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In computing, a bubble or pipeline stall is a delay in execution of an instruction in an instruction pipeline in order to resolve a hazard.
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During the decoding stage, the control unit will determine if the decoded instruction reads from a register that the instruction currently in the execution stage writes to. If this condition holds, the control unit will stall the instruction by one clock cycle. It also stalls the instruction in the fetch stage, to prevent the instruction in that stage from being overwritten by the next instruction in the program.
To prevent new instructions from being fetched when an instruction in the decoding stage has been stalled, the value in the PC register and the instruction in the fetch stage are preserved to prevent changes. The values are preserved until the bubble has passed through the execution stage.
The execution stage of the pipeline must always be performing an action. A bubble is represented in the execution stage as a NOP instruction, which has no effect other than to stall the instructions being executed in the pipeline.
Timeline
The following is two executions of the same four instructions through a 4-stage pipeline but, for whatever reason, a delay in fetching of the purple instruction in cycle #2 leads to a bubble being created delaying all instructions after it as well.
Classic RISC pipeline
The below example shows a bubble being inserted into a classic RISC pipeline, with five stages (IF = Instruction Fetch, ID = Instruction Decode, EX = Execute, MEM = Memory access, WB = Register write back). In this example, data available after the MEM stage (4th stage) of the first instruction is required as input by the EX stage (3rd stage) of the second instruction. Without a bubble, the EX stage (3rd stage) only has access to the output of the previous EX stage. Thus adding a bubble resolves the time dependence without needing to propagate data backwards in time (which is impossible).