Using Nested For Loops and an Array in MIPS

Question

This is a homework assignment, I've written the whole program myself, run through it in the debugger, and everything plays out the way I mean it to EXCEPT for this line:

sw $t1, counter($a3)

The assignment is to convert this snippet of C code to MIPS

    for(i = 0; i < a; i++) {
       for(j = 0; j < b; j++) {
          C[2 * i] = i – j; } }

All the registers change values the way they should in my program except for $a3 - It never changes.

Changes: An array needed to be declared and "pointed to" by a register and a label can't be used for an offset in the manner I started with

EDIT: Here's the finished, working code

Finished

Why did you expect $a3 to change ? It should be the address of your array, and thus it should not change. Actually I don't understand what is your problem. — ElderBug
– ElderBug, Commented Feb 12, 2015 at 9:14
@ElderBug I'm trying to store the result of i - j in each index of $a3, so shouldn't I be seeing some value popping up in the $a3 register value space? I'm not sure how better to explain myself, would a screenshot of my mars compiler help? I can show you what I'm looking at — Captain Hook
– Captain Hook, Commented Feb 12, 2015 at 9:49
If you have the line sw $t1, x($a3) with x=8, the value of t1 will be stored at $a3[2] (3rd int of your array, pointed by $a3). If it is not the case, then the line is never executed. — ElderBug
– ElderBug, Commented Feb 12, 2015 at 10:49
I'm not sure about the MIPS syntax, but to declare it should be like myarray: .space 40 for 10 words. Then you have to load the address into $a3, with la $a3, myarray, if I'm not mistaken. — ElderBug
– ElderBug, Commented Feb 12, 2015 at 11:35
Actually, you are right on this point, I forgot that you use i-1 later. Anyway, your loops are still wrong. Try to translate the while, that's really straightforward. Just translate what the while do step by step. The only not direct point is that the condition is a "continue" condition, when you need an "exit" condition (but that's just the opposite). And translate one loop at a time, it doesn't even matter if you have duplicate labels. — ElderBug
– ElderBug, Commented Feb 12, 2015 at 20:51

ElderBug · Accepted Answer · 2015-02-12 22:12:59Z

3

Recap answer from the comments

Your $a3 register, is supposed to be loaded with the address of an array defined in the .data section.

One big problem with your code is how you constructed your loops. The best way is to translate your loops step by step, and one loop at a time. Also, remember that :

for( i = 0; i < a; i++ )
{
    loop_content;
}

Is equivalent to :

i = 0;
while( i < a )
{
    loop_content;
    i++;
}

Which is easier to translate in assembly. The condition just have to be negated, has you need an "exit" condition, and not a "continue" condition as in a while loop. Your code will be much clearer and easier to understand (and less error prone).

Your "out of range" error comes from here : sw $t1, counter($a3). Here counter is a label, and therefore an address. Thus counter($a3) is doing "$a3 (=0x10010008) + address of counter (=0x100100f8)", giving 0x20020100, which is clearly not what you want (and non-sense).

Oh, and in the sw $r, offset($a) MIPS instruction, offset MUST be a 16-bit CONSTANT. Here, you use a 32-bit address, but it's just that the assembler kindly translate sw $t1, counter($a3) to $x = $a3 + counter; sw $t1, 0($x), which is why you may see a sw with 0 as offset.

answered Feb 12, 2015 at 22:12

ElderBug

6,16518 silver badges28 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Captain Hook Over a year ago

You're the man, that makes sense and I have a much better understanding of what is happening. Will post revised, polished code later for reference, let me know if there is anything I can do to make it more efficient or "cleaner". Thanks again

Collectives™ on Stack Overflow

Using Nested For Loops and an Array in MIPS

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related