Question

What are various Pipeline Hazards. Explain each hazard with some common solutions that arebeing employed today?

Answer 1

Given that

Pipeline Hazards:

There are situations, called hazards, that prevent the next instruction in the instruction stream from being executing during its designated clock cycle. Hazards reduce the performance from the ideal speedup gained by pipelining.

Here 3 types of pipeline hazards:

1) Data Hazards

2) Control Hazards or instruction Hazards

3) Structural Hazards

1) Data Hazards:

They arise from the pipelining of branches and other instructions that change the PC.

A data hazard is any condition in which either the source or the destination operands of an instruction are not available at the time expected in the pipeline. As a result of which some operation has to be delayed and the pipeline stalls. Whenever there are two instructions one of which depends on the data obtained from the other.

A=3+A

B=A*4

2) Control Hazards or instruction Hazards:

The instruction fetch unit of the CPU is responsible for providing a stream of instructions to the execution unit. The instructions fetched by the fetch unit are in consecutive memory locations and they are executed.

However the problem arises when one of the instructions is a branching instruction to some other memory location. Thus all the instruction fetched in the pipeline from consecutive memory locations are invalid now and need to removed(also called flushing of the pipeline).This induces a stall till new instructions are again fetched from the memory address specified in the branch instruction.

3)Structural Hazards:

  They arise from resource conflicts when the hardware cannot support all possible combinations of instructions in simultaneous overlapped execution.

This situation arises mainly when two instructions require a given hardware resource at the same time and hence for one of the instructions the pipeline needs to be stalled