MIPS 跳转指令共分为三类：基于PC的相对跳转、基于PC区域的相对跳转、基于寄存器的绝对跳转。其中基于 PC 区域的相对跳转也就是我们要说的 J 类指令。
J类指令有长达26位的指令 index 编码域，因为指令都是4字节对齐的，所有表示的范围是 256M（28位）。那么J类跳转的目标地址是如何计算的呢？
目标PC = 延迟槽指令PC的28位以上的高位 || (J类指令26位的立即数 << 2)
是不是不易想像范围，看看图示吧：
j-class |: 265M 边界 j: j 指令位置 t: 可行的跳转目标位置 ----------------|--------------------------------|--------------------------------|---------------- ---------------j|tttttttttttttttttttttttttttttttt|--------------------------------|---------------- ----------------j-ttttttttttttttttttttttttttttttt|--------------------------------|---------------- ----------------|j-tttttttttttttttttttttttttttttt|--------------------------------|---------------- ----------------|tttttttttttttttj-ttttttttttttttt|--------------------------------|---------------- ----------------|ttttttttttttttttttttttttttttttj-|--------------------------------|---------------- ----------------|-------------------------------j|tttttttttttttttttttttttttttttttt|---------------- ----------------|--------------------------------j-ttttttttttttttttttttttttttttttt|---------------- |
Over!
哥们,配图啥意思呀?
就是J类指令所处的位置与其可跳转目标间的关系呀。:)