The ENTER instruction creates the stack frame required by most block-structured high-level languages. The first operand specifies the number of bytes of dynamic storage allocated on the stack for the routine being entered. The second operand gives the lexical nesting level (0 to 31) of the routine within the high-level language source code. It determines the number of stack frame pointers copied into the new stack frame from the preceding frame.
Both the operand-size attribute and the stack-size attribute are used to determine whether BP or EBP is used for the current frame pointer and SP or ESP is used for the stack pointer.
If the operand-size attribute is 16-bits, the processor uses the BP register as the frame pointer and the SP register as the stack pointer, unless the stack-size attribute is 32-bits in which case it uses EBP for the frame pointer and ESP for the stack pointer. If the operand-size attribute is 32-bits, the processor uses the EBP register for the frame pointer and the ESP register for the stack pointer, unless the stack-size attribute is 16-bits in which case it uses BP for the frame pointer and SP for the stack pointer.
If the second operand is 0, the ENTER instruction pushes the frame pointer (BP or EBP register) onto the stack; the ENTER instruction then subtracts the first operand from the stack pointer and sets the frame pointer to the current stack-pointer value.
For example, a procedure with 12 bytes of local variables would have an ENTER 12,0 instruction at its entry point and a LEAVE instruction before every RET instruction. The 12 local bytes would be addressed as negative offsets from the frame pointer.