You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
85 lines
1.9 KiB
85 lines
1.9 KiB
5 years ago
|
# 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"
|