The traditional operating system perspective of a process is as one entity that pursues a single activity. However, in the context of distributed systems, and also in the context of uniprocessor systems that required concurrency, this traditional view was found, starting in the early 1980s, to be obsolete and inadequate. The traditional model of a process thus had to be enhanced so that it could be associated with multiple activities in the computer system taking place at once.

Nowadays, therefore, a process is considered to be an environment of execution, together with one or more threads--a thread being an abstraction for a "thread of execution" of a certain activity within the operating system. The execution environment is a collection of local resources managed by the operating system kernel, to which the threads have access. An execution environment has to consist of:

• an address space;
• thread management...