MIPS assembly addition program

Question

I am new to assembly and have made a simple addition program to sum two integers read from the keyboard. The program outputs correctly, but I want to know if there is a way to streamline my code. It seems a bit cumbersome for such a simple program and I may have instructions that are unnecessary.

# Author: Evan Bechtol
# Description: This program prompts the user to enter 2 integers and computes their sum.
#---------------------------------------------------------------------------------------#
        .data
A:          .word       # Store the number 4 as an integer in var1  # $t0 is used
B:          .word       # Store the number 2 as an integer in var2  # $t1 is used
S:          .word       # Store the sum of A and B          # $t2 is used
Prompt1:    .asciiz "Please enter first number: "
Prompt2:    .asciiz "Please enter second number: "
Result:     .asciiz "The sum of A and B is: "
        .text
main:
    #--------------------------------------------------------#
    #Display first prompt
    li  $v0, 4      # Load instruction "print string"
    la  $a0, Prompt1    # Load prompt into $a0
    syscall

    #Read first integer
    li  $v0, 5      # Read 1st integer
    la  $t0, A      # $t0 = A
    syscall

    #Store first integer into memory
    move    $t0, $v0    # Move contents in $v0 to $t0
    sw  $t0, A      # A = value at $t0 
    #--------------------------------------------------------#

    #Display second prompt
    li  $v0, 4      # Load instruction "print string"
    la  $a0, Prompt2    # Load prompt into $a0
    syscall

    #Read second integer
    li  $v0, 5      # Read 1st integer
    la  $t1, B      # $t0 = A
    syscall

    #Store second integer into memory
    move    $t1, $v0    # Move contents in $v0 to $t0
    sw  $t1, B      # A = value at $t0 
    #--------------------------------------------------------#

    #Add the two variables
    la  $t2, S      # $t2 = S   
    add     $t2, $t0, $t1   # $t2 = $t0 + $t1
    sw  $t2, S      # S = value at $t2

    #Display the Result prompt
    la  $a0, Result # Loads Output label to be printed
    li  $v0, 4      # Sysycall to print string
    syscall

    #Display the sum
    lw  $a0, S      # $a0 = value at S
    li  $v0, 1      # Syscall to print integer
    syscall

    #Exit the program
    li  $v0, 10     # Load exit code to $v0
    syscall

Answer 1

• The comments are misleading:

  #Read second integer
  li  $v0, 5      # Read 1st integer
  la  $t1, B      # $t0 = A
  

  umm... are we reading second or 1st? Bottomline is, do not overcomment the code.

• syscall 5 leaves a value in $v0. The contents of $t0 (or $t1) is irrelevant during the syscall. Set them up when you need them:

  li $v0, 5
  syscall
  la $t0, A
  move    $t0, $v0
  

• You store data to memory just to load them back. This is very anti-assembly. Generally you want to use registers as much as possible, and avoid memory as much as possible:

  li $v0, 5
  syscall
  move $t0, $v0
  ...
  li $v0, 5
  syscall
  # At this moment you have first integer in $t0, and the second in $v0.
  # Just add them together. No memory access is necessary.
  

  Consult your documentation on which registers are guaranteed to survive a syscall (I suspect, all of them besides $v0).

• Nothing to simplify reading and printing.