Mips program throws bad adress exception

Question

I have the following mips code:

.text
c:
    li $8, 0
    addu $sp, $sp, -4
    sw $8, 0($sp)
    jr $ra
a:
    li $8, 0
    addu $sp, $sp, -4
    sw $8, 0($sp)
    addu $sp, $sp, -4
    sw $8, 0($sp)
    addu $sp, $sp, -8
    sw $fp, 0($sp)
    sw $ra, 4($sp)
    jal c
    lw $fp, 0($sp)
    lw $ra, 4($sp)
    addu $sp, $sp, 8
    lw $8, 0($sp)
    addu $sp, $sp, 4
    jr $ra
main:
    addu $sp, $sp, 0
    addu $sp, $sp, -8
    sw $fp, 0($sp)
    sw $ra, 4($sp)
    jal a
    lw $fp, 0($sp)
    lw $ra, 4($sp)
    addu $sp, $sp, 8
    addu $sp, $sp, 0
    j exit

I get " Exception 6 [Bad instruction address]", but I can't see anything wrong with the code.

How can I know where is it happening? I run the code using spim command. I suspect the return adress or the stack pointer is not being restored' but it looks right to me, what am I doing wrong?