第一讲 流水线数据通路和控制逻辑第二讲 流水线冒险处理第....ppt

MIPS I has not the hardware solution and therefore leave the interlock checking work to the user. Notice that although the branch instruction is fetched during Cycle 4, its target address is NOT written into the Program Counter until the end of clock Cycle 7. Consequently, the branch target instruction is not fetched until clock Cycle 8. In other words, there is a 3-instruction delay between the branch instruction is issued and the branch effect is felt in the program. This is referred to as Branch Hazard in the text book. And as we will show in the next lecture, by clever design techniques, we can reduce the delay to ONE instruction. That is if the branch instruction in Location 12 is issued in Cycle 4, we will only execute one more sequential instruction (Location 16) before the branch target is executed. +2 = 71 min. (Y:51) Here are the timing diagrams showing the differences between the single cycle, multiple cycle, and pipeline implementations. For example, in the pipeline implementation, we can finish executing the Load, Store, and R-type instruction sequence in seven cycles. In the multiple clock cycle implementation, however, we cannot start executing the store until Cycle 6 because we must wait for the load instruction to complete. Similarly, we cannot start the execution of the R-type instruction until the store instruction has completed its execution in Cycle 9. In the Single Cycle implementation, the cycle time is set to accommodate the longest instruction, the Load instruction. Consequently, the cycle time for the Single Cycle implementation can be five times longer than the multiple cycle implementation. But may be more importantly, since the cycle time has to be long enough for the load instruction, it is too long for the store instruction so the last part of the cycle here is wasted. +2 = 77 min. (X:57) Let’s follow a Load instruction down the pipeline and see how everything works. Every instruction starts at the Instruction Fetch stage which: (a