a thread that can be used to execute Nachos kernel code

The IRIX kernel, since release 6.4, is structured as a set of independent, interruptible, lightweight threads. These "kthreads" (kernel threads) are used to implement system functions and to handle interrupts and such background tasks as disk buffering. Some kthreads are used to execute the code of threads in user-level programs. However, a kthread and a thread are quite different in their abilities and execution environment.