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
print_character 11 $a0 = character
read_character 12 character (in $v0)
open 13 $a0 = filename, file descriptor (in $v0)
$a1 = flags, $a2 = mode
read 14 $a0 = file descriptor, bytes read (in $v0)
$a1 = buffer, $a2 = count
write 15 $a0 = file descriptor, bytes written (in $v0)
$a1 = buffer, $a2 = count
close 16 $a0 = file descriptor 0 (in $v0)
exit2 17 $a0 = value

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 $a0$\ldots$$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:
  str:  .asciiz "the answer = "
        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. print_character prints a single ASCII character.

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. read_character reads a single ASCII character. 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. This pointer is word aligned. exit stops a program from running. exit2 stops the program from running and takes an argument, which is the value that spim or xspim uses in its call on exit.

open, read, write and close behave the same as the Unix system calls of the same name. They all return $-1$ on failure.

Ian Moor 2009-03-11