MIPS instruction of an array of 32 bits integer

Question

so I am currently stuck on a question and I would be really glad to see if someone can elaborate on my question.

The question is listed below

Let register $s1 be the base of the array A,

register $v0 be the variable X,

register $t0 be the index i,

then write a MIPS instructions for the following statements

K = A[i] + A[i+3];

The solution is listed as below

sll $t1, $t0, 2
add $t2, $s1, $t1
lw $t3, 0($t2)
lw $t4, 12($t2)
add $v0, $t3, $t4

My solution for this question was

addi $t1, $t0, 3  # for the $t1 <- i+3
lw $t2, 0($t1)    # for the $t2 <- A[i+3]
lw $t3, 0($t0)    # for the $t3 <- A[i]
add $v0, $t2, $t3 # for the K <- A[i+3] + A[i]

So the question is,

I don't really understand why the solution didn't add index by 3 (i+3) then load from added index.

Also the 2nd line from the solution, add $t2, $s1, $t1, how is it possible to add a base with an index(i.e. $t2 <- A+i)? Doesn't array represents the memory stack and the index as the location address for the stack?

Thanks in advance.

Any elaboration for this questions with similar question would be very grateful.

AlexKoren AlexKoren · Accepted Answer · 2015-09-17T22:32:29

Remember that each index of A is not 32bits. Each index of A is only 8 bits. The code above does the following:

sll $t1, $t0, 2   # take $t0 and shift it over 2 bits
                  # (equivalent to multiply by 4)
add $t2, $s1, $t1 # from the 0 index, add on $t1 to get into the register for A[i]
lw $t3, 0($t2)    # load 4 bytes (the entire 32bits) from A[i]
lw $t4, 12($t2)   # load 4 bytes (the entire 32bits) from A[i + 3] 
                  # where 3 is actually 12 because it's 8bits per index
                  # so multiply by 4
add $v0, $t3, $t4

The big thing to realize was the 8bits (byte) vs. 32bits (word) indexing of the "array."

Looking at your code:

addi $t1, $t0, 3  # actually just moves this over 3 bytes, not 3 words
lw $t2, 0($t1)    # gets the word at A[i / 4]
lw $t3, 0($t0)    # gets the word at A[(i + 3) / 4]
add $v0, $t2, $t3 # and now you can see why this would be wrong.

MIPS instruction of an array of 32 bits integer

1 Answers