Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 6-21
PROCEDURE CALLS, INTERRUPTS, AND EXCEPTIONS
the EBP register on the stack, copies the contents of the ESP register into the EBP
register, and subtracts the first operand from the contents of the ESP register to allo-
cate dynamic storage. The non-nested form differs from the nested form in that no
stack frame pointers are copied. The nested form of the ENTER instruction occurs
when the second parameter (lexical level) is not zero.
The following pseudo code shows the formal definition of the ENTER instruction.
STORAGE is the number of bytes of dynamic storage to allocate for local variables,
and LEVEL is the lexical nesting level.
PUSH EBP;
FRAME_PTR ← ESP;
IF LEVEL > 0
THEN
DO (LEVEL − 1) times
EBP ← EBP − 4;
PUSH Pointer(EBP); (* doubleword pointed to by EBP *)
OD;
PUSH FRAME_PTR;
FI;
EBP ← FRAME_PTR;
ESP ← ESP − STORAGE;
The main procedure (in which all other procedures are nested) operates at the
highest lexical level, level 1. The first procedure it calls operates at the next deeper
lexical level, level 2. A level 2 procedure can access the variables of the main
program, which are at fixed locations specified by the compiler. In the case of level 1,
the ENTER instruction allocates only the requested dynamic storage on the stack
because there is no previous display to copy.
A procedure that calls another procedure at a lower lexical level gives the called
procedure access to the variables of the caller. The ENTER instruction provides this
access by placing a pointer to the calling procedure's stack frame in the display.
A procedure that calls another procedure at the same lexical level should not give
access to its variables. In this case, the ENTER instruction copies only that part of the
display from the calling procedure which refers to previously nested procedures
operating at higher lexical levels. The new stack frame does not include the pointer
for addressing the calling procedure’s stack frame.
The ENTER instruction treats a re-entrant procedure as a call to a procedure at the
same lexical level. In this case, each succeeding iteration of the re-entrant procedure
can address only its own variables and the variables of the procedures within which it
is nested. A re-entrant procedure always can address its own variables; it does not
require pointers to the stack frames of previous iterations.
By copying only the stack frame pointers of procedures at higher lexical levels, the
ENTER instruction makes certain that procedures access only those variables of
higher lexical levels, not those at parallel lexical levels (see Figure 6-6).