Read two numbers and print their sum

This program asks for two decimal integers and prints their sum. Select Run. When service 5 waits for input, enter each number and press Enter; execution then continues after that syscall.

.data
first:  .asciiz "First number: "
second: .asciiz "\nSecond number: "
answer: .asciiz "\nThe sum is "

.text
.globl main
main:
    li $v0, 4
    la $a0, first
    syscall
    li $v0, 5           # service 5: read an integer into $v0
    syscall
    move $t0, $v0       # keep the first number before reusing $v0

    li $v0, 4
    la $a0, second
    syscall
    li $v0, 5
    syscall
    add $t0, $t0, $v0   # use the second number before reusing $v0

    li $v0, 4
    la $a0, answer
    syscall
    li $v0, 1           # service 1: print a signed number
    move $a0, $t0
    syscall

    li $v0, 10
    syscall

The first read leaves its result in $v0. The program copies it to $t0 before loading another service number into $v0. After the second read, add uses that new result directly from $v0. It leaves the sum in $t0, ready to copy to $a0 for service 1. If you skip the first move, the next li $v0, 4 replaces the first number with a service number.

Each prompt is printed with service 4 before service 5 waits for its number. Service 5 reads a decimal integer; it does not print a prompt. The \n at the start of second and answer puts those strings on new lines. With 17 and 25, the console displays:

First number:
Second number:
The sum is 42

Change add $t0, $t0, $v0 to sub $t0, $t0, $v0 and, with 17 and 25, the console displays The sum is -8. Now change the li $v0, 1 further down to li $v0, 36 and run it again with the same two numbers: the answer becomes 4294967288. The subtraction leaves the same 32 bits, 0xFFFFFFF8, in $t0 either way. Service 1 interprets those bits as signed -8; service 36 interprets them as unsigned 4294967288.

Try it yourself

Write the two reads and the subtraction below. Save the first result before selecting service 5 again. Then put the difference in $a0 and use service 1 to print it. Select Test to check the negative result. After it passes, try 25 then 17 with Run; the difference should be 8.

.data
label: .asciiz "Difference: "

.text
.globl main
main:
    # Read two integers and leave first minus second in $t0.

    li $v0, 4
    la $a0, label
    syscall
    # Print $t0 as a signed integer.

    li $v0, 10
    syscall

.data
label: .asciiz "Difference: "

.text
.globl main
main:
    li $v0, 5
    syscall
    move $t0, $v0

    li $v0, 5
    syscall
    sub $t0, $t0, $v0

    li $v0, 4
    la $a0, label
    syscall
    li $v0, 1
    move $a0, $t0
    syscall

    li $v0, 10
    syscall