ENTER.htm

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<TITLE>80386 Programmer's Reference Manual -- Opcode ENTER</TITLE>
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<H1>ENTER -- Make Stack Frame for Procedure Parameters</H1>

<PRE>
Opcode      Instruction        Clocks     Description

C8 iw 00    ENTER imm16,0      10         Make procedure stack frame
C8 iw 01    ENTER imm16,1      12         Make stack frame for procedure
                                          parameters
C8 iw ib    ENTER imm16,imm8   15+4(n-1)  Make stack frame for
                                          procedure parameters
</PRE>

<H2>Operation</H2>

<PRE>
level := level MOD 32
IF OperandSize = 16 THEN Push(BP) ELSE Push (EBP) FI;
   (* Save stack pointer *)
frame-ptr := eSP
IF level > 0
THEN (* level is rightmost parameter *)
   FOR i := 1 TO level - 1
   DO
      IF OperandSize = 16
      THEN
         BP := BP - 2;
         Push[BP]
      ELSE (* OperandSize = 32 *)
         EBP := EBP - 4;
         Push[EBP];
      FI;
   OD;
   Push(frame-ptr)
FI;
IF OperandSize = 16 THEN BP := frame-ptr ELSE EBP := frame-ptr; FI;
IF StackAddrSize = 16
THEN SP := SP - First operand;
ELSE ESP := ESP - ZeroExtend(First operand);
FI;
</PRE>

<H2>Description</H2>

ENTER creates the stack frame required by most block-structured
high-level languages. The first operand specifies the number of bytes of
dynamic storage allocated on the stack for the routine being entered.
The second operand gives the lexical nesting level (0 to 31) of the routine
within the high-level language source code. It determines the number of
stack frame pointers copied into the new stack frame from the preceding
frame. BP (or EBP, if the operand-size attribute is 32 bits) is the current
stack frame pointer.
<P>
If the operand-size attribute is 16 bits, the processor uses BP as the
frame pointer and SP as the stack pointer. If the operand-size attribute is
32 bits, the processor uses EBP for the frame pointer and ESP for the stack
pointer.
<P>
If the second operand is 0, ENTER pushes the frame pointer (BP or
EBP) onto the stack; ENTER then subtracts the first operand from the
stack pointer and sets the frame pointer to the current stack-pointer
value.
<P>
For example, a procedure with 12 bytes of local variables would have an
ENTER 12,0 instruction at its entry point and a 
<A HREF="LEAVE.htm">LEAVE</A> instruction
before every <A HREF="RET.htm">RET</A>. 
The 12 local bytes would be addressed as negative
offsets from the frame pointer.

<H2>Flags Affected</H2>

None

<H2>Protected Mode Exceptions</H2>

#SS(0) if SP or ESP would exceed the stack limit at any point during
instruction execution; #PF(fault-code) for a page fault

<H2>Real Address Mode Exceptions</H2>

None

<H2>Virtual 8086 Mode Exceptions</H2>

None


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