ECEN3350-Project 3 Functions and Conventions Solved

Introduction
In this project, you will use the Nios II application binary interface to make function calls in assembly and C.

Objectives:

•   Learn the importance of calling conventions / ABIs in assembly

•   Use callee and caller saved registers and the stack

•   Understand buffer overflows and their consequences

Part 1. Application Binary Interface
In this part, you’ll write three ABI-compliant functions in Nios II assembly.

Part 1.1 Calling Convention
In Nios II assembly, write an ABI-compliant function that takes 2 arguments, and returns the sum of them. You should use signed arithmatic, and your function should have the type signature: int sum_two(int a, int b)

(Note: this is just the function prototype; you’ll write your function entirely in assembly).

Part 1.2 Saving Registers
We’ve defined a new mathmatical operation, ◦, and written an ABI-complaint function that implements it. The operation is binary: it takes two inputs, and produces one output. We won’t tell you what the operation is (it’s a mystery!), but we can say that ◦ is a commutative and associative operation. This means that a◦b = b◦a, and that (a◦b)◦c = a◦(b◦c). Our function is named op_two, and takes two 32-bit inputs a and b, and returns a single 32-bit value a◦b.

Your task is to implement an ABI-compliant op_three function in Nios II assembly, that inputs three 32-bit inputs, a, b, and c, and returns a◦b◦c.

The type signature of the given function is int op_two(int a, int b), and your function’s type signature should be int op_three(int a, int b, int c).

Note that you don’t know what operation ◦ is, so you’ll have to rely on calling op_two to perform that for you! Your function should work for any operation we implement in op_two. At the very least, you should test it with your above sum_two function as the op_two function, but we encourage you to try it with other commutative/associative functions, such as multiplication, minimum integer, bitwise xor, etc.

Make sure you don’t leave a op_two function in your submission! We’ll define one, and use it to test your program.

Part 1.3 Recursvie functions
In Nios II assembly, write an ABI-compliant recursive function that calculates the Fibonacci sequence for a given input index number. The Fibonacci sequence is defined as the sum of the previous two numbers in the sequence, with the first two numbers being 0 and 1 (i.e. fibonacci(n) = fibonacci(n-1) + fibonacci(n-2), with fibonacci(0) = 0 and fibonacci(1) = 1).

Thus, for example, if given index 0, your function should return 0. Input index 1 should return 1, and input index 7 should return 13.

Your function’s type signature should look like int fibonacci(int n).

What to submit          A single file, part1.s, that includes all three functions you wrote in Part 1.1, Part 1.2, and Part 1.3 combined. Your file should not define a _start label (and it also should not define an op_two function).

Your file could look something like this:

.text .global sum_two sum_two:

# implement Part 1.1 here

.global op_three op_three:

# implement Part 1.2 here

.global fibonacci fibonacci:

# implement Part 1.3 here

You may want to test your program (we recommend you do!!). You can do this by adding part1.s to your project, and defining your _start function (along with other test functions, e.g. op_two) in another file (e.g. main.s) that is also added to your project. Then, only turn in the part1.s file.


Part 2. Buffer Overflows
In this section, you’ll provide different data to two programs we’ve written to determine your grade.

Part 2.1
In the first part, we’ve written a simple program in C that reads your name over the JTAG UART port, and then assigns a grade, writing the result back over the UART terminal.

Download the C file at https://ecen3350.rocks/static/proj3-part2-1.c

An equivalent assembly file can be found at https://ecen3350.rocks/static/proj3-part2-1. s and can be loaded into the CPUlator: https://cpulator.01xz.net/?sys=nios-de10-lite& maxmem=67108864.

You’ll note that the code appears to assign an F- for everyone. Your job is to provide a specific name that will make your grade an A+ instead. It doesn’t matter what’s printed in the name field, but it should print Your grade is: A+. You’ll provide your input name in a text file for this part (part2.txt), on its own line, following the submission template below.

Please note: we will grade your assignment with compiler optimizations turned off (in your project’s Program Settings, “Additional compiler flags” should not have -O1 in it).

Part 2.2
In the second part, we’ve learned better than to store grades on the stack.

Instead, we have a hardcoded assembly program that assigns the grades, and the name is provided with its length as a global variable STUDENT_NAME.

Download the assembly file at https://ecen3350.rocks/static/proj3-part2-2.s

Your task is to change the values of STUDENT_NAME and STUDENT_NAME_LEN to improve your grade. Hint: there is a print_good_grade function in the program, but nothing calls it. How can you make sure that code gets used?

You may NOT change the value of any other data or code in the program, besides STUDENT_NAME and STUDENT_NAME_LEN.

You may use the CPUlator or your DE-10 for this part.