Pipeline hazards based on the material prepared by arvind and krste asanovic. Pipelining is an implementation technique that exploits parallelism among the instructions in a sequential instruction stream. Data hazards control hazards what is the next instruction to execute if a branch is taken. Pipeline is divided into stages and these stages are. Pipeline hazards are situations that prevent the next instruction in the instruction stream from executing during its designated clock cycles. Control hazards control hazards instructions are fetched in stage 1 if branch and jump decisions occur in stage 3 ex i. Control hazards can cause a greater performance loss for dlx pipeline than data hazards. Multiple execution units alus several instructions executed simultaneously cpi control hazards control hazards instructions are fetched in stage 1 if branch and jump decisions occur in stage 3 ex i. Pipelining jumps i 1 096 add i 2 100 j 200 i 3 104 add i 4 304 add kill i 2 i 1 104 stall ir. Schedule programmer explicitly avoids scheduling instructions that would create data hazards. Control hazards issue description the presence of a conditional branch alters the sequential. If a branch changes the pc to its target address, it is a taken branch.
Pipelining hazards pipeline hazards prevent next instruction from executing during designated clock cycle there are 3 classes of hazards. As long as we do that, everything else works the same as before. Pipelining 1 cis 501 introduction to computer architecture unit 6. They arise when an instruction depends on the result of a previous instruction in a way that is exposed by the overlapping of instructions in the pipeline. Jul 04, 2018 control hazards hazards in pipelining duration.
Like any other optimization, it should not change the semantics. Pipelining break instructions into steps work on instructions like in an assembly line allows for more instructions to be executed in less time a nstage pipeline is n times faster than a non pipeline processor in theory 3. Possible solutions stall until the branch is revolved. Handling hazards generally introduces bubbles into pipeline and reduces ideal cpi 1. Control hazards this is lecture from my old class notes. Occur when given instruction depends on data from an. Computer organization and architecture pipelining set 2. On many instruction architectures, the processor will not know the target address of these instructions when it needs to insert the new instruction into the pipeline.
Hazards during pipelining operand forwarding and delay the pipe technique duration. Thus, every branch instruction incurs a penalty equal to the number of stall cycles. Perfect pipelining with no hazards an instruction completes every cycle total cycles num instructions. Spot the data dependencies in the code below and figure out how forwarding can resolve data hazards. Control hazards instructions that disrupt the sequential flow of control present problems for pipelines. Deal with data and control hazards pipelining is an optimization to the implementation. A pipeline is correct only if the resulting machine satis. Hazards that impact pipelining situations that prevent starting the next instruction in the next cycle structure hazards a required resource is busy data hazard need to wait for previous instruction to complete its data readwrite control hazard deciding on control action depends on previous instruction. Pipelining break instructions into steps work on instructions like in an assembly line allows for more instructions to be executed in less time a nstage pipeline is n times faster than. As a result of which some operation has to be delayed and the pipeline stalls.
Three common types of hazards are data hazards, structural hazards, and control hazards branching hazards. Control dependency branch hazards this type of dependency occurs during the transfer of control instructions such as branch, call, jmp, etc. Pipelining is not suitable for all kinds of instructions. Pipeline hazards 1 pipeline hazards are situations that prevent the next instruction in the instruction stream from executing in its designated clock cycle hazards reduce the performance from the ideal speedup gained by pipelining three types of hazards structural hazards data hazards control hazards pipeline hazards 2 hazards in pipeline can make the pipeline to stall. A hazard describes any situation where the processor may need to stall due to lack of a certain resource or changes in control flow. Pipeline control hazards and instruction variations. 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. Arise from the pipelining of conditional branches and other. Cse 141, s206 jeff brown pipelining and exceptions exceptions represent another form of control dependence. Pipelining basicsstructural hazards data hazards overview of data hazards i data hazards occur when one instruction depends on a data value produced by an preceding instruction still in the pipeline i approaches to resolving data hazards. Control hazards simple techniques to handle control hazard stalls. Control hazards control hazards occur when we dont know which instruction to execute next mostly caused by branches strategies for dealing with them stall guess. When some instructions are executed in pipelining they can stall the pipeline or flush it totally.
The following are solutions that have been proposed for mitigating aspects of control hazards. A control hazard occurs if there is a control instruction e. When a branch is executed, it may or may not change the pc program counter to something other than its current value plus 4. Hazards in pipelines can make it necessary to stall the pipeline. Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising.
Arise from hardware resource conflicts when the available control hazards. Pipelining basics structural hazardsdata hazards an ideal pipeline stage 1 stage 2 stage 3 stage 4 i all objects go through the same stages i no sharing of resources between any two stages i propagation delay through all pipeline stages is equal i scheduling of a transaction entering pipeline is not affected by transactions in other stages i these conditions generally. Dec 11, 2017 pipeline hazards in computer architecture ppt. Pipelining for instruction execution is similar to construction of factor assembly line for product manufacturing. In a typical computer program besides simple instructions, there are branch instructions, interrupt operations, read and write instructions. Pipeline hazards 1 pipeline hazards are situations that prevent the next instruction in the instruction stream from executing in its designated clock cycle hazards reduce the performance from the ideal speedup gained by pipelining three types of hazards structural hazards data hazards control hazards pipeline hazards 2 hazards in pipeline can make the. Leads to speculation flushing the pipeline strategies for making better guesses understand the difference between stall and. Pipelined processors are great for speed, but by their very nature they have multiple instructions in flight at. Freeze the pipeline until the branch outcome and target are known, then proceed with fetch. Computer organization and architecture pipelining set. Concept of pipelining computer architecture tutorial.
Pipelining is a technique where multiple instructions are overlapped during execution. First, take note of the fact that we have no data hazards since no instruction depends on data from a previous instruction. The stall does not occur until after id stage where we know that the instruction is a branch this control hazards stall must be implemented differently from a data hazard, since the if cycle of the instruction following the branch must be repeated as soon as we know the branch outcome. Therefore, they create a potential branch hazard exceptions must be recognized early enough in the pipeline that subsequent instructions can be flushed before they change any permanent state.
Any condition that causes a stall in the pipeline operations can be called a hazard. What is pipelining hazard in computer organization and. They arise from the pipelining of branches and other instructions that change the pc. Cs 152 computer architecture and engineering cs252. Lecture 10 control hazards and advanced pipelinning. Structural hazards in pipelining pdf structural hazards. Control hazards what do we need to calculate next pc. As long as we do that, everything else works the same. We also have no control hazards since we are not branching or jumping. Irs and control points ir ir ir 31 pc a b y r md1 md2 addr inst inst memory 0x4 add ir imm ext alu rd1 gprs rs1 rs2 ws wdrd2 we wdata. Thus, the first if cycle is essentially a stall because it never performs useful work, which comes to.
It allows storing and executing instructions in an orderly process. Pipelining is the process of accumulating instruction from the processor through a pipeline. Hazards during pipelining operand forwarding and delay the pipe technique. Pipelining improves performance by increasing instruction throughput. Beq because the program counter pc following the control instruction is not known until the control instruction computes if the branch should be taken or not. Pipelining hazards a hazard is a situation that prevents starting the next instruction in the next clock cycle 1 structural hazard a required resource is busy e. Cs 152 computer architecture and engineering cs252 graduate.
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