Assignment 1: Flood in MIPS

version: 1.0 last updated: 2025-06-?? ??:??:??

Aims

  • to give you experience writing MIPS assembly code
  • to give you experience translating C to MIPS
  • to give you experience with data and control structures in MIPS

Getting Started

Create a new directory for this assignment called flood, change to this directory, and fetch the provided code by running these commands: 

mkdir -m 700 flood
cd flood
1521 fetch flood

If you're not working at CSE, you can download the provided files as a zip file or a tar file.

This will add the following files into the directory:

  • flood.s: a stub MIPS assembly file to complete.
  • flood.c: a reference implementation of flood in C.
  • flood.simple.c: a copy of the reference implementation of flood, for you to simplify.
  • input.txt: example input file.
  • flood.mk: a make(1) fragment for compiling flood.c.

Flood: The Game

Flood is an implementation of Simon Tatham's Flood, a game where players fill a coloured grid with repeated flood-fill operations on the top left cell.

The provided C program (and eventually, your MIPS translation) implement the following commands:

  • w/a/s/d move the selected cell up/left/down/right.
  • h prints instructions.
  • e performs a flood fill.
  • c prints a (relatively) optimal solution.
  • q exits flood.
Flood is won when the grid is all the same colour.

Flood is lost if you run out of steps.

The number of allowed steps is chosen based on an optimal solution, with a few extra to make the game a bit easier.

To get a feel for this game, try it out in a terminal: 

dcc flood.c -o flood
./flood

You should read through flood.c. There are comments throughout it that should help you understand what the program is doing [citation needed] — which you'll need for the next part of the assignment.

flood.s: The Assignment

Your task in the assignment is to translate the provided flood.c program into MIPS assembly.

You have to add code to the provided flood.s file so that it executes exactly the same as flood.c.

Any modifications to the .data section are NOT PERMITTED, and will break autotest.

All global variables and strings are already present in the provided .data section.

You have to implement the following functions in MIPS assembly:

  • main
  • print_welcome
  • in_bounds
  • game_loop
  • initialise_game
  • game_finished
  • do_fill
  • initialise_board
  • initialise_fill_in_progress
  • find_optimal_solution
  • invalid_step
  • print_optimal_solution
  • rate_choice
  • find_adjacent_cells
  • fill
  • solve_next_step
  • copy_mem

The following functions have already been translated to MIPS assembly for you:

  • random_in_range
  • initialise_solver
  • simulate_step
  • initialise_solver_adjacent_cells
  • print_board
  • print_board_bottom
  • print_board_row
  • print_board_inner_line
  • print_board_seperator_line
  • process_command

You MUST obey the MIPS calling conventions discussed in lectures.

When translating functions calls, you should avoid making assumptions about any side effects of the called function.

If you make assumptions about the behaviour of called functions that are not made in the provided C implementation, you may be penalised.

Subsets

This assignment is split into four subsets. Later subsets will involve more complex translation.

Subset Functions Performance Weight
Subset 0
  • main
  • print_welcome
10%
Subset 1
  • in_bounds
  • game_loop
  • initialise_game
  • game_finished
  • do_fill
45%
Subset 2
  • initialise_board
  • initialise_fill_in_progress
  • find_optimal_solution
  • invalid_step
  • print_optimal_solution
25%
Subset 3
  • rate_choice
  • find_adjacent_cells
  • fill
  • solve_next_step
  • copy_mem
  • whole program autotesting
 20%

Please note that the weighting for Subset 3 includes whole_prog automarking.

Running & Testing

To run your MIPS code, simply enter the following in your terminal: 

1521 mipsy flood.s

Recommended testing approach

The recommended approach to testing is using Function tests to determine whether your code is correct as you progress through the assignment.

After you have completed all the functions and are passing all of the Function tests, it is recommended to use the Whole program tests to determine if your solution has any bugs not picked up by autotest.

It is particularly common to find bugs relating to your conformance to the MIPS calling convention in Subset 3 (use of clobbered register, overwriting saved register), as the provided autotests for Subset 3 do not check for this.

Automated testing

There are two types of autotests for this assignment:

  1. Whole program tests
  2. Function tests

Whole program tests run your entire flood.s file and compare the observed output against the expected output. These tests are generally not practical until most or all of your functions have been implemented in MIPS.

Function tests only test individual functions from flood.s. These tests isolate a function and run it separately to the rest of your solution. For example, you can test the main function with: 

1521 autotest flood main
The result of this test is based entirely on the implementation of the main function. It does not depend on the correctness of any other functions that main calls, such as print_welcome, initialise_game, or game_loop.

Manual Testing

The game takes a lot of input, so it's a good idea to write a file with the input you want to test, and then pipe that into your program using the shell commands below.

You have been given a file called input.txt to use for this. 

