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 a "trap 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. (There is, unfortunately, no Ebert 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 trap 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 
                  # 

Different services use different registers, and not all services return values to the caller.