SPIM runs under Linux, and needs to interact with environment

So, the initialisation code (that invokes main) sets up argc/argv:

    lw    $a0 0($sp)   # argc 
    addiu $a1 $sp 4    # argv 
    ...
    jal   main 
    nop 
    li    $v0 10 
    syscall            # syscall 10 (exit) 

Note: we are ignoring envp (environment pointer)