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

6-20 Vol. 1
PROCEDURE CALLS, INTERRUPTS, AND EXCEPTIONS
ENTER and LEAVE offer two benefits:
• They provide machine-language support for implementing block-structured
languages, such as C and Pascal.
• They simplify procedure entry and exit in compiler-generated code.
6.5.1 ENTER Instruction
The ENTER instruction creates a stack frame compatible with the scope rules typically
used in block-structured languages. In block-structured languages, the scope of a
procedure is the set of variables to which it has access. The rules for scope vary
among languages. They may be based on the nesting of procedures, the division of
the program into separately compiled files, or some other modularization scheme.
ENTER has two operands. The first specifies the number of bytes to be reserved on
the stack for dynamic storage for the procedure being called. Dynamic storage is the
memory allocated for variables created when the procedure is called, also known as
automatic variables. The second parameter is the lexical nesting level (from 0 to 31)
of the procedure. The nesting level is the depth of a procedure in a hierarchy of
procedure calls. The lexical level is unrelated to either the protection privilege level or
to the I/O privilege level of the currently running program or task.
ENTER, in the following example, allocates 2 Kbytes of dynamic storage on the stack
and sets up pointers to two previous stack frames in the stack frame for this proce-
dure:
ENTER 2048,3
The lexical nesting level determines the number of stack frame pointers to copy into
the new stack frame from the preceding frame. A stack frame pointer is a doubleword
used to access the variables of a procedure. The set of stack frame pointers used by
a procedure to access the variables of other procedures is called the display. The first
doubleword in the display is a pointer to the previous stack frame. This pointer is
used by a LEAVE instruction to undo the effect of an ENTER instruction by discarding
the current stack frame.
After the ENTER instruction creates the display for a procedure, it allocates the
dynamic local variables for the procedure by decrementing the contents of the ESP
register by the number of bytes specified in the first parameter. This new value in the
ESP register serves as the initial top-of-stack for all PUSH and POP operations within
the procedure.
To allow a procedure to address its display, the ENTER instruction leaves the EBP
register pointing to the first doubleword in the display. Because stacks grow down,
this is actually the doubleword with the highest address in the display. Data manipu-
lation instructions that specify the EBP register as a base register automatically
address locations within the stack segment instead of the data segment.
The ENTER instruction can be used in two ways: nested and non-nested. If the lexical
level is 0, the non-nested form is used. The non-nested form pushes the contents of