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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Opcode RET</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="REP.htm"> REP/REPE/REPZ/REPNE/REPNZ Repeat Following String Operation</A><BR><B>next:</B><A HREF="SAHF.htm"> SAHF Store AH into Flags</A><P><HR><P><H1>RET -- Return from Procedure</H1><PRE>Opcode Instruction Clocks DescriptionC3 RET 10+m Return (near) to callerCB RET 18+m,pm=32+m Return (far) to caller, same privilegeCB RET pm=68 Return (far), lesser privilege, switch stacksC2 iw RET imm16 10+m Return (near), pop imm16 bytes of parametersCA iw RET imm16 18+m,pm=32+m Return (far), same privilege, pop imm16 bytesCA iw RET imm16 pm=68 Return (far), lesser privilege, pop imm16 bytes</PRE><H2>Operation</H2><PRE>IF instruction = near RETTHEN; 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 RETTHEN; 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 RETTHEN 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. Theaddress is usually placed on the stack by a <A HREF="CALL.htm">CALL</A> instruction, and thereturn 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 returnaddress is popped. These items are typically used as input parameters to theprocedure called.<P>For the intrasegment (near) return, the address on the stack is a segmentoffset, which is popped into the instruction pointer. The CS register isunchanged. For the intersegment (far) return, the address on the stackis 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, anintersegment return causes the processor to check the descriptoraddressed by the return selector. The AR byte of the descriptor mustindicate a code segment of equal or lesser privilege (or greater or equalnumeric value) than the current privilege level. Returns to a lesserprivilege level cause the stack to be reloaded from the value saved beyondthe parameter block.<P>The DS, ES, FS, and GS segment registers can be set to 0 by the RETinstruction during an interlevel transfer. If these registers refer tosegments that cannot be used by the new privilege level, they are set to0 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) fora page fault<H2>Real Address Mode Exceptions</H2>Interrupt 13 if any part of the operand would be outside the effectiveaddress space from 0 to 0FFFFH<H2>Virtual 8086 Mode Exceptions</H2>Same exceptions as in Real Address Mode; #PF(fault-code) for a pagefault<P><HR><P><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="REP.htm"> REP/REPE/REPZ/REPNE/REPNZ Repeat Following String Operation</A><BR><B>next:</B><A HREF="SAHF.htm"> SAHF Store AH into Flags</A></BODY>⌨️ 快捷键说明
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