Introduction to Branching in ARM Assembly

Question

Open the factorial.s file, and open it up in a text editor of your choice. Note that word processors (e.g., Microsoft Word, Pages, Google Docs) will probably not work for this purpose, as you must save your file as plain text. You must write ARM assembly code that will compute the factorial of an input number, along with showing intermediate values along the way. Psuedocode showing what you need to implement is shown below:

n = <<first integer read from "one_int.txt">>

accum = 1

while (n != 0) {

accum = accum * n

print accum

print "\n"

n = n - 1

}

print accum

print "\n"

A sample run of the program with the given one_int.txt file is shown below:

5

20

60

120

120

120

The code you write will likely look similar to the ARM assembly code in while_loop.s, which is a translation of the Java code in WhileLoop.java.

Your code must be able to work with more than just 5. With this in mind, be sure to test your solution! At the very least, you should test values between 1 and 10. You may assume that no negative inputs will be given as inputs.

Answer 1

there's a translator on godbolt dot org

there are many versions of the ARM assembler from which to choose...

It is interesting to see how the assembler handles the recursion with two push calls...

I went from C++ to ARM v 11 clang.

Here's the source for the factorial.

using namespace std;

#include <iostream>

long factorial ( long n)

{

    long longFactorialReturn=-1;

    if (n==0)

    {

        longFactorialReturn=1;

    }

    else

    {

        longFactorialReturn = n * factorial(n-1);

        cout << longFactorialReturn << endl;

    }

    return(longFactorialReturn);

}

int main()

{

    long n=5;

    cout << "Factorial :  n!  = " << factorial(n) << endl;

}