It has two parts: a memory address and an index register. 它包括两个部分:一个内存地址以及一个索引寄存器。
Any memory access that is not available by other modes ( the address can be calculated by other means and stored in the register, which is then used for the access) 使用其他模式无法进行的内存访问(地址可以通过其他方式进行计算,并存储到寄存器中,然后就使用这个值来访问内存)