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。
