其他常见的格式还有 X-Form（接受两个加在一起的寄存器）和 A-Form（可保存一个常量或一个常量相对偏移量地址）。
The other popular formats are the X-Form, which takes two registers which are added together, or the A-Form, which can hold a constant or a constant relative offset address.
LK位指定了下一个顺序指令的地址是否作为子例程调用的返回地址保存在链接寄存器中。
The LK bit specifies whether the address of the next sequential instruction is saved in the Link Register as a return address for a subroutine call.
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