CSC263 Computer Org & Arch I

System Services

The MARS simulator supports I/O through a set of system services. The code for the desired service is loaded into register $v0. The requirements for using the services and the results are summarized in the following table.

Service	Code in $v0	Arguments	Result
print integer	1	$a0 = integer to print
print float	2	$f12 = float to print
print double	3	$f12 = double to print
print string	4	$a0 = address of null-terminated string to print
read integer	5		$v0 contains integer read
read float	6		$f0 contains float read
read double	7		$f0 contains double read
read string	8	$a0 = address of input buffer $a1 = n, the maximum number of characters to read	string read in can be no longer than n-1 chars; if less than that, adds newline to end, then pads with null byte
sbrk (allocate heap memory)	9	$a0 = number of bytes to allocate	$v0 contains address of allocated memory
exit (terminate execution)	10
print character	11	$a0 = character to print	prints ASCII character for value in low order byte
read character	12		$v0 contains character read
open file	13	$a0 = address of null-terminated string containing filename $a1 = flags $a2 = mode	$v0 contains file descriptor (negative if error). See note below table
read from file	14	$a0 = file descriptor $a1 = address of input buffer $a2 = maximum number of characters to read	$v0 contains number of characters read (0 if end-of-file, negative if error). See note below table
write to file	15	$a0 = file descriptor $a1 = address of output buffer $a2 = number of characters to write	$v0 contains number of characters written (negative if error). See note below table
close file	16	$a0 = file descriptor

Examples

Print an Integer

    # put the number to be printed into reg $a0
    li    $v0, 1             # system call code for print int
    syscall                  # print the integer in $a0

Print a String

strout: .asciiz "Here is the string to print"

    li    $v0, 4             # system call code for print string
    la    $a0, strout        # load addr of string to print in $a0
    syscall                  # print strout

Read an Integer

    li    $v0, 5             # system call code for read int
    syscall                  # read integer into $v0
    # the value just read is in reg $v0

Read a String

strin: .space 40
$a0 = address of input buffer $a1 = maximum number of characters to read

    li    $v0, 8             # system call code for read string
    la    $a0, strin         # load addr of string to print in $a0
    li    $a1, 40            # read at most 39 characters
    syscall                  # read into strin

Exit from Program

    li    $v0, 10            # system call code to exit program
    syscall	                 # return control to system