MIPS: fetch address not aligned on word boundary 0x400fffff - mips

Question

I'm writing a mips program to calculate exponent, but get an error:
Runtime exception at 0x00400158: fetch address not aligned on word boundary 0x400fffff
Any ideas?
The problem line is lw $v0, 0($t0) # $v0 = rev_binary[idx]
.data
prompt1: .asciiz "Enter a base: "
prompt2: .asciiz "Enter an exponential: "
b: .word 0
e: .word 0
rev_binary: .word 0
i: .word 0
result: .word 1
.text
main:
la $a0, prompt1 # print prompt1
addi $v0, $0, 4
syscall
addi $v0, $0, 5 # read b
syscall
move $t0, $v0 # $t0 = b
la $a0, prompt2 # print prompt2
addi $v0, $0, 4
syscall
addi $v0, $0, 5 # read e
syscall
move $t1, $v0 # $t1 = e
addi $sp, $sp, -8 # 2 * 4 = 8 bytes
sw $t1, 4($sp)
sw $t0, 0($sp)
jal power
move $a0, $v0
addi $v0, $0, 1
syscall # print the return value
j end
power:
# save $ra and $fp
addi $sp, $sp, -8
sw $ra, 4($sp)
sw $fp, 0($sp)
#copy $sp to $fp
addi $fp, $sp, 0
# allocate local variables
addi $sp, $sp, -20
# save $sp into $fp
move $fp, $sp
#rev_binary = [ 0 ] * e
lw $a0, 32($sp)
sll $a0, $a0, 2 # $a0 = 4 * e(e << 2)
addi $a0, $a0, 4 # $a0 = 4 * e + 4
addi $v0, $0, 9 # $v0 = 9
syscall # allocate memory
sw $v0, 0($sp)
lw $t0, 32($sp) # $t0 = e
sw $t0, 0($v0) #store the size of rev_binary
sw $0, 4($sp)
while0:
# while e != 0
lw $t0, e # $t0 = e
beq $t0, $0, while0_end # break if e == 0
# e_half = e >> 1
lw $t0, 32($sp) # allocate e_half
srl $t1, $t0, 1 # $t1 = $t0 >> 1
sw $t1, 8($sp) # e_half = $t1
# rev_binary[i] = e - 2 * e_half
lw $t0, 8($sp) # $t0 = e_half
sll $t0, $t0, 1 # $t0 = e_half * 2
lw $t1, e # $t1 = e
sub $t0, $t1, $t0 # $t0 = e - e_half * 2
lw $t1, 4($sp) # $t1 = i
sll $t1, $t1, 2
addi $t1, $t1, 4 # $t1 = i * 4 + 4
lw $t2, rev_binary # $t3 = the address of rev_binary
add $t1, $t1, $t2 # $t1 = the address of rev_binary[i]
sw $t0, 0($t1) # rev_binary[i] = $t0
# i += 1
lw $t0, 4($sp) # $t0 = i
addi $t0, $t0, 1 # $t0 += 1
sw $t0, 4($sp) # i = $t0
# e = e_half
lw $t0, 8($sp) # $t0 = e_half
sw $t0, 32($sp) # e = $t0
addi $sp, $sp, 4 # release local variable n_half
j while0
while0_end:
# rev_binary = rev_binary[:i]
lw $t0, 0($sp) # $t0: the address of rev_binary
lw $t1, 4($sp) # $t1 = i
sw $t1, 0($t0) # set the size of rev_binary to i
# result = 1
addi $t0, $0, 1 # $t0 = 1
sw $t0, 12($sp) # result = $t0
# for j in range(i - 1, -1, -1):
addi $sp, $sp, -4 # allocate j
lw $t0, i # $t0 = i
addi $t0, $t0, -1 # $t0 -= 1
sw $t0, 0($sp) # j = $t0 = i - 1
# idx = len(rev_binary) - 1
lw $t0, 4($sp) # $t0 = i = len(rev_binary)
addi $t0, $t0, -1 # $t0 -= 1
sw $t0, 16($sp) # idx = $t0
while1: # while idx >= 0
lw $t0, 16($sp) # idx = $t0
blt $t0, $0, while1_end # break if idx < 0
# result = result * result
lw $t0, 12($sp) # t0 = result
mul $t0, $t0, $t0
sw $t0, 12($sp) # $t0 = result
# if rev_binary[idx]
lw $t0, 16($sp) # $t0 = idx
sll $t0, $t0, 2 # $t0 = idx * 4
addi $t0, $t0, 4 # $t0 = idx * 4 + 4
lw $t1, 0($sp) # $t1 = the address of rev_binary
add $t0, $t1, $t0 # $t0 = the address of rev_binary[idx]
lw $v0, 0($t0) # $v0 = rev_binary[idx]
if:
beq $t0, $0, end_if # end if if (idx == 0)
# result = result * b
lw $t0, 12($sp) # $t0 = result
lw $t1, 28($sp) # $t1 = b
mul $t0, $t0, $t1 # $t0 = $t0 * $t1
sw $t0, 12($sp) # result = $t0
end_if:
# idx = idx - 1
lw $t0, 16($sp) # $t0 = idx
addi $t0, $t0, -1 # $t0 -= 1
sw $t0, 16($sp) # idx = $t0
j while1 # loop
while1_end:
lw $v0, 12($sp) # $v0 = result
lw $t0, 20($sp) # reset old $fp
move $fp, $t0
lw $t0, 24($sp) # reset old $ra
move $ra, $t0
addi $sp, $sp, 36 # release local variables
jr $ra # return
end:
# exit

Answer 1

You can only load words from aligned addresses. The address also looks strange since it has 0xffff in the end.
Looking at the code you have
lw $t1, 0($sp) # $t1 = the address of rev_binary
but before that you store j to (0)$sp, so that can't contain the address of rev_binary. Maybe you need
la $t1, rev_binary # $t1 = the address of rev_binary
instead?