To use SPIM, connect to grid1.cs.uwm.edu using telnet, ssh (putty) or X, and at the shell prompt type spim. With the default exception handler, you must not put __start in your code or it will error out. The exception handler calls a main proceedure that you must write. Thus your code should look like:
.text main: # your main code here jr $ra # this exits the program # other procedures here