Description
Introduction and Objectives
This assignment is all about the Y86 pipeline. You will work with an implementation of
the pipeline: Y86-PipeMinus, that handles hazards by stalling (in the following assignments, you will work with Y86-Pipe, that handles hazards using data forwarding and jump
prediction).
The objective of this assignment is to help you read and understand the pipeline control
logic for stalling and to prepare you for making more advanced changes to the pipelined
CPU. You will achieve this by finding and fixing three bugs in the Y86-PipeMinus implementation of the CPU provided with the assignment. The PipeMinus implementation is in
the class arch.y86.machine.pipeminus.student.CPU. The main method for this implementation is in the class SimpleMachine$Y86PipeMinusStudent.
Problem
The arch.y86.machine.pipeminus.student.CPU class we provided you with has three
bugs. These bugs were introduced by simply deleting a section of code. No other changes
were made. The three bugs are all in method pipelineHazardControl, or in its helper
isDataHazardOnReg.
1. Use the provided program pipe-test.s to identify the symptoms of the bugs.
To find a bug, first write down the values that the registers should have at the end of
one of the tests. Run the simulator on that test, and then compare the values actually
stored in the registers. If they differ (i.e. some of the tests are OK and don’t trigger
any of the bugs) from the expected values you may have found a bug. To run a test,
double-click on the address of the first instruction of the test (this should set the value
of the pc register in the Fetch stage to this address), and then click on Run or Step
as usual.
Carefully describe the erroneous execution and explain your theory for what is happening. When you find a bug, describe it carefully, fix it in the simulator code, and
describe your solution. Re-run the test to demonstrate that you correctly fixed the
bug. The descriptions you provide of the symptom, theory, bug and solution are as
important as fixing the bugs.
Please run the tests in the order in which they appear in file pipe-test.s, and fix
each bug before moving on to the next test (some of the later tests might fail not only
because of the bug they are meant to reveal, but also because of those revealed by
earlier tests). Note that once the file is loaded in the simulator you can double click
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on the address of the first instruction you want to execute to set the initial program
counter value.
2. Use the cCnt and iCnt processor registers to document the pipeline efficiency for executing the programs sum.s, max.s (both included with the simulator’s source code
and with the posting) and heapsort-student.s (your completed version from assignment #1; if you could not get it to work, you can copy a friend’s solution). The cCnt
field is incremented once per clock cycle and the iCnt field is incremented whenever
an instruction other than a bubble is retired. Present pipeline efficiency as average
number of cycles per instruction (CPI) for each program.
Deliverables
You should use the handin program to submit the first part of this assignment. The
assignment name is a3, and the files to submit for this part are:
1. The corrected CPU.java file (with comments to indicate the code you added).
2. A plain text ASCII file that contains the following information:
• Your name and student number.
• For each of the three bugs the registers values expected, the actual values, a
description of the erroneous execution (i.e the symptoms), your theory of what the
problem is and how to fix it and finally an explanation of what was actually wrong
with the code in the CPU.java file, and one sentence describing your solution.
• Your CPI results from question 2 and the heapsort-student.s code you used. If
you did not use your own solution indicate whose code you used.
• How long it took you to complete this assignment (not including any time you
may have spent revising before starting to work on it).
Each bug found, explained and corrected will be worth 9 marks, the CPI results will be
worth 2 marks, and the amount of time you spent on the assignment 1 mark, for a total of
30 marks.
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