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Hazards in pipeline
The document discusses different types of hazards that can occur in an instruction pipeline: data hazards, control hazards, and structural hazards. Data hazards include RAW (read after write), WAR (write after read), and WAW (write after write) and occur when there are dependencies between instructions. Control hazards occur due to incorrect branch predictions. Structural hazards happen when multiple instructions need the same functional unit or resource. These hazards can be avoided through techniques like operand forwarding, renaming, branch prediction, and increasing resources or latency.
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Hazards in pipeline
- 1. Hazards in Pipeline Preparedby : Ms. Snehalata Agasti CSE department
- 2. Hazards Hazards meansproblem occurs in instruction pipeline (or) if two or more microoperations occurred at same time than hazards occurs. It is of three types. -Data hazards -Control hazards -Structural hazards e.g. multiple instructions wants to access single ALU or memory. These are called stall in pipeline. ➢ Hazards can be avoided by using operand forwarding, renaming , branch prediction technique or by introducing some stall cycles.
- 3. Structural hazards Structuralhazards occurs when more than one instructions in pipeline and they need same resource. It is also known as resource hazards. This problem can be solved by increasing the resources. F D W F D W F D W t0 t1 t2 I1 I2 I3 Both micro operations need memory operations
- 4. Control hazards Whenbranch instruction is executed and some wrong predictions happened then some instructions brought into pipelining can be discarded, that is called control hazards/branch hazards. E.g. 100: goto lb: 101: Instruction1; 102: Instruction2; 103: Instruction3; 104lb: Instruction4; By using branch predictions or by increasing latency , control hazards can be avoided. Instruction1 and Instruction2 is loaded in pipeline Wrong prediction
- 5. Data hazards Whendata dependency is present in instructions that is called data hazards. Data is modified in different stages. It is of three types -RAW -WAR -WAW E.g. instruction-1: C=A+B; instruction-2: E=C+D Data hazards can be avoided by using Operand forwarding.
- 6. RAW(Read After Write) RAW data hazards occurs when our required result is not found after the execution of instructions. E.g I1 -: R2 R7 + R5 I2 -: R4 R2 + R3 Instrution-2 should be executed after the complete execution of instruction- 1. If R2 in I2 is read before writing operation performed in I1 then RAW data hazards can be occurred.
- 7. WAR(Wite After Read) WAR data hazards occurs when our awaited result is not found after the execution of instructions. E.g I1 -: R2 R7 + R5 I2 -: R5 R2 + R3 Instrution-2 should be executed after the complete execution of instruction- 1. If R5 in I2 is stored before reading operation performed in I1 then WAR conflict can be occurred.
- 8. WAW(Write After Write) WAR data hazards occurs when our proper result is not found after the complete execution of instructions. E.g I1 -: R4 R1 + R5 I2 -: R4 R1 + R3 Instrution-2 should be executed after the complete execution of instruction- 1. If R4 in I2 is stored before writing operation performed in I1 then WAW data hazards can be occurred.