hooleyhoop / pk_do_not_be_afraid

ASM really isn't that different!

Do Not Be Afraid

For those worrying about CSC 2510 next term, assembly language is much easier to master if you let go of your expectations.

V1

This program enumerates argv, printing each entry. It does not use a for loop counting over argc. Rather, it leverages the fact that argv is a NULL terminated array.

#include <iostream>

using namespace std;

int main(int argc, char * argv[]) {
	while (*argv) {
		cout << *(argv++) << endl;
	}
	return 0;
}

V2

In this version, we lose the while loop, replacing it with:

• a label
• an if
• a goto (this might be the only time you'll see goto used!)

#include <iostream>

using namespace std;

int main(int argc, char * argv[]) {
	top:
		if (*argv) {
			cout << *(argv++) << endl;
			goto top;
		}
	return 0;
}

V3

In this version we lose the cout / endl. At this point, we're no longer C++, just C.

#include <stdio.h>

int main(int argc, char * argv[]) {
	top:
		if (*argv) {
			puts(*(argv++));
			goto top;
		}
	return 0;
}

V4

In this version we have no brackets at all (except enclosing main).

This version is just about assembly language!

#include <stdio.h>

int main(int argc, char * argv[]) {
	top:
		if (*argv == NULL)
			goto bottom;
		puts(*(argv++));
		goto top;

	bottom:
		return 0;
}

V5

Here is the same code expressed in ARM V8 assembly language.

	.global		main
main:
	stp		x21, x30, [sp, -16]!   // push onto stack
	mov		x21, x1                // argc -> x0, argv -> x1

top:
	ldr		x0, [x21], 8           // argv++, old dereferenced value in x0
	cbz		x0, bottom             // if *argv == NULL goto bottom
	bl		puts                   // puts(*argv)
	b		top                    // goto top

bottom:
	ldp		x21, x30, [sp], 16     // pop from stack
	mov		x0, xzr	               // return 0
	ret

Of interest are the lines between top and bottom - notice how naturally the assembly language flows from the high level language. It is also interesting how braces can become labels and branches.

Bottom line

That which you resist, persists.

CSC 2510 will be much easier if you relax and keep an open mind.

Free your mind, and you'll soon see things the way they really are.