The part of a two-part memory address that remains constant and provides a reference point from which the location of a byte of data can be calculated.
保持恒定的两部分内存地址的一部分并提供一个基准点，从这里可以计算一个字节数据的位置。
2
The final address is calculated using this formula
使用这个公式计算最终的地址
3
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).