>> compile the C implementation <<
dcc flood.c -o flood
>> run the C implementation with the inputs in input.txt, storing output in c.out <<
cat input.txt | ./flood | tee c.out
>> run the your mips implementation with the inputs in input.txt, storing output in mips.out <<
cat input.txt | 1521 mipsy flood.s | tee mips.out
>> automatically compare the two output files and report differences <<
diff -s c.out mips.out
Files c.out and mips.out are identical

Try this with different sequences of inputs (with different inputs stored inside input.txt).

You are not required to use this testing method if it is too confusing. Inputting manually is perfectly fine, it is just a bit tedious.

Simplified C code

You are encouraged to simplify your C code to remove any loop constructs and if-else statements, and test that your simplified code works correctly before translating it to MIPS, in a separate file flood.simple.c.

This file will not be marked - you do not need to submit it.

In order to allow you to check that your simplified code works correctly, we have provided a simple set of automated tests.

You can run these tests by running the following command: 

1521 autotest flood.simple

Hints

  • You should implement all the functions from one subset before moving on to the next.

  • You may find the provided function implementations to be useful guidance for your implementation including comments, label names, indentation and register usage.

An example game of Flood

1521 mipsy flood.s
Welcome to flood!
To move your cursor up/down, use w/s
To move your cursor left/right, use a/d
To see this message again, use h
To perform flood fill on the grid, use e
To cheat and see the 'optimal' solution, use c
To exit, use q
Enter the grid width: 4
Enter the grid height: 4
Enter a random seed: 1521
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| ... | ``` | *** | ``` |
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Filled 13 cells!
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> e
Filled 15 cells!
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| xxx | xxx | xxx | xxx |
| xxx | xxx | xxx | xxx |
|-----+-----+-----+-----|
| xxx | xxx | xxx | xxx |
| xxx | xxx | xxx | xxx |
| xxx | xxx | xxx | xxx |
|-----+-----+-----+-----|
                         
                     ^   
                     |   
11/12 steps
You win!

Assumptions, Clarifications, and Restrictions

Like all good programmers, you should make as few assumptions as possible.

  • Your submitted code must be hand-written MIPS assembly, which you yourself have written.
    You may not submit code in other languages.
    You may not submit compiled output.

  • You may not copy a solution from an online source. e.g. GitHub.

  • Your functions will be tested individually. They must exactly match the behaviour of the corresponding C function and they must follow the MIPS calling conventions described below.

  • You may assume the input provided to flood.s will be correctly formatted. For example, where a number is expected the input will not be a letter.

  • The C code defines constants using #define. Your MIPS translation should use the corresponding provided named constants, in the places where a #define is used in the C code. You should not use a #define constant in your MIPS translation if it is not used in the corresponding part of the C code.

  • There will be a correctness penalty for assignments that do not follow standard MIPS calling conventions including:

    • Function arguments are passed in registers $a0..$a3.

    • Function return values are passed in register $v0

    • Values in registers $s0..$s7 are preserved across function calls.
      If a function changes these registers, it must restore the original value before returning.

    • The only registers' values that can be relied upon across a function call are $s0..$s7, $gp, $sp, and $fp.
      All other registers must be assumed to be have an undefined value after a function call, except $v0 which has the function return value.

  • If you need clarification on what you can and cannot use or do for this assignment, ask in the class forum.

  • You are required to submit intermediate versions of your assignment. See below for details.

Change Log

Version 1.0
(2025-06-?? ??:??:??)
  • Initial release

Assessment

Testing

We have provided some automated tests to help you check the correctness of your translation. To run all the provided tests, execute the following command: 
1521 autotest flood
Some of these tests check only a specific function, and some test your whole program. To run all the tests for a specific function, pass the name of the function to autotest. For example, to run all the tests for the main function, run the command: 
1521 autotest flood main
You can also run all the autotests for a particular subset. For example, to run all the autotests for subset 1, run the command: 
1521 autotest flood S1
To run the autotests which test your program as a whole, run the command: 
1521 autotest flood whole_prog
Some tests are more complex than others. If you are failing more than one test, you are encouraged to focus on solving the first of those failing tests. To do so, you can run a specific test by giving its name to the autotest command: 
1521 autotest flood main_S0_0

Submission

When you are finished working on the assignment, you must submit your work by running give: 

give cs1521 ass1_flood flood.s

You must run give before Week 5 Friday 18:00:00 to obtain the marks for this assignment. Note that this is an individual exercise, the work you submit with give must be entirely your own.

You can run give multiple times.
Only your last submission will be marked.

If you are working at home, you may find it more convenient to upload your work via give's web interface.

You cannot obtain marks by emailing your code to tutors or lecturers.

You can check your latest submission on CSE servers with: 

1521 classrun check ass1_flood

You can check the files you have submitted here.

Manual marking will be done by your tutor, who will mark for style and readability, as described in the Assessment section below. After your tutor has assessed your work, you can view your results here; The resulting mark will also be available via give's web interface.

Due Date

This assignment is due Week 5 Friday 18:00:00 (2025-07-04 18:00:00).

The UNSW standard late penalty for assessment is 5% per day for 5 days - this is implemented hourly for this assignment.

Your assignment mark will be reduced by 0.2% for each hour (or part thereof) late past the submission deadline.

For example, if an assignment worth 60% was submitted half an hour late, it would be awarded 59.8%, whereas if it was submitted past 10 hours late, it would be awarded 57.8%.

Beware - submissions 5 or more days late will receive zero marks. This again is the UNSW standard assessment policy.

Assessment Scheme

This assignment will contribute 15 marks to your final COMP1521 mark.

80% of the marks for assignment 1 will come from the performance of your code on a large series of tests.

20% of the marks for assignment 1 will come from hand marking. These marks will be awarded on the basis of clarity, commenting, elegance and style. In other words, you will be assessed on how easy it is for a human to read and understand your program.

An indicative assessment scheme for performance follows.
The lecturer may vary the assessment scheme after inspecting the assignment submissions, but it is likely to be broadly similar to the following:

100% for performance implements all behaviours perfectly,
following the spec exactly.
85% for performance implements all simple and most difficult functions correctly.
65% for performance implements all simple and
some moderate difficulty functions correctly.
≤ 50% for performance good progress,
simple functions work correctly.

An indicative assessment scheme for style follows.
The lecturer may vary the assessment scheme after inspecting the assignment submissions, but it is likely to be broadly similar to the following:

100% for style perfect style
90% for style great style, almost all style characteristics perfect.
80% for style good style, one or two style characteristics not well done.
70% for style good style, a few style characteristics not well done.
60% for style ok style, an attempt at most style characteristics.
≤ 50% for style an attempt at style.

An indicative style rubric follows.
The lecturer may vary the assessment scheme after inspecting the assignment submissions, but it is likely to be broadly similar to the following:

  • Formatting (8/20):
    • Whitespace
    • Indentation (consistent, tabs or spaces are okay)
    • Line length (below 120 characters unless very exceptional)
    • Line breaks (using vertical whitespace to improve readability)
  • Documentation (12/20):
    • Header comment (with name, zID, description of program)
    • Function comments (above each function with a description)
    • Sensible commenting throughout the code
    • Descriptive label names, indicating structure

Note that the following penalties apply to your total mark for plagiarism:

0 for
assignment 1		 knowingly providing your work to anyone
and it is subsequently submitted (by anyone).
0 FL for
COMP1521		 submitting any other person's work; this includes joint work.
academic
misconduct		 submitting another person's work without their consent;
paying another person to do work for you.

Intermediate Versions of Work

You are required to submit intermediate versions of your assignment.

Every time you work on the assignment and make some progress you should copy your work to your CSE account and submit it using the give command above. It is fine if intermediate versions do not compile or otherwise fail submission tests. Only the final submitted version of your assignment will be marked.

Assignment Conditions

  • Joint work is not permitted on this assignment.

    This is an individual assignment. The work you submit must be entirely your own work: submission of work even partly written by any other person is not permitted.

    Do not request help from anyone other than the teaching staff of COMP1521 — for example, in the course forum, or in help sessions.

    Do not post your assignment code to the course forum. The teaching staff can view code you have recently submitted with give, or recently autotested.

    Assignment submissions are routinely examined both automatically and manually for work written by others.

    Rationale: this assignment is designed to develop the individual skills needed to produce an entire working program. Using code written by, or taken from, other people will stop you learning these skills. Other CSE courses focus on skills needed for working in a team.

  • The use of code-synthesis tools, such as GitHub Copilot, ChatGPT, Google Bard, etc. are not permitted on this assignment.

    Rationale: this assignment is designed to develop your understanding of basic concepts. Using synthesis tools will stop you learning these fundamental concepts, which will significantly impact your ability to complete future courses.

  • Sharing, publishing, or distributing your assignment work is not permitted.

    Do not provide or show your assignment work to any other person, other than the teaching staff of COMP1521. For example, do not message your work to friends.

    Do not publish your assignment code via the Internet. For example, do not place your assignment in a public GitHub repository.

    Rationale: by publishing or sharing your work, you are facilitating other students using your work. If other students find your assignment work and submit part or all of it as their own work, you may become involved in an academic integrity investigation.

  • Sharing, publishing, or distributing your assignment work after the completion of COMP1521 is not permitted.

    For example, do not place your assignment in a public GitHub repository after this offering of COMP1521 is over.

    Rationale: COMP1521 may reuse assignment themes covering similar concepts and content. If students in future terms find your assignment work and submit part or all of it as their own work, you may become involved in an academic integrity investigation.

Violation of any of the above conditions may result in an academic integrity investigation, with possible penalties up to and including a mark of 0 in COMP1521, and exclusion from future studies at UNSW. For more information, read the UNSW Student Code, or contact the course account.