Final answer:
To ensure correct execution in a pipelined datapath, NOPs need to be inserted to account for data hazards if no forwarding or hazard detection is applied. Code can be rearranged to avoid the need for NOPs. If forwarding is applied, data can be directly forwarded to the stages that require it, eliminating the need for NOPs.
Step-by-step explanation:
In the given sequence of instructions, the following pipelined datapath diagram can be drawn:
a) If there is no forwarding or hazard detection
To ensure correct execution in this case, we need to insert NOPs to account for the pipeline stalls caused by data hazards. A data hazard occurs when an instruction depends on the result of a previous instruction that has not been completed yet. In this case, we need to add NOPs after the load instructions (ld) to allow enough time for the data to be read from the memory and written to the destination register.
b) Re-arrange the code to avoid NOPs
To avoid the need for NOPs, we can rearrange the code by moving the instructions that cause hazards or dependencies apart. For example, we can move the add instruction before the two load instructions to allow more time for the data to be read from the memory and avoid data hazards.
c) If forwarding is applied
With forwarding, we can eliminate the need for NOPs by allowing data to be forwarded directly from the previous stages to the stages where it is needed. In this case, the forwarding path would involve connecting the output of the ALU (Arithmetic Logic Unit) stage to the input of the following stages that require the data. For example, the result of the add instruction can be forwarded to the input of the or instruction, eliminating the need for a stall.
The question addresses computation in a five-stage pipelined processor, including handling data hazards with NOPs, code reordering to reduce stalls, and representing forwarding paths in the pipeline.
The student's question involves computations related to a five-stage instruction pipeline in a processor. The instructions provided are to be executed in such a way that the right data is used at the right time, despite potential data hazards. There are three parts to this question:
In the absence of forwarding or hazard detection mechanisms, we need to insert No-Operation (NOPs) instructions to prevent data hazards.
Next, we must rearrange the code to avoid using NOPs.
Finally, we need to illustrate how forwarding would allow the instructions to execute without stalls by showing the connections between pipeline stages where data gets forwarded.
It is crucial to understand the concepts of pipeline hazards, how NOPs work, instruction scheduling to reduce stalls, and the mechanics of forwarding in pipeline architecture to answer this question correctly.