Editing SPIM (section)

== The SPIM operating system ==
The SPIM simulator comes with a rudimentary operating system, which allows the programmer usage of common used functions in a comfortable way. Such functions are invoked by the {{mono|syscall}}-instruction. Then the OS acts depending on the values of specific registers.

{| class=wikitable
|+ '''Examples of system calls (used by SPIM)'''
|-
! service || Trap code || Input || Output || Notes
|-
! print_int
| $v0 = 1 || $a0 = integer to print || prints $a0 to standard output || base = 10
|-
! print_string
| $v0 = 4 || $a0 = address of first character ||  || prints a character string to standard output
|-
! read_int
| $v0 = 5 ||   || integer read from standard input placed in $v0 || base = 10
|-
! sbrk
| $v0 = 9 || $a0 = number of bytes required || $v0= address of allocated memory || Allocates memory from the heap
|-
! exit
| $v0 = 10 ||  ||  ||
|-
! file_open
| $v0 = 13 || $a0 = full path, $a1 = flags, $a2 = UNIX octal file mode || $v0 = file descriptor || example; there are read/write/close functions, too
|}

The SPIM OS expects a label named {{mono|main}} as a handover point from the OS-preamble.