RET.htm

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<TITLE>80386 Programmer's Reference Manual -- Opcode RET</TITLE>
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<H1>RET -- Return from Procedure</H1>

<PRE>
Opcode     Instruction  Clocks         Description

C3         RET          10+m           Return (near) to caller
CB         RET          18+m,pm=32+m   Return (far) to caller, same
                                       privilege
CB         RET          pm=68          Return (far), lesser privilege,
                                       switch stacks
C2  iw     RET imm16    10+m           Return (near), pop imm16 bytes of
                                       parameters
CA  iw     RET imm16    18+m,pm=32+m   Return (far), same privilege, pop
                                       imm16 bytes
CA  iw     RET imm16    pm=68          Return (far), lesser privilege, pop
                                       imm16 bytes
</PRE>


<H2>Operation</H2>

<PRE>
IF instruction = near RET
THEN;
   IF OperandSize = 16
   THEN
      IP := Pop();
      EIP := EIP AND 0000FFFFH;
   ELSE (* OperandSize = 32 *)
      EIP := Pop();
   FI;
   IF instruction has immediate operand THEN eSP := eSP + imm16; FI;
FI;

IF (PE = 0 OR (PE = 1 AND VM = 1))
   (* real mode or virtual 8086 mode *)
   AND instruction = far RET
THEN;
   IF OperandSize = 16
   THEN
      IP := Pop();
      EIP := EIP AND 0000FFFFH;
      CS := Pop(); (* 16-bit pop *)
   ELSE (* OperandSize = 32 *)
      EIP := Pop();
      CS := Pop(); (* 32-bit pop, high-order 16-bits discarded *)
   FI;
   IF instruction has immediate operand THEN eSP := eSP + imm16; FI;
FI;

IF (PE = 1 AND VM = 0) (* Protected mode, not V86 mode *)
   AND instruction = far RET
THEN
   IF OperandSize=32
   THEN Third word on stack must be within stack limits else #SS(0);
   ELSE Second word on stack must be within stack limits else #SS(0);
   FI;
   Return selector RPL must be >= CPL ELSE #GP(return selector)
   IF return selector RPL = CPL
   THEN GOTO SAME-LEVEL;
   ELSE GOTO OUTER-PRIVILEGE-LEVEL;
   FI;
FI;

SAME-LEVEL:
   Return selector must be non-null ELSE #GP(0)
   Selector index must be within its descriptor table limits ELSE
      #GP(selector)
   Descriptor AR byte must indicate code segment ELSE #GP(selector)
   IF non-conforming
   THEN code segment DPL must equal CPL;
   ELSE #GP(selector);
   FI;
   IF conforming
   THEN code segment DPL must be <= CPL;
   ELSE #GP(selector);
   FI;
   Code segment must be present ELSE #NP(selector);
   Top word on stack must be within stack limits ELSE #SS(0);
   IP must be in code segment limit ELSE #GP(0);
   IF OperandSize=32
   THEN
      Load CS:EIP from stack
      Load CS register with descriptor
      Increment eSP by 8 plus the immediate offset if it exists
   ELSE (* OperandSize=16 *)
      Load CS:IP from stack
      Load CS register with descriptor
      Increment eSP by 4 plus the immediate offset if it exists
   FI;

OUTER-PRIVILEGE-LEVEL:
   IF OperandSize=32
   THEN Top (16+immediate) bytes on stack must be within stack limits
      ELSE #SS(0);
   ELSE Top (8+immediate) bytes on stack must be within stack limits ELSE
      #SS(0);
   FI;
   Examine return CS selector and associated descriptor:
      Selector must be non-null ELSE #GP(0);
      Selector index must be within its descriptor table limits ELSE
         #GP(selector)
      Descriptor AR byte must indicate code segment ELSE #GP(selector);
      IF non-conforming
      THEN code segment DPL must equal return selector RPL
      ELSE #GP(selector);
      FI;
      IF conforming
      THEN code segment DPL must be <= return selector RPL;
      ELSE #GP(selector);
      FI;
      Segment must be present ELSE #NP(selector)
   Examine return SS selector and associated descriptor:
      Selector must be non-null ELSE #GP(0);
      Selector index must be within its descriptor table limits
         ELSE #GP(selector);
      Selector RPL must equal the RPL of the return CS selector ELSE
         #GP(selector);
      Descriptor AR byte must indicate a writable data segment ELSE
         #GP(selector);
      Descriptor DPL must equal the RPL of the return CS selector ELSE
         #GP(selector);
      Segment must be present ELSE #NP(selector);
   IP must be in code segment limit ELSE #GP(0);
   Set CPL to the RPL of the return CS selector;
   IF OperandMode=32
   THEN
      Load CS:EIP from stack;
      Set CS RPL to CPL;
      Increment eSP by 8 plus the immediate offset if it exists;
      Load SS:eSP from stack;
   ELSE (* OperandMode=16 *)
      Load CS:IP from stack;
      Set CS RPL to CPL;
      Increment eSP by 4 plus the immediate offset if it exists;
      Load SS:eSP from stack;
   FI;
   Load the CS register with the return CS descriptor;
   Load the SS register with the return SS descriptor;
   For each of ES, FS, GS, and DS
   DO
      IF the current register setting is not valid for the outer level,
         set the register to null (selector := AR := 0);
      To be valid, the register setting must satisfy the following
         properties:
         Selector index must be within descriptor table limits;
         Descriptor AR byte must indicate data or readable code segment;
         IF segment is data or non-conforming code, THEN
            DPL must be >= CPL, or DPL must be >= RPL;
      FI;
   OD;
</PRE>

<H2>Description</H2>

RET transfers control to a return address located on the stack. The
address is usually placed on the stack by a 
<A HREF="CALL.htm">CALL</A> instruction, and the
return is made to the instruction that follows the 
<A HREF="CALL.htm">CALL</A>.
<P>
The optional numeric parameter to RET gives the number of stack bytes
(OperandMode=16) or words (OperandMode=32) to be released after the return
address is popped. These items are typically used as input parameters to the
procedure called.
<P>
For the intrasegment (near) return, the address on the stack is a segment
offset, which is popped into the instruction pointer. The CS register is
unchanged. For the intersegment (far) return, the address on the stack
is a long pointer. The offset is popped first, followed by the selector.
<P>
In real mode, CS and IP are loaded directly. In Protected Mode, an
intersegment return causes the processor to check the descriptor
addressed by the return selector. The AR byte of the descriptor must
indicate a code segment of equal or lesser privilege (or greater or equal
numeric value) than the current privilege level. Returns to a lesser
privilege level cause the stack to be reloaded from the value saved beyond
the parameter block.
<P>
The DS, ES, FS, and GS segment registers can be set to 0 by the RET
instruction during an interlevel transfer. If these registers refer to
segments that cannot be used by the new privilege level, they are set to
0 to prevent unauthorized access from the new privilege level.

<H2>Flags Affected</H2>

None

<H2>Protected Mode Exceptions</H2>

#GP, #NP, or #SS, as described under "<H2>Operation</H2>" above; #PF(fault-code) for
a page fault

<H2>Real Address Mode Exceptions</H2>

Interrupt 13 if any part of the operand would be outside the effective
address space from 0 to 0FFFFH

<H2>Virtual 8086 Mode Exceptions</H2>

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


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Chapter 17 -- 80386 Instruction Set</A><BR>
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