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# Simple routine to demo functions
# USING a stack in this example to preserve
# values of calling function
# ------------------------------------------------------------------
.text
.globl main
main:
# Register assignments
# $s0 = x
# $s1 = y
# Initialize registers
lw $s0, x # Reg $s0 = x
lw $s1, y # Reg $s1 = y
# Call function
move $a0, $s0 # Argument 1: x ($s0)
jal fun # Save current PC in $ra, and jump to fun
move $s1,$v0 # Return value saved in $v0. This is y ($s1)
# Print msg1
li $v0, 4 # print_string syscall code = 4
la $a0, msg1
syscall
# Print result (y)
li $v0,1 # print_int syscall code = 1
move $a0, $s1 # Load integer to print in $a0
syscall
# Print newline
li $v0,4 # print_string syscall code = 4
la $a0, lf
syscall
# Exit
li $v0,10 # exit
syscall
# ------------------------------------------------------------------
# FUNCTION: int fun(int a)
# Arguments are stored in $a0
# Return value is stored in $v0
# Return address is stored in $ra (put there by jal instruction)
# Typical function operation is:
fun: # This function overwrites $s0 and $s1
# We should save those on the stack
# This is PUSH'ing onto the stack
addi $sp,$sp,-4 # Adjust stack pointer
sw $s0,0($sp) # Save $s0
addi $sp,$sp,-4 # Adjust stack pointer
sw $s1,0($sp) # Save $s1
# Do the function math
li $s0, 3
mul $s1,$s0,$a0 # s1 = 3*$a0 (i.e. 3*a)
addi $s1,$s1,5 # 3*a+5
# Save the return value in $v0
move $v0,$s1
# Restore saved register values from stack in opposite order
# This is POP'ing from the stack
lw $s1,0($sp) # Restore $s1
addi $sp,$sp,4 # Adjust stack pointer
lw $s0,0($sp) # Restore $s0
addi $sp,$sp,4 # Adjust stack pointer
# Return from function
jr $ra # Jump to addr stored in $ra
# ------------------------------------------------------------------
# Start .data segment (data!)
.data
x: .word 5
y: .word 0
msg1: .asciiz "y="
lf: .asciiz "\n"
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