CODEWITHSUNDEEP
assemblyalignmentmipsruntime-error
Alyssa
Alyssa

Reputation: 36

Fetch address not aligned on word boundary (MIPS)

Any ideas why I'm getting an alignment error? The error happens when executing la $t0, mainF, even though the similar line la $t0, mainB executes just fine. This is my first time coding in MIPS so I did some research and have a vague idea of what the address alignment means, but the compiler doesn't even get to the part where I add 4 before throwing me this runtime exception. 

.data

mainF:
    .byte   1
mainB:
    .byte   1

mainN:
   (has '.word's, generic tests for the program itself)

newline:
    .asciiz "\n"
textFw:
    .asciiz "The integers in order:\n"
textBw:
    .asciiz "The integers in backwards order:\n"        

.text        

main:
    # Function prologue 
    addiu   $sp, $sp, -24        # allocate stack space -- default of 24 here
    sw      $fp, 0($sp)          # save caller's frame pointer
    sw      $ra, 4($sp)          # save return address
    addiu   $fp, $sp, 20         # setup main's frame pointer

    # Put mainF into $s0
    la      $t0, mainF
    lw      $s0, 0($t0)

    # Put mainB into $s1
    la      $t0, mainB
    lw      $s1, 0($t0)

         ...

Upvotes: 1

Views: 4019

Answers (1)

Gabe
Gabe

Reputation: 86718

You have the following declarations:

mainF:
    .byte   1
mainB:
    .byte   1

Assuming that mainF gets assigned the address 0, mainB will get assigned the address 1. Since the address 1 is clearly not word-aligned (i.e. 1 is not a multiple of 4), attempting to load it will cause an exception.

Upvotes: 2

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