Question

I am attempting to reverse a string of ints when given size and string. Here is what I have so far. I know my problem is in the loop section, mainly in figuring out how to get a pointer to point at the back of the string and move what is contained to where it should go

Any help would be appreciated:

        .data

Size: .word 9 # Size is 9

Vals: .word 1, 2, 3, 4, 5, 6, 7, 8, 9 # Vals[9]={1,2,..}

        .text
main:

la $t1, Size # get the address of variable Size
la $t2, Vals # get the address of variable Vals
lw $t3, 0($t1) # fetch the value of Size to register $t3
sra $t4, $t3, 1 # right shift arithematic, now $t4 contains Size/2
sll $t3, $t3, 2 # left shift logic, now $t3 contains Size*4

loop:
lb $t5, 0($t2)
sb $t0, 36($t2)
sb  $t0, ($t5)
addi $t0, $t0, 4
addi $t5, $t5, -4
bne $t5, $t4, end
j loop

end:

la $t0, Vals # get the address of Vals to $t0
la $t1, Size # get the address of Size to $t1
lw $t3, 0($t1) # get Size to $t3
sll $t3, $t3, 2 # left shift logic, now $t3 contains Size*4
add $t1, $t0, $t3 # $t1=Vals+Size*4 => array bound
li $v0, 1       # service 1 is print integer

lab4:
lw $a0, 0($t0)  # load desired value into argument register $a0
syscall         # print the value in $a0
addi $t0, $t0, 4 # increase array index
bne $t0, $t1, lab4 # check if reach array bound

Answer 1

There are some issues in that loop:

• You are reading bytes instead of words (use lw/sw instead of lb/sb
• You are mixing contents with addresses (usage of register $t5)
• You are adding/subtracting 4 to the contents read instead of the pointers to the array
• The way you are trying to reverse the array seems wrong as adding 36 (=9×4) would point to a location after the end of the array

What you should do is use 2 pointers: one which starts pointing to the beginning of the array and another that starts pointing to the end of the array, then read both items and interchange its contents. Now move the the next element in each pointer (one pointer moves ahead and the other move behind), and repeat the process until all the items are processed (that is, when both pointers crosses).

E.g (snip):

addu $t7, $t2, $t3 # $t5 contains address of end of string

loop:
  addi $t7, $t7, -4 # moves pointer backwards
  lw $t5, 0($t2)
  lw $t0, 0($t7)
  sw $t5, ($t7)    # interchange
  sw  $t0, ($t2)   # contents
  addi $t2, $t2, 4 # moves pointer forward
  ble $t2, $t7, loop