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Unformatted text preview: time that instruction 2 collects its operands. Instruction 2 must therefore stall until the result is available, giving the behaviour shown in Figure 1.14 on page 23. This is a read-after-write pipeline hazard. Figure 1.13 Pipelined instruction execution. Processor design trade-offs 23 Branch instructions result in even worse pipeline behaviour since the fetch step of the following instruction is affected by the branch target computation and must therefore be deferred. Unfortunately, subsequent fetches will be taking place while the branch is being decoded and before it has been recognized as a branch, so the fetched instructions may have to be discarded. If, for example, the branch target calculation is performed in the ALU stage of the pipeline in Figure 1.13, three instructions will have been fetched from the old stream before the branch target is available (see Figure 1.15). It is better to compute the branch target earlier in the pipeline if possible, even though this will probably require dedicated hardware. If branch instructions have a fixed format, the target may be computed speculatively (that is, before it has been determined that the instruction is a branch) during the 'dec' stage, thereby reducing the branch latency to a single cycle, though note that in this pipeline there may still be hazards on a conditional branch due to dependencies on the condition code result of the instruction preceding the branch. Some RISC architectures (though not the ARM) Figure 1.14 Read-after-write pipeline hazard. Figure 1.15 Pipelined branch behaviour. 24 An Introduction to Processor Design define that the instruction following the branch is executed whether or not the branch is taken. This technique is known as the delayed branch. Pipeline efficiency Though there are techniques which reduce the impact of these pipeline problems, they cannot remove the difficulties altogether. The deeper the pipeline (that is, the more pipeline stages there are), the worse the problems get. For rea...
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- Spring '09