Due:: Friday, September 26, 10pm
Starter code:: See Assignment 3 on Canvas for the Github Classroom link.
Submission:: Submit the contents of your repository via Gradescope. See Deliverables below for what to submit.

This is an individual assignment.

This assignment will take you through 2 tasks implementing and using functions in assembly. You will be also asked to write some C (pseudo-)code as a blueprint for your assembly. The C code does not need to compile or run, but needs to reflect the functionality. Feel free to rely on your Java knowledge here. If in doubt, leave comments.

This assignment will also introduce separate compilation. For one of the tasks you will write a function called by an already implemented main function. For the other, your task will be to write a main function for an already implemented function. Read the instruction carefully to make sure you understand what is being asked in each of the tasks.

For each of the required functions, you need to follow the Assembly Design Recipe which we went, or will go over in class. You can optimize your code once you’ve written it – if, for example, there is some repetition that can be trivially removed, but it still needs to be clear that the recipe was followed.

Task 1: Mystery function

You are provided with a “mystery library”. This pre-compiled library contains a single function, crunch, which takes two signed long integers and returns a long integer.

Your task is to complete the mystery program by implementing the main function in mystery-main.s, which needs to call the provided crunch function. This is also called a “driver”. When compiled, the program should have the following features and behaviour:

Accept exactly two arguments. You can assume that, when provided, these arguments are valid signed long integers.
The provided crunch function should be called with these two numbers, converting them from strings as necessary.
Based on the result from the function, the program should print one of the following strings (with a newline at the end) and exit with an exit status of 0:
- hat, if crunch returned a negative integer
- tea, if crunch returned 0
- beer, if crunch returned a positive integer.
If fewer or more than 2 arguments are provided, the program should print “Two arguments required.”, followed by a newline, and exit with a status of 1.

First, write the main function in C and save it in mystery-main.c, where we provided a “stub” for you. The program does not have to compile, but it should be a fairly accurate high-level representation of the assembly program. Do not spend too much time on this, but give it your best shot. Try to use your knowledge of Java and provide comments if you are struggling with C.

Second, implement an assembly version of the main function in mystery-main.s. Your program must compile using the provided Makefile by running make mystery. The command make mystery will combine libmystery.a (provided by us) and mystery-main.s (written by you) into the executable mystery.

Sample test cases with mystery:

$ $ ./mystery 1 2
tea
$ ./mystery 2344 12
beer
$ ./mystery -1345 321
hat
$ ./mystery 1
Two arguments required.

The provided library libmystery.a should work in Codespaces (and Gradescope, of course). We cannot guarantee that it will also work on your local Linux, but please let us know if it does work in, e.g., WSL2.

Task 2: The Maximum of an Array

The second task is to write the function:

unsigned long array_max(unsigned long n, unsigned long *items)

This function will return the maximum value of an array of long integers ≥ 0. The first argument provided is the number of elements, the second argument is the address of the first element.

You do not need to write a C version of this function, but we recommend doing so.

We have provided the driver program in array-max-main.c, which processes the command line arguments and calls the array_max function. The Makefile will compile both array-max-main.c and your implementation of the array_max function to produce the executable array-max. Once compiled, the interactions with array-max should look as follows:

$ ./array-max 1 2
2
$ ./array-max 42 1
42
$ ./array-max 3 1 5 8 2 4 8 20 1
20

Your program must compile without any modification to the provided array-max-main.c file, using the provided Makefile. The command make array-max will combine array-max-main.c (provided by us) and array-max.s (written by you) into the executable array-max.

Using the Makefile

We have provided a Makefile for you. You can use it as follows on the command line:

make: compile all programs (mystery, array-max)
make clean: basic cleanup, remove binaries
make mystery: compile the mystery program using mystery-main.s and libmystery.a
make array-max: compile the array-max program using array-max.s and array-max-main.c

Deliverables

Task 1: Modify the files mystery-main.s (implementation) and mystery-main.c (blueprint) and commit them to your repository.

Task 2: Modify the file array-max.s and commit it to your repository.

Do not include any executables, object files, or any other binary, intermediate or hidden files.
Finally, go to our Gradescope and submit a ZIP archive of your repository (which can be downloaded from Github).

Note

While inspecting C compiler output to learn about assembly is a good approach, you are not, under any circumstances, allowed to submit compiler (or any other machine-generated) output as your .s files. Doing so will result in an automatic 0 for the assignment. The code must be clearly written by you, following the Assembly Design Recipe

Hints and Tips

Read the assignment carefully and make sure you understand what is being asked in each Task.
Start early. This doesn’t mean you start writing code right away, but you should at least read the description, clone the assignment repository, and look for “missing links”.
Make sure you understand the provided starter code. Ask questions early if not.
Each argument to a program is a string, that is, the value is actually a memory address pointing to the first character of the string in memory.
The above is reflected in the signature of main, which is int main(int argc, char *argv[]) – argc is the argument count, and argv is an array of strings. Each string is an array of characters, ending with a null byte/character (i.e., a character whose numeric ASCII value is 0). An array is represented as the address of (= a pointer to) its first element.
The first element of argv is always the path and name of the executable. This means the actual arguments start at argv[1].
The return value of main is the exit code or exit status of the program. We return 0 by default to signal success.
You can use standard C functions like atol where needed.
Pay close attention to assembly calling conventions and the use of registers when calling C functions, or when writing functions.