Spim is a software that runs on a number of platforms to simulate the MIPS machines. You can test and run your assembly program directly without compilation. But mixing C and MIPS assembly programs are not possible.
Two versions of spim are provided in this lab. the program xspim has a nice window interface, while spim is text based. You can start xspim by just typing xspim at the Unix prompt, assuming that /usr/local/bin is in your search path.
Appendix A of the textbook has a good description about spim.
Spim source and documentation can be obtained from University of Wisconsin. A PC version run under Windows is also available. But I think it is not as nice as the UNIX version.