MIPS bad address/ Exception

Question

The code is a recursive block for calculating the factorial of a number. I am getting an exception at PC (bad address in text). I am taking a look at the trace but am not sure where the problem really is, other than the fact that my PC is showing garbage values. I am attaching the code down below.

.data
    prompt: .asciiz "Enter the n"
.text
    main:
        #User I/O
        li $v0, 4
        la $a0, prompt
        syscall
        li $v0, 5
        syscall
        add $a0, $v0, $zero
        jal fact
        j finish
    fact:
        addi $sp, $sp, -8  #adding two elements (reserve space), since stacks start from higher mem address
        sw $ra, 0($sp)
        sw $a0, 4($sp)

        slti $t0, $a0, 1
        beq $t0, $zero, loop

        addi $sp, $sp, 8
        addi $v0, $zero, 1

        jr $ra

    loop:
        addi $a0, $a0, -1
        jal fact    #stores address of the lw instruction and goes to label-->fact

    lw $a0, 0($sp)
    lw $ra, 4($sp)
    addi $sp, $sp, 8 #deleting 2 elements

    mul $v0, $v0, $a0
    jr $ra #until main ra, recursively go through fact(n)=fact(n-1)*n

    finish:

Simulator Used : QtSpim

Any help is appreciated. Thanks! Also am attaching the PC and register values in case it helps. Register values at the time of the error

As you deleted your other question with bubble sort attempt, while I was trying to write one for my own fun, I'm posting it here, sorry: https://pastebin.com/bHFPdgbr — Ped7g, Aug 20 '18 at 16:40
Sorry about that, wanted to re-write the entire code. Thanks for the contribution, here is the link of the GitHub repo were I am posting the code for the basics of MIPS, I can add your code there if you fancy it(PR). link: https://github.com/rahulkrishnan98/computer-organization-and-architecture — Rahul Krishnan, Aug 21 '18 at 02:28
Feel free to include it in your git if you think it is worth it (I didn't specify license, but by posting it on SO it can be treated as PD or copyleft piece of code of course (I should have mention that in that pastebin). But especially try to understand what/how it does, and if you have any question about it, feel free to ask me here (use @Ped7g to notify me). GL. :) — Ped7g, Aug 21 '18 at 08:24

score 3 · Accepted Answer · answered Aug 20 '18 at 06:16

Single-step with your debugger to find out what went wrong when lw $ra, 4($sp), and what you actually loaded and jumped to with jr $ra. That instruction just jumps to whatever address in $ra, and is the most likely candidate for setting the PC to something bogus.

It looks like you save $ra to 0($sp), but restoring it from 4($sp), so you're swapping the arg and the return address.

Also, your main isn't saving its $ra on entry at all, so you won't be able to return from main. You run a jal before saving $ra, so the only save/restore is inside fact (whose definition is mixed in with main? Don't do that. Instead of jumping over the definition of another function, just put code to return from main or make an exit system call at the bottom of main.)

Also you might be crashing when you j finish, which will fall off the end of your program into non-instructions.

Thank you for spotting that. Single stepping with my debugger helped verify that I was indeed mixing up the return address. — Rahul Krishnan, Aug 20 '18 at 06:31

MIPS bad address/ Exception

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