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- 1. Pipeline Performance F1 F2 F3 I1 I2 I3 E1 E2 E3 D1 D2 D3 W1 W2 W3 Instruction F4 D4 I4 Clock c y cle 1 2 3 4 5 6 7 8 9 Figure 8.3. Effect of an e xecution operation taking more than one clock c ycle. E4 F5 I5 D5 Time E5 W4
- 2. Pipeline Performance The previous pipeline is said to have been stalled for two clock cycles. Any condition that causes a pipeline to stall is called a hazard. Data hazard – any condition in which either the source or the destination operands of an instruction are not available at the time expected in the pipeline. So some operation has to be delayed, and the pipeline stalls. Instruction (control) hazard – a delay in the availability of an instruction causes the pipeline to stall. Structural hazard – the situation when two instructions require the use of a given hardware resource at the same time.
- 3. Pipeline Performance F1 F2 F3 I1 I2 I3 D1 D2 D3 E1 E2 E3 W1 W2 W3 Instruction Figure 8.4. Pipeline stall caused by a cache miss in F2. 1 2 3 4 5 6 7 8 9 Clock c y cle (a) Instruction execution steps in successiv e clock cy cles 1 2 3 4 5 6 7 8 Clock c y cle Stage F: Fetch D: Decode E: Execute W: Write F1 F2 F3 D1 D2 D3 idle idle idle E1 E2 E3 idle idle idle W1 W2 idle idle idle (b) Function perf ormed by each processor stage in successiv e clock cy cles 9 W3 F2 F2 F2 Time Time Idle periods – stalls (bubbles) Instruction hazard
- 4. Pipeline Performance F1 F2 F3 I1 I2 (Load) I3 E1 M2 D1 D2 D3 W1 W2 Instruction F4 I4 Clock c y cle 1 2 3 4 5 6 7 Figure 8.5. Effect of a Load instruction on pipeline timing. F5 I5 D5 Time E2 E3 W3 E4 D4 Load X(R1), R2 Structural hazard
- 5. Data Hazards We must ensure that the results obtained when instructions are executed in a pipelined processor are identical to those obtained when the same instructions are executed sequentially. Hazard occurs A ← 3 + A B ← 4 × A No hazard A ← 5 × C B ← 20 + C When two operations depend on each other, they must be executed sequentially in the correct order. Another example: Mul R2, R3, R4 Add R5, R4, R6
- 6. Data Hazards F1 F2 F3 I1 (Mul) I2 (Add) I3 D1 D3 E1 E3 E2 W3 Instruction Figure 8.6. Pipeline stalled by data dependenc y between D 2 and W 1. 1 2 3 4 5 6 7 8 9 Clock c y cle W1 D2A W2 F4 D4 E4 W4 I4 D2 Time Figure 8.6. Pipeline stalled by data dependency between D2 and W1.
- 7. Operand Forwarding Instead of from the register file, the second instruction can get data directly from the output of ALU after the previous instruction is completed. A special arrangement needs to be made to “forward” the output of ALU to the input of ALU.
- 9. Handling Data Hazards in Software Let the compiler detect and handle the hazard: I1: Mul R2, R3, R4 NOP NOP I2: Add R5, R4, R6 The compiler can reorder the instructions to perform some useful work during the NOP slots.
- 10. Side Effects The previous example is explicit and easily detected. Sometimes an instruction changes the contents of a register other than the one named as the destination. When a location other than one explicitly named in an instruction as a destination operand is affected, the instruction is said to have a side effect. (Example?) Example: conditional code flags: Add R1, R3 AddWithCarry R2, R4 Instructions designed for execution on pipelined hardware should have few side effects.
- 11. Overview Whenever the stream of instructions supplied by the instruction fetch unit is interrupted, the pipeline stalls. Cache miss Branch
- 12. Unconditional Branches F2 I2 (Branch) I3 Ik E2 F3 Fk Ek Fk+1 Ek+1 Ik+1 Instruction Figure 8.8. An idle c ycle caused by a branch instruction. Execution unit idle 1 2 3 4 5 Clock c y cle Time F1 I1 E1 6 X
- 13. Branch Timing Figure 8.9. Branch timing. Fk+1 Dk+1 Ik+1 Ek+1 (b) Branch address computed in Decode stage - Branch penalty - Reducing the penalty
- 14. Instruction Queue and Prefetching F : Fetch instruction E : Execute instruction W : Write results D : Dispatch/ Decode Instruction queue Instruction fetch unit Figure 8.10. Use of an instruction queue in the hardware organization of Figure 8.2b. unit
- 15. Conditional Braches A conditional branch instruction introduces the added hazard caused by the dependency of the branch condition on the result of a preceding instruction. The decision to branch cannot be made until the execution of that instruction has been completed. Branch instructions represent about 20% of the dynamic instruction count of most programs.