i386/i386.reference/ENTER.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode ENTER</TITLE>
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   8 Chapter 17 -- 80386 Instruction Set</A><BR>
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  11 <P>
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  13 <P>
  14 <H1>ENTER -- Make Stack Frame for Procedure Parameters</H1>
  15
  16 <PRE>
  17 Opcode      Instruction        Clocks     Description
  18
  19 C8 iw 00    ENTER imm16,0      10         Make procedure stack frame
  20 C8 iw 01    ENTER imm16,1      12         Make stack frame for procedure
  21                                           parameters
  22 C8 iw ib    ENTER imm16,imm8   15+4(n-1)  Make stack frame for
  23                                           procedure parameters
  24 </PRE>
  25
  26 <H2>Operation</H2>
  27
  28 <PRE>
  29 level := level MOD 32
  30 IF OperandSize = 16 THEN Push(BP) ELSE Push (EBP) FI;
  31    (* Save stack pointer *)
  32 frame-ptr := eSP
  33 IF level > 0
  34 THEN (* level is rightmost parameter *)
  35    FOR i := 1 TO level - 1
  36    DO
  37       IF OperandSize = 16
  38       THEN
  39          BP := BP - 2;
  40          Push[BP]
  41       ELSE (* OperandSize = 32 *)
  42          EBP := EBP - 4;
  43          Push[EBP];
  44       FI;
  45    OD;
  46    Push(frame-ptr)
  47 FI;
  48 IF OperandSize = 16 THEN BP := frame-ptr ELSE EBP := frame-ptr; FI;
  49 IF StackAddrSize = 16
  50 THEN SP := SP - First operand;
  51 ELSE ESP := ESP - ZeroExtend(First operand);
  52 FI;
  53 </PRE>
  54
  55 <H2>Description</H2>
  56
  57 ENTER creates the stack frame required by most block-structured
  58 high-level languages. The first operand specifies the number of bytes of
  59 dynamic storage allocated on the stack for the routine being entered.
  60 The second operand gives the lexical nesting level (0 to 31) of the routine
  61 within the high-level language source code. It determines the number of
  62 stack frame pointers copied into the new stack frame from the preceding
  63 frame. BP (or EBP, if the operand-size attribute is 32 bits) is the current
  64 stack frame pointer.
  65 <P>
  66 If the operand-size attribute is 16 bits, the processor uses BP as the
  67 frame pointer and SP as the stack pointer. If the operand-size attribute is
  68 32 bits, the processor uses EBP for the frame pointer and ESP for the stack
  69 pointer.
  70 <P>
  71 If the second operand is 0, ENTER pushes the frame pointer (BP or
  72 EBP) onto the stack; ENTER then subtracts the first operand from the
  73 stack pointer and sets the frame pointer to the current stack-pointer
  74 value.
  75 <P>
  76 For example, a procedure with 12 bytes of local variables would have an
  77 ENTER 12,0 instruction at its entry point and a
  78 <A HREF="LEAVE.htm">LEAVE</A> instruction
  79 before every <A HREF="RET.htm">RET</A>.
  80 The 12 local bytes would be addressed as negative
  81 offsets from the frame pointer.
  82
  83 <H2>Flags Affected</H2>
  84
  85 None
  86
  87 <H2>Protected Mode Exceptions</H2>
  88
  89 #SS(0) if SP or ESP would exceed the stack limit at any point during
  90 instruction execution; #PF(fault-code) for a page fault
  91
  92 <H2>Real Address Mode Exceptions</H2>
  93
  94 None
  95
  96 <H2>Virtual 8086 Mode Exceptions</H2>
  97
  98 None
  99
 100
 101 <P>
 102 <HR>
 103 <P>
 104 <B>up:</B> <A HREF="c17.htm">
 105 Chapter 17 -- 80386 Instruction Set</A><BR>
 106 <B>prev:</B><A HREF="DIV.htm"> DIV Unsigned Divide</A><BR>
 107 <B>next:</B><A HREF="HLT.htm"> HLT Halt</A>
 108 </BODY>