Question 1. Pipeline Stages

Consider a five stage pipelined (fetch, decode, execute, memory, write back) processor.

Question 1.A Baseline

Fill out the first row in the table above for the given pipelined processor. Cycle time is the clock period at which this machine can run. Max clock frequency is the maximum frequency clock that can be run given the cycle time (lower frequencies are possible). Latency of instruction is the time it takes to get the first output after it starts. Throughput is the rate at which output is produced.

Question 1.B Faster Memory

Suppose there is an optimization that reduces the Memory stage latency by 100 ps. However, the optimization in memory stage results in 100ps increase in the Write back stage latency. How much would this improve the overall performance of the 8-stage pipeline? Fill out the second row in the table above for this new pipeline.

Question 1.C Proposed Scheme

If you had the option to rearrange the pipeline, how would your arrange the pipeline to achieve maximum frequency?

Question 2. Datapath Bypassing

In this problem we will be investigating the implications of using a two-cycle pipelined integer ALU. Our new pipelined processor will have the following six stages:
• F - instruction fetch
• D - decode and read registers
• X0 - first half of the ALU operation
• X1 - second half of the ALU operation
• M - data memory read/write
• W - write registers

The figure on the next page shows the datapath of the new six-stage datapath. Notice that this datapath currently does not allow any data bypassing. Also notice that conditional branches are resolved by the end of the X1 stage. Assume that this instruction set does not include a branch delay slot.

1 add r3, r1, r2
2 xor r7, r3, r4
3 and r5, r2, r3
4 xor r8, r8, r8
5 add r8, r7, r3
6 sw r5, 400(r3)

Question 2.A Implementing Bypassing

Modify the figure on the next page to implement the di↵erent types of bypassing.

Question 2.B Stalling

For the above assembly code, draw a pipeline diagram using stalls to resolve any data de- pendencies that you find.

Question 2.C Bypassing

For the above assembly code, draw a pipeline diagram using whichever bypass you think is valid for that case. Indicate which type of bypass you have used for each case.

Question 3. Pipelining and Hazzards

Consider the MIPS assembly code given below. We want to run this code on a 5-stage pipelined processor, with some modifications. The processor is a typical 5-stage pipeline (F-D-X-M-W), with the following exceptions:

• The multiplier block used to execute the mul instruction is pipelined into three stages as shown:

This means that a multiply instruction runs through the pipeline as follows: F-D-X0- X1-X2-M-W and up to three multiply instructions maybe in-flight at a time. All other instruction types are blocked from the execute stage while any of the multiply stages are being used.

• The divider block used to execute the div instruction is iterative and takes five cycles as shown:

This means that a divide instruction runs through the pipeline as follows: F-D-X0- X0-X0-X0-X0-M-W. All other instructions are blocked from the execute stage while a division is being done.

1 xor r0, r1, r1
2 addiu r1, r0, 16
3 j L1
4 loop: lw r3, 0(r2)
5 mul r4, r3, r3
6 div r3, r4, r3
7 mul r3, r3, r1
8 mul r3, r3, r0
9 addiu r0, r0, 2
10 sw r3, 0(r2)
11 addiu r2, r2, 4
12 L1: bne r0, r1, -9

Question 3.A Structural Hazards

Draw a pipeline diagram showing the execution of the MIPS code through the first iteration of the loop, without bypassing. Assume data hazards and structural hazards are resolved using only stalling. Assume the processor assumes branches are not taken, until they are resolved. What is the CPI of the entire program?

Question 3.B Data Hazards

Draw a pipeline diagram similar to Part A, but now assume the processor has data bypassing. What is the CPI of the entire program?

Question 3.C Control Hazards

Will the assembly code shown above lead to control hazards? If yes, Where does it occur and how can it be solved?

Question 4. Out-of-Order Execution

This question is based on the same pipeline and code as question 3.

Consider the MIPS assembly code given below. We want to run this code on a 5-stage pipelined processor, with some modifications. The processor is a typical 5-stage pipeline (F-D-X-M-W), with the following exceptions:

• The multiplier block used to execute the mul instruction is pipelined into three stages as shown:

This means that a multiply instruction runs through the pipeline as follows: F-D-X0- X1-X2-M-W and up to three multiply instructions maybe in-flight at a time. All other instruction types are blocked from the execute stage while any of the multiply stages are being used.

• The divider block used to execute the div instruction is iterative and takes five cycles as shown:

This means that a divide instruction runs through the pipeline as follows: F-D-X0- X0-X0-X0-X0-M-W. All other instructions are blocked from the execute stage while a division is being done.

• This processor supports out of order execution.
1 xor r0, r1, r1
2 addiu r1, r0, 16
3 j L1
4 loop: lw r3, 0(r2)
5 mul r4, r3, r3
6 div r3, r4, r3
7 mul r3, r3, r1
8 mul r3, r3, r0
9 addiu r0, r0, 2
10 sw r3, 0(r2)
11 addiu r2, r2, 4
12 L1: bne r0, r1, -9

Question 4.A Out-of-Order with no Bypassing

Draw a pipeline diagram showing the out-of-order execution of the MIPS code through the first iteration of the loop, without bypassing. Assume data hazards and structural hazards are resolved using only stalling. Assume the processor assumes branches are not taken, until they are resolved.

Question 4.B Out-of-Order with Bypassing

Draw a pipeline diagram similar to Part A, showing the out-of-order execution, but now assume the processor has data bypassing.

Question 4.C Program Latency

Determine the number of cycles it takes to execute all iterations of the loop for both the scenario in Part A and the scenario in Part B. Justify your answer.