The paper is a discussion of the issues that arose in