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Reoder buffer(ROB)
为了解决异常问题
instruction
的decode阶段被写入缓冲区的条目,指令完成的话,向缓冲区写入结果,最早decode的指令(程序顺序)如果没有被标记异常的话,写入reg file
register rename
是为了解决假的冲突,什么是假的冲突呢,请看下面这三个例子
只有第一个是冲突的。 Output and anti dependencies are not true dependencies
改名可以这样改
Register ID ——》 ROB entry ID Architectural register ID ---》 Physical
register ID
整体流程
Reorder buffer的trade off
Out of Order Execution
设计初衷
面临的问题,需要做到的事
过程
比如r8 + r9 ---> r10 r10在这个指令结束前都不会得到,那么我们就要将valid标记为0,然后tag就是保留站的offset.
完成的话就broadcast这个tag和value
如果广播后有指令的源reg都valid的话,就wake up这个指令
out of order store/load问题
memory很难rename,因为地址太多了,同时在5级流水线中,要aluout作为address,是动态而不是静态的依赖判断
summary