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Improved MIPS Pipeline Architecture: Analysis of Dual ALUs and Adder Integration

This review examines a proposed enhancement to the existing MIPS pipeline architecture by replacing the ALU in the execute stage with a simple adder while positioning the ALU in the memory stage. It compares the behavior of the pipeline under typical instruction sequences to identify scenarios that lead to pipeline bubbles in the traditional setup but not in the revised architecture, and vice versa. Additionally, we discuss the advantages and disadvantages of this new dual ALU datapath and the impact on instruction execution efficiency while evaluating further improvements to eliminate hazards.

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Improved MIPS Pipeline Architecture: Analysis of Dual ALUs and Adder Integration

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  1. CMPE 421 REVIEW: MIDTERM

  2. IR A addr inst Y Inst Memory B R A2 we rs1 rd1 rs2 we PC ws addr add wd rd2 GPRs rdata Data Memory Imm Ext MD1 MD2 wdata wdata ALU B2 A MODIFIED FIVE-Stage Pipeline

  3. Question • Consider a new datapath to improve the performance of our existing pipeline datapath that we have seen in class • In the new datapath the ALU the Execute stage is replaced by a simple adder and the original ALU is moved from the Execute stage to the Memory stage. • The adder in the 3rdstage is used only for address calculations involving load/store instructions. For all other instructions, the data is simply forwarded to the 4th stage. • The ALU is running in parallel with the data memory in the 4th stage of the pipeline. • During a load/store instruction, the ALU is inactive, while the data memory is inactive during the ALU instructions. • In this problem we will ignore jump and branch instructions.

  4. Question • Give an example sequence of MIPS instructions (five or fewer instructions) that would cause a pipeline bubble in the original datapath, but not in the new datapath. • Give an example sequence of MIPS instructions (five or fewer instructions) that would cause a pipeline bubble in the new datapath, but not in the original datapath. • Compare the advantages and disadvantages of the new datapath.

  5. Question • Further improvements to the fully bypassed 5-stage MIPS processor pipelines presented in page. In this new pipeline also the Adder with a proper ALU, with the goal of eliminating all hazards • The Dual ALU Pipeline has two ALUs: • ALU1 is in the 3rd pipeline stage (EX1) • ALU2 is in the 4th pipeline stage (EX2/MEM). A memory instruction always uses ALU1 to compute its address. An ALU instruction uses either ALU1 or ALU2, but never both. • If an ALU instruction’s operands are available (either from the register file or the forwarding path) by the end of the ID stage, the instruction uses ALU1; otherwise, the instruction uses ALU2.

  6. IR A addr inst Y Inst Memory B R IF/ID ID/EX EX2/MEM MEM/WB A2 we rs1 rd1 rs2 we PC ws addr ALU1 wd rd2 GPRs rdata Data Memory Imm Ext MD1 MD2 wdata wdata ALU2 B2 Modified-Modified Datapath

  7. Question • For the following instruction sequence, indicate which ALU each add instruction uses. • Assume that the pipeline is initially idle (for example, it has been executing nothing but nop instructions).

  8. Indicate whether each of the following instruction sequences causes a stall in the pipeline and a short summary of the reason why. Consider each sequence separately and assume that the pipeline is initially idle.

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