The SunOS 5.7 system supports multiple threads in the kernel, and multiple CPUs. A thread is a sequence of instructions being executed by a program. In the SunOS 5.7 system, there are application threads, and there are kernel threads. Kernel threads are used to execute kernel code, and are the threads of concern to the driver writer.
Interrupts are also handled as threads. Because of this, there is less of a distinction between the tophalf and bottomhalf of a driver than there was in the SunOS 4.1 system. All driver code is executed by a thread, which may be running in parallel with threads in other (or the same) part of a driver. The distinction now is whether these threads have user context.
See Chapter 4, Multithreading, for more information.