GNU Info

(nasm.info)Section B.4.65

---

Next: Section B.4.66 Prev: Section B.4.64 Up: Section B.4

---

Enter node , (file) or (file)node

B.4.65. `ENTER': Create Stack Frame
-----------------------------------

     ENTER imm,imm                 ; C8 iw ib             [186]

   `ENTER' constructs a `stack frame' for a high-level language
procedure call. The first operand (the `iw' in the opcode definition
above refers to the first operand) gives the amount of stack space to
allocate for local variables; the second (the `ib' above) gives the
nesting level of the procedure (for languages like Pascal, with nested
procedures).

   The function of `ENTER', with a nesting level of zero, is equivalent
to

               PUSH EBP            ; or PUSH BP         in 16 bits
               MOV EBP,ESP         ; or MOV BP,SP       in 16 bits
               SUB ESP,operand1    ; or SUB SP,operand1 in 16 bits

   This creates a stack frame with the procedure parameters accessible
upwards from `EBP', and local variables accessible downwards from `EBP'.

   With a nesting level of one, the stack frame created is 4 (or 2)
bytes bigger, and the value of the final frame pointer `EBP' is
accessible in memory at `[EBP-4]'.

   This allows `ENTER', when called with a nesting level of two, to look
at the stack frame described by the _previous_ value of `EBP', find the
frame pointer at offset -4 from that, and push it along with its new
frame pointer, so that when a level-two procedure is called from within
a level-one procedure, `[EBP-4]' holds the frame pointer of the most
recent level-one procedure call and `[EBP-8]' holds that of the most
recent level-two call. And so on, for nesting levels up to 31.

   Stack frames created by `ENTER' can be destroyed by the `LEAVE'
instruction: see *Note Section B.4.136::.