正确理解 LEAL (Load Effective Address) 指令

LEAL: leal S, D    ->    D ← &S

在 CSAPP (Computer Systems: A Programmer’s Perspective) 中，对 LEAL 指令用作简单算术运算的情况，给出了一个例子：

For example, if register %edx contains value x, leal 7(%edx,%edx,4), %eax will set register %eax to 5x + 7.

正确理解逻辑为：

1. 设%edx的值为x

2. 7(%edx,%edx,4) 为变质寻址，目标内存地址为5x+7

3. 设储存在地址5x+7的值为y。然而LEAL并不寻址，它只取y的地址。y的地址即为5x+7。因此，LEAL的左操作数为5x+7

CSAPP描述为:

It has the form of an instruction that reads from memory to a register, but it does not reference memory at all. Its first operand appears to be a memory reference, but instead of reading from the designated location, the instruction copies the effective address to the destination.

4. LEAL将y的地址，5x+7，赋给%eax

至此，LEAL实现了简单算术功能。