The Pilot operating system is designed for the personal computing environment. It provides a basic set of services within which higher-level programs can more easily serve the user and/or communicate with other programs on other machines. Pilot omits certain functions sometimes associated with “complete” operating systems, such as character-string naming or user-command interpretation; higher-level software provides such facilities as needed. On the other hand, Pilot provides a higher level of service than that normally associated with the “kernel” or “nucleus” of an operating system. Pilot is closely coupled to the Mesa programming language and runs on a rather powerful personal computer, which would have been thought sufficient to support a substantial timesharing system of a few years ago. The primary user interface is a high resolution bit-map display, with a keyboard and a pointing device. Secondary storage generally takes the form of a sizable local disk. A local packet network provides a high bandwidth connection to other personal computers, and to server systems offering such remote services as printing and shared file storage.

The Chi/OS operating system, the latest large-scale software effort of Chi Corporation, has had an excellent reliability record since its installation in November 1973. Although the system design is vital to the reliability of Chi/OS, several environmental factors are equally vital. After a brief presentation of the substantial work load supported by Chi/OS, this paper deals with those environmental factors which contribute to the reliability of the software.

A detailed examination of binary search trees reveals that the probability of making precisely i comparisons in placing the (n−1)th item in the tree is related to the (n−i)th symmetric function of the integers 1, …, n. A recurrence relation for the moments of this distribution of comparisons is derived, and formulas for the mean number of comparisons and its variance are displayed. These are shown to be in accord with previously published values.