A Topaz application consists of one or more communicating processes. One way to think of a process is as a virtual computer. Like a real computer, it has components for storage, processing, and input/output. Compared with a real computer, the components of a process are intended to be easier to use; in exchange, some potential performance is given up. At any time there may be one or more processes running on a given computer (Firefly or VAX). These processes are reasonably isolated from each other with respect to programming and operator errors. Just as the Firefly computer contains multiple processors connected to a single store, so a Topaz process can contain multiple threads of control (or threads for short) within its single address space. A thread in one process can communicate with another process on the same or a different computer by making a remote procedure call (RPC). The RPC mechanism lets processes communicate in space; the file system lets them communicate in both time and space. The file system allows access to local and remote disk files, to tty-like devices, and to interprocess pipes. The file system provides a hierarchically organized name space to facilitate sharing of files among programs and users.