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COMP2100

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COMP 2100/2500/6442: Lab 8 - Code Review

One of the learning outcomes for this course is to make modifications (including source code design, implementation, and testing) within a moderate-sized Java program system (1000 - 10000 lines of code), given a documented specification, design and implementation of the system. Both the assignments partly address this learning outcome. However, effective code review of a sizable project is a crucial learning outcome which we aim to address in this lab session.

rPeANUt, a Java based project of about 4500 lines of code, is a simulation of a simple computer which is used in teaching Introduction to Computer Systems. As with many systems it began with a small focused code base. Initially, rPeANUt simply simulated a simple CPU using a GUI. However, over time its code base incrementally grew with a host of features and refinements. This is evident in the design and implementation. rPeANUt should not be seen as an ideal design/implementation, rather, it represents real-world software composed of both average and high quality components.

Pre-lab Tasks - Understanding rPeANUt [2 marks]

The pre-lab task involves downloading and running rPeANUt, gaining an understanding of the design and implementation, and then getting a more detailed understanding of the code that displays the register contents.

Setup rPeANUt by completing the following tasks:

• Clone the rPeANUt git repository available at:
• Get the rPeANUt development environment set up on a lab machine, so you can develop and modify rPeANUt
• Compile and run rPeANUt using your favourite IDE
• Load in the "hello.s" assembly code available in the examples directory, press "Assemble", and then press "Run". You should see the program print "Hello" to the simulated terminal. If you wish to re-run your code you can press "Assemble" again and then press "Run"
• You can run the program step by step by pressing the "Assemble" button and then pressing the "Step" button. You should see the CPU execute each instruction step-by-step and see register R1 change as different characters of the "Hello" string are loaded into it. (Note you do not need to understand the rPeANUt computer/instruction set, rather, this lab's focus is on the implementation of rPeANUt)

Read over the code and answer the following questions (Answers can be hand written or typed):

• Which is the class that has the "main" method that rPeANUt is run from?
• Which class holds the state of the simulated computer (the memory, register, program counter, etc.)?
• Which class has the code that assembles the program (takes the text of the program, generates the machine code and sets the memory for that program)? To work this out you may wish to follow the "Assemble" button, and see what code is executed when assemble is pressed.
• Which class is responsible for simulating the instructions?
• How would your describe the design approach? (just a 1 or 2 sentence summary is expected)
• Which class is responsible for displaying the register's values (this is the list on the right of the window)?
• How does this class for displaying a register value work? The register values are just 32 bit integers and they are displayed in their hex form, how is this achieved?

Mini-test - Simple Modifications to rPeANUt [2 marks]

Guidelines

• Any code that is modified in this mini-test must be committed and pushed to your mini_tests gitlab repository as a git diff compared to the master branch of the rpeanut gitlab repository before it is evaluated by your tutor
• DO NOT push your modifications to the rPeANUt git repo
• Use your favourite IDE in a guest account on a lab machine (personal laptops not allowed)
• Make sure you do not have code written beforehand in your account and simply import it during the test. Your tutor will keep vigil for this. If this is found to happen, you will receive a mark of 0 for the entire lab

Your tutor will select one task at random from the three options below and ask you to complete it in the duration of the lab time. You may ask your tutor for a problem as soon as your pre-lab exercise is marked off. Start coding as soon as you are handed a task!

In order to attain full marks, your solution must be correct. Your tutor may also decide to award some marks for a good attempt, in case of an incorrect or unfinished solution.

Remember, this is an individual exercise and must not be attempted collaboratively. However, feel free to discuss solutions to these problems with your colleagues before you attend this lab session.

Currently, the CPU register values in rPeANUt are displayed as hexadecimal values.

Task 1

The instruction count displays the number of instructions that have been executed since a program starts running. It is located under the register displays.

The instruction profile counts are enabled by Profile in the Code menu and are shown in the memory display section of the interface. If you assemble and run a program such as:

0x0100 : load #4 R3         jump 0x0100

You should see these counts increase rapidly.

• Modify rPeANUt such that both the instruction count and profile instruction counts are displayed using decimal numbers with comma separated thousands.

Hint: To format a number with comma separated 000's use: String.format("%,d",thenumber)

Task 2

• Modify rPeANUt such that the current count of non-zero memory addresses is displayed just below the Count: label. Use the label Mem:.

Hints: To add such a label into the display follow the same approach as the Count: label. To work out the number of elements in main memory that are non-zero, add a method that loops through all the memory locations incrementing a counter for the locations that have values that are non-zero.

Task 3

• In the latest version of rPeANUt when the file being edited is modified but not saved, a "*" appears after file name in the title. Change this such that (modified) appears rather than the "*".

The image below is a screenshot of rPeANUt with all 3 tasks implemented (remember you only need to do the one you are allocated during this mini test):

rPeANUT Screenshot

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