The process has long been the workhorse of the UNIX environment. Modern UNIX operating systems have adopted the thread as the new schedulable entity. Chapter 4 examines both the process and the thread, and their relationship to resources and task management. One of the most important aspects of understanding the process/thread environment is in flow of control followed when a system call is requested. The transistion from user mode to system mode and the return to user mode are examined in detail in Chapter 4.