MIPS system calls

(from SPIM S20: A MIPS R2000 Simulator, James J. Larus, University of Wisconsin-Madison)

SPIM provides a small set of operating-system-like services through the MIPS system call (syscall) instruction. To request a service, a program loads the system call code (see Table below) 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).
Service System Call Code Arguments Result
print integer 1 $a0 = value (none)
print float 2 $f12 = float value (none)
print double 3 $f12 = double value (none)
print string 4 $a0 = address of string (none)
read integer 5 (none) $v0 = value read
read float 6 (none) $f0 = value read
read double 7 (none) $f0 = value read
read string 8 $a0 = address where string to be stored
$a1 = number of characters to read + 1
memory allocation 9 $a0 = number of bytes of storage desired $v0 = address of block
exit (end of program) 10 (none) (none)
print character 11 $a0 = integer (none)
read character 12 (none) char in $v0
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