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In a standard five-stage pipeline, during the decoding stage, the control unit will determine whether the decoded instruction reads from a register to which the currently executed instruction writes. If this condition holds, the control unit will stall the instruction by one clock cycle.
The pipeline could stall, or cease scheduling new instructions until the required values are available. This results in empty slots in the pipeline, or bubbles , in which no work is performed. An additional data path can be added that routes a computed value to a future instruction elsewhere in the pipeline before the instruction that produced ...
Bubbling the pipeline, also termed a pipeline break or pipeline stall, is a method to preclude data, structural, and branch hazards. As instructions are fetched, control logic determines whether a hazard could/will occur. If this is true, then the control logic inserts no operation s (NOP s) into the pipeline. Thus, before the next instruction ...
The processor's pipeline will normally have already read the next instruction, the write, by the time the ALU has calculated which path it will take. This is known as a branch hazard. If it has to return to the top, the write instruction has to be discarded and the read instruction read from memory instead. That takes one full instruction cycle ...
When a next-line predictor points to aligned groups of 2, 4, or 8 instructions, the branch target will usually not be the first instruction fetched, and so the initial instructions fetched are wasted. Assuming for simplicity, a uniform distribution of branch targets, 0.5, 1.5, and 3.5 instructions fetched are discarded, respectively.
Avoid illegal or semantically ambiguous operations (typically involving subtle instruction pipeline timing issues or non-interlocked resources). The pipeline stalls can be caused by structural hazards (processor resource limit), data hazards (output of one instruction needed by another instruction) and control hazards (branching).
Operand forwarding (or data forwarding) is an optimization in pipelined CPUs to limit performance deficits which occur due to pipeline stalls. [1] [2] A data hazard can lead to a pipeline stall when the current operation has to wait for the results of an earlier operation which has not yet finished.
The main common concept of each design is a five-stage execution instruction pipeline. During operation, each pipeline stage works on one instruction at a time. Each of these stages consists of a set of flip-flops to hold state, and combinational logic that operates on the outputs of those flip-flops.