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INSTRUCTION SET REFERENCE
ENTERMake Stack Frame for Procedure Parameters
Description
This instruction creates a stack frame for a procedure. The first operand (size operand) specifies
the size of the stack frame (that is, the number of bytes of dynamic storage allocated on the stack
for the procedure). The second operand (nesting level operand) gives the lexical nesting level (0
to 31) of the procedure. The nesting level determines the number of stack frame pointers that are
copied into the display area of the new stack frame from the preceding frame. Both of these
operands are immediate values.
The stack-size attribute determines whether the BP (16 bits) or EBP (32 bits) register specifies
the current frame pointer and whether SP (16 bits) or ESP (32 bits) specifies the stack pointer.
The ENTER and companion LEAVE instructions are provided to support block structured
languages. The ENTER instruction (when used) is typically the first instruction in a procedure
and is used to set up a new stack frame for a procedure. The LEAVE instruction is then used at
the end of the procedure (just before the RET instruction) to release the stack frame.
If the nesting level is 0, the processor pushes the frame pointer from the EBP register onto the
stack, copies the current stack pointer from the ESP register into the EBP register, and loads the
ESP register with the current stack-pointer value minus the value in the size operand. For nesting
levels of one or greater, the processor pushes additional frame pointers on the stack before
adjusting the stack pointer. These additional frame pointers provide the called procedure with
access points to other nested frames on the stack. Refer to Section 4.5., Procedure Calls for
Block-Structured Languages in Chapter 4, Procedure Calls, Interrupts, and Exceptions of the
Intel Architecture Software Developers Manual, Volume 1, for more information about the
actions of the ENTER instruction.
Opcode
Instruction
Description
C8 iw 00
ENTER imm16,0
Create a stack frame for a procedure
C8 iw 01
ENTER imm16,1
Create a nested stack frame for a procedure
C8 iw ib
ENTER imm16,imm8Create a nested stack frame for a procedure
