Please see Set 1 for Execution, Stages and Performance (Throughput) and Set 3 for Types of Pipeline and Stalling. Dependencies in a pipelined processor There are mainly three types of dependencies possible in a pipelined processor. These are : 1) Structural Dependency 2) Control Dependency 3) Data Dependency These dependencies may introduce stalls in the pipeline. Stall : A stall is a cycle in the pipeline without new input. Structural dependency This dependency arises due to the resource conflict in the pipeline. A resource conflict is a situation when more than one instruction tries to access the same resource in the same cycle. A resource can be a register, memory, or ALU. Example:
To minimize data dependency stalls in the pipeline, operand forwarding is used. Operand Forwarding : In operand forwarding, we use the interface registers present between the stages to hold intermediate output so that dependent instruction can access new value from the interface register directly. Operand Forwarding can avoid stalls only if the dependent instructions are ALU type instructions. Considering the same example: I1 : ADD R1, R2, R3 I2 : SUB R4, R1, R2
| Instruction / Cycle | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| I1 | IF | ID | EX | DM |
| I2 | IF | ID | EX |
Data Hazards Data hazards occur when instructions that exhibit data dependence, modify data in different stages of a pipeline. Hazard cause delays in the pipeline. There are mainly three types of data hazards: 1) RAW (Read after Write) [Flow/True data dependency] 2) WAR (Write after Read) [Anti-Data dependency] 3) WAW (Write after Write) [Output data dependency] Let there be two instructions I and J, such that J follow I. Then,
