Read two numbers and print their sum

This program asks for two numbers and prints their sum. Press Run. At the first prompt, type a decimal integer in the box below the console and press Enter. The program pauses at that ecall until then.

Service 5 turns the text you entered into an integer and places that result in a0.

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

.text
.globl main
main:
    li a7, 4
    la a0, first
    ecall
    li a7, 5            # service 5: read an integer into a0
    ecall
    mv t0, a0           # keep the first number

    li a7, 4
    la a0, second
    ecall
    li a7, 5
    ecall
    add t0, t0, a0      # add the second number

    li a7, 4
    la a0, answer
    ecall
    li a7, 1            # service 1: print it as a signed number
    mv a0, t0
    ecall

    li a7, 10
    ecall

Service 5 places its result in a0. Save the first result before a setup instruction such as la a0, second replaces it. Here mv t0, a0 keeps that number in t0. After the second read, add t0, t0, a0 puts the sum in t0; then la a0, answer can load the address for the label, and mv a0, t0 can load the sum for service 1.

For each number, the program prints its prompt with service 4 and then calls service 5 to read the number.

The \n at the front of second and answer is a newline inside the string, so it starts the next output on a new line. The text and newline in .asciz "\nSecond number: " occupy 16 bytes. .asciz adds the zero terminator, so the declaration occupies 17 bytes in total.

Type 17 and 25 and the console reads The sum is 42. Service 5 reads a decimal number. In this Playground, input that is not a valid decimal integer ends the run.

Turn that add into a sub and the console reads The sum is -8, because service 1 prints its argument as a signed number. After that change, t0 contains the bits FFFFFFF8. Change the li a7, 1 under it to li a7, 36; service 36 prints those same bits as the unsigned value 4294967288.

Your turn: read two numbers and print their difference

Complete the program so it prints the first number minus the second. After the first read, save the value before loading the address of the second prompt into a0.

.data
first:      .asciz "First number: "
second:     .asciz "\nSecond number: "
difference: .asciz "\nDifference: "

.text
.globl main
main:
    # print first, read the first number, and save it in t0
    # print second and read the second number
    # subtract the second number from t0 and print the result
    # end the run

.data
first:      .asciz "First number: "
second:     .asciz "\nSecond number: "
difference: .asciz "\nDifference: "

.text
.globl main
main:
    li a7, 4
    la a0, first
    ecall
    li a7, 5
    ecall
    mv t0, a0

    li a7, 4
    la a0, second
    ecall
    li a7, 5
    ecall
    sub t0, t0, a0

    li a7, 4
    la a0, difference
    ecall
    li a7, 1
    mv a0, t0
    ecall

    li a7, 10
    ecall