All kernels for all Silicon Graphics CPUs support the spinlock functions (although these functions perform no actual work on single-processor systems). Therefore, a fully semaphored multiprocessor device driver can run unmodified on a single-processor system.
Note: Because IRIX kernels cannot, as a rule, be preempted, any driver that sits in a loop waiting for some condition to be satisfied may tie a processor up for as long as it wants. Real-time processes, such as audio, are very sensitive to such delays.