add 1 and 2 in MIPS Assembly https://www.cs.ucsb.edu/~franklin/64/lectures/mipsassemblytutorial.pdf

add.mips

.text

main:
    li       $t1, 1        # load immediate value 1 into $t1
    add      $t0, $t1, 2   # $t1 + 2 and put result into $t0

    li       $v0, 10       # 10 is the the exit syscall num
    syscall


# spim
# load "add.asm"
# run
# (spim) print $t1
# Reg 9 = 0x00000001 (1)
# (spim) print $t0
# Reg 8 = 0x00000003 (3)
# (spim) print $t2
# Reg 10 = 0x00000000 (0)
# (spim) exit

add2.asm

# A program that computes and prints the sum
# of two numbers specified at runtime by the user.
# Registers used:
#   $t0 - used to hold the first number.
#   $t1 - used to hold the second number.
#   $t2 - used to hold the sum of $t1 and $t2.
#   $v0 - syscall parameter.
#   $a0 - syscall parameter.

main:
    li $v0, 5    # read_int syscall
    syscall
    move $t0, $v0

    li $v0, 5    # read_int syscall
    syscall
    move $t1, $v0

    add $t2, $t0, $t1

    move $a0, $t2
    li $v0, 1    # print_int syscall
    syscall

    li $v0, 10   # exit syscall
    syscall

# Example usage
#   (spim) load "add2.mips"
#   (spim) run
#   1
#   2
#   3(spim) run
#   4
#   5
#   9(spim) exit

hello.asm

# Hello World!

# Registers used:
#   $a0 - syscall param -- the string to print.
#   $v0 - syscall param and return value.

# Example:
#   (spim) load "hello.asm"
#   (spim) run
#   Hello World

    .text

main:
    la    $a0, hello_msg  # load the addr of hello_msg into $a0
    li    $v0, 4          # print_string syscall
    syscall

    li    $v0, 10         # exit syscall
    syscall


# Data for the program:
    .data

hello_msg:    .asciiz "Hello World\n"