Question

I’m working on a homework assignment that requires me to compute the grayscale value of a series of hex values in an array. That’s the part I understand. I need to loop through this array of values until a -1 is encountered. Here’s what I’ve got:

 # --------------------------------
# Below is the expected output.
# 
# Converting pixels to grayscale:
# 0
# 1
# 2
# 34
# 5
# 67
# 89
# Finished.
# -- program is finished running --
#---------------------------------

.data 0x0
    startString:    .asciiz "Converting pixels to grayscale:\n"
    finishString:   .asciiz "Finished."
    newline:    .asciiz "\n"
    pixels:     .word   0x00010000, 0x010101,   0x6,        0x3333, 
                0x030c,     0x700853,   0x294999,   -1

.text 0x3000

main:
    ori $v0, $0, 4              #System call code 4 for printing a string
    ori $a0, $0, 0x0            #address of startString is in $a0
    syscall                 #print the string

LOOP:   ori $a0, $0, 0x0 
    lw $t1, 48($a0)
    beq $t1 -1, exit
    addi $t4, $0, 3
    sll $t2, $t1, 8
    srl $s1, $t2, 24    #$s1 becomes red value
    sll $t2, $t1, 16
    srl $s2, $t2, 24    #$s2 becomes green value
    sll $t2, $t1, 24
    srl $s3, $t2, 24    #$s3 become blue value
    add $t1, $s1, $s2
    add $t1, $t1, $s3
    div $t1, $t4
    mflo $s4        #$s4 becomes grayscale value
    or $a0, $0, $s4
    ori $v0, $0, 1
    syscall
    ori $v0, $0, 4
    ori $a0, $0, 43
    syscall
    j LOOP

exit:

    ori $v0, $0, 4              #System call code 4 for printing a string
    ori $a0, $0, 33             #address of finishString is in $a0; we computed this
                        #  simply by counting the number of chars in startString,
                        #  including the \n and the terminating \0

    syscall                 #print the string

    ori $v0, $0, 10             #System call code 10 for exit
    syscall                 #exit the program

I know that 48 needs to be incremented by 4 for every iteration of the loop, I just have no idea how to do this in MIPS. Any help is much appreciated!

Answer 1

What you should do is use some register to hold the value of the index of the array you are working with, and in each iteration increment that register with 4.

It is also a bad idea to just put the constant where your array is located, because if you later change the location in memory of your array you have to update that constant too. Instead, use a label and let the assembler figure out the actual location.

Suppose we use register $a1 to hold the index. Then, we just need to make some minor changes to your code:

    ori $a1, $0, 0x0   # Initialize index with 0
LOOP:  
    lw $t1, pixels($a1)  # We use the label name instead of the actual constant
         ...
         ...
    addi $a1, $a1, 4   # Increment index by 4
    j LOOP