I am having an issue with byte alignment and im not sure what to change or how to fix it (MIPS)

Question

I've tried several variations and nothing is working correctly - i keep getting an error from qtspim

I am trying to parse the header from a ppm file and store the relevant quantities as integers so that i can later on use them in calculations.

I read from the file and my entire header is included in a buffer, and i now need to break this up and actually retrieve values for my width, height, and max value.

The problem is as follows: When i run through the buffer bit by bit, i try to store the relevant bits that would form, in this code snippet, the width value. I keep getting an error on this line:

sb $t2, 0($t7)

Ive attached the rest of the code below.

ive tried different variations on $t7, including having another temporary buffer to load into $t7, different space allocations, etc. i believe it to be an issue regarding bit alignment, but im at a loss as to where to start on fixing that.

any help would be greatly appreciated!

.data
    header_buffer:  .space 256     # Buffer to store the header
    width:          .word 0
    height:         .word 0 
    max_val:        .word 0
    filename: .asciiz "input.ppm" 

.text
    .globl main
    
    
main:
    # Open the file
    li $v0, 13              # Open syscall code
    la $a0, filename        # Load the filename into $a0
    li $a1, 0               # Read-only mode
    syscall
    move $s0, $v0           # Save the file descriptor in $s0

    # Read the first 4 lines of the header
    la $t1, header_buffer   # Load the header buffer address
    

# Read the first line of the file - should be P2/P3
read_head:
    li $v0, 14              # Read syscall code
    move $a0, $s0           # File descriptor
    move $a1, $t1           # Buffer address
    li $a2, 1               # Maximum length
    syscall
    
    #Check if this is the end of the first line
    lb $t4, 0($t1)              # Load the read character
    addi $t1, $t1, 1
    beq $t4, 10, read_comment   # End of line (newline character)
    j read_head
    
# Read the second line of the file, check if its a comment 
read_comment:
    li $v0, 14              # Read syscall code
    move $a0, $s0           # File descriptor
    move $a1, $t1           # Buffer address
    li $a2, 1               # Maximum length
    syscall
    
    #Check if this a comment line
    lb $t4, 0($t1)                      # Load the read character
    addi $t1, $t1, 1
    bne $t4, 35, read_width     # Branch to next line if the first char isnt a #
    
read_comment_loop:
    li $v0, 14              # Read syscall code
    move $a0, $s0           # File descriptor
    move $a1, $t1           # Buffer address
    li $a2, 1               # Maximum length
    syscall
    
    #Check if this a comment line
    lb $t4, 0($t1)                      # Load the read character
    addi $t1, $t1, 1
    beq $t4, 10, read_width     # End of line (newline character)
    j read_comment_loop

# Read the width and height and max value of the file
init_values:
    li $t7, 0       # Use to store width
    li $t8, 0       # Use to store height
    li $t9, 0       # Use to store max val
    
read_width: 
    li $v0, 14              # Read syscall code
    move $a0, $s0           # File descriptor
    move $a1, $t1           # Buffer address
    li $a2, 1               # Maximum length
    syscall
    
    lb $t2, ($t1)                       # Load the read character
    sb $t2, 0($t7)
    addi $t1, $t1, 1
    beq $t2, 32, store_width            # Check if char is whitespace 
    j read_width
    
store_width:
    move $t0, $t7
    jal str_to_int
    sw $t6, width