nasm中的enter

nasm手册：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 theprevious 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.

  翻译并增加些解释：

 ENTER指令为高级语言的调用过程构造所必须的栈帧。第一个操作数（机器指令码中的iw(也就是伪指令中的imm)）给出为本地参数所需要的栈帧空间的大小（此子函数没有用到本地局部变量，股此处的栈帧控件大小为0），第二个参数给出进程调用的嵌套级别（对于一些编程语言，像C（或者原文中的Pascal）语言之类的，都是支持函数的嵌套调用的）。

  enter指令的功能，对于指定嵌套级别为0，功能等价于一下几条汇编指令：

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

上述指令构建出一个栈帧，栈帧大小为operand1.在EBP之上，是进程的参数，在EBP之下，是本地变量。

对于嵌套级别为1的函数的栈帧，栈帧要比嵌套级别为0的大4（或2，对于16为的来说）个字节，最后一个嵌套层的EBP可以在第0层的EBP地址减小4位后找到（注意栈是向下增长的）。

当enter调用嵌套级别为2的内嵌函数时，通过查找寄存器EBP描述的栈帧指针的以前的值，查找栈指针偏移量为-4的地址值，把这个值和新的栈指针值压栈，如此一来，当嵌套级别为2的进程调用嵌套级别为1的进程时，[EBP-4]保存最近的嵌套级别为1的栈指针，[EBP-8]保存级别为2的指针。按照这个规律，可以支持的最大嵌套级别为31层。