) If the return code or stack segment selector NULL. If the return instruction pointer is not within the return code segment limit
#GP(selector) If the RPL of the return code segment selector is less then the CPL. If the return code or stack segment selector index is not within its descriptor table limits. If the return code segment descriptor does not indicate a code segment. If the return code segment is non-conforming and the segment selector’s DPL is not equal to the RPL of the code segment’s segment selector If the return code segment is conforming and the segment selector’s DPL greater than the RPL of the code segment’s segment selector If the stack segment is not a writable data segment. If the stack segment selector RPL is not equal to the RPL of the return code segment selector. If the stack segment descriptor DPL is not equal to the RPL of the return code segment selector.
#SS(0) If the top bytes of stack are not within stack limits. If the return stack segment is not present.
#NP(selector) If the return code segment is not present.
#PF(fault-code) If a page fault occurs.
#AC(0) If an unaligned memory access occurs when the CPL is 3 and alignment checking is enabled.

Real-Address Mode Exceptions

#GP If the return instruction pointer is not within the return code segment limit
#SS If the top bytes of stack are not within stack limits.

Virtual-8086 Mode Exceptions

#GP(0) If the return instruction pointer is not within the return code segment limit
#SS(0) If the top bytes of stack are not within stack limits.
#PF(fault-code) If a page fault occurs.
#AC(0) If an unaligned memory access occurs when alignment checking is enabled.

Compatibility Mode Exceptions

Same as 64-bit mode exceptions.

64-Bit Mode Exceptions

#GP(0) If the return instruction pointer is non-canonical. If the return instruction pointer is not within the return code segment limit. If the stack segment selector is NULL going back to compatibility mode. If the stack segment selector is NULL going back to CPL3 64-bit mode. If a NULL stack segment selector RPL is not equal to CPL going back to non-CPL3 64-bit mode. If the return code segment selector is NULL.
#GP(selector) If the proposed segment descriptor for a code segment does not indicate it is a code segment. If the proposed new code segment descriptor has both the D-bit and L-bit set. If the DPL for a nonconforming-code segment is not equal to the RPL of the code segment selector. If CPL is greater than the RPL of the code segment selector. If the DPL of a conforming-code segment is greater than the return code segment selector RPL. If a segment selector index is outside its descriptor table limits. If a segment descriptor memory address is non-canonical. If the stack segment is not a writable data segment. If the stack segment descriptor DPL is not equal to the RPL of the return code segment selector. If the stack segment selector RPL is not equal to the RPL of the return code segment selector.
#SS(0) If an attempt to pop a value off the stack violates the SS limit. If an attempt to pop a value off the stack causes a non-canonical address to be referenced.
#NP(selector) If the return code or stack segment is not present.
#PF(fault-code) If a page fault occurs.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.