syscall

In even the simplest computer, putting a character on the screen involves many instructions and a detailed knowledge of the video card. Let's leave this topic until later (perhaps years later). SPIM includes an "exception handler" that simulates a tiny operating system that can do input from the keyboard and output to the monitor.

Assembly language programs request operating system services using the syscall instruction. The syscall instruction transfers control to the operating system which then performs the requested service. Then control (usually) returns to the program. (This description leaves out many details).

syscall          # ask the operating system to perform a service

Different operating systems use this instruction in different ways. For the SPIM exception handler it is used like this:

li  $v0,code      # Load $v0 with the "code" number of an OS service.

.......           # Put parameters for the service in
.......           # registers $a0, $a1 or $f12 (see below).

syscall           # Invoke the operating system.

                  # Return value (if any) is in $v0 or $f0 
                  # 

The syscall instruction causes an exception, which transfers control to an exception handler. An exception handler is the part of an operating system that receives requests for operating system services. Different services expect data in different registers, and not all services return values to the caller.