What, in the context of pipelined processors, is a bubble and why is it detrimental to the performance of a pipelined processor?
What will be an ideal response?
As an instruction flows through a pipeline, various operations are applied to it. For example, in the first stage it is
fetched from memory and it may be decoded. In the second stage any operands it requires are read from the
register file, and so on. Sometimes, it is not possible to perform an operation on an instruction. For example, if an
operand is required and that operand is not ready, the stage processing the operand cannot continue. This results in a bubble or a stall when ‘nothing happens’. Equally, bubbles appear when a branch is taken and instructions following the branch are no longer going to be executed. So, a bubble is any condition that leads to a stage in the pipeline not performing its normal operation because it cannot proceed. A bubble is detrimental to performance because it means that an operation that could be executed is not executed and its time slot is wasted.
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