System Calls


Table: System services.
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


SPIM provides a small set of operating-system-like services through the system call (syscall) instruction. To request a service, a program loads the system call code (see Table [*]) into register $v0 and the arguments into registers $a0…$a3 (or $f12 for floating point values). System calls that return values put their result in register $v0 (or $f0 for floating point results). For example, to print ``the answer = 5'', use the commands: 
        .data
  str:  .asciiz "the answer = "
        .text
        li $v0, 4        # system call code for print_str
        la $a0, str      # address of string to print
        syscall          # print the string

        li $v0, 1        # system call code for print_int
        li $a0, 5        # integer to print
        syscall          # print it

print_int is passed an integer and prints it on the console. print_float prints a single floating point number. print_double prints a double precision number. print_string is passed a pointer to a null-terminated string, which it writes to the console.

read_int, read_float, and read_double read an entire line of input up to and including the newline. Characters following the number are ignored. read_string has the same semantics as the Unix library routine fgets. It reads up to n - 1 characters into a buffer and terminates the string with a null byte. If there are fewer characters on the current line, it reads through the newline and again null-terminates the string. Warning: programs that use these syscalls to read from the terminal should not use memory-mapped IO (see Section [*]).

sbrk returns a pointer to a block of memory containing n additional bytes. exit stops a program from running.