System Calls

In order to perform I/O with the console, SPIM provides a small library of system calls. In general, system calls are set up by placing a system call in register $v0, and any arguments in register $a0 and $a1. Returned values are placed in register $v0. See the table and the example program below for usage.

Service	System Call Code	Arguments	Result
Print_int	1	$a0 = integer
Print_float	2	$f12 = float
Print_double	3	$f12 = double
Print_string	4	$a0 = string
Read_int	5		Integer (in $v0)
Read_float	6		Float (in $f0)
Read_double	7		Double (in $f0)
Read_string	8	$a0 = buffer, $a1 = length
Sbrk	9	$a0 = amount	Address (in $v0)
exit	10

# This program takes input from the user and echoes it back

.data

# Constant strings to be output to the terminal

promptInt: .asciiz "Please input an integer: "

resultInt: .asciiz "Next integer is: "

linefeed: .asciiz "\n"

enterkey: .asciiz "Press any key to end program."

.text

main:

# prompt for an integer

li $v0,4 # code for print_string

la $a0,promptInt # point $a0 to prompt string

syscall #print the prompt

# get an integer from the user

li $v0,5 # code for read_int

syscall #get int from user --> returned in $v0

move $t0,$v0 # move the resulting int to $t0

# compute the next integer

addi $t0, $t0, 1 # t0 <-- t0 + 1

# print out text for the result

li $v0,4 #code for print_string

la $a0,resultInt # point $a0 to result string

syscall # print the result string

# print out the result

li $v0,1 # code for print_int

move $a0,$t0 # put result in $a0

syscall # print out the result

# print out a line feed

li $v0,4 # code for print_string

la $a0,linefeed # point $a0 to linefeed string

syscall # print linefeed

# wait for the enter key to be pressed to end program

li $v0,4 # code for print_string

la $a0,enterkey # point $a0 to enterkey string

syscall # print enterkey

# wait for input by getting an integer from the user (integer is ignored)

li $v0,5 # code for read_int

syscall #get int from user --> returned in $v0

# All done, thank you!

li $v0,10 # code for exit

syscall # exit program