Von Neumann architecture
In a Von Neumann architecture which uses the program counter (PC) register to determine the current instruction being fetched in the pipeline, 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 instruction causing the conflict has passed through the execution stage. Such an event is often called a bubble, by analogy with an air bubble in a fluid pipe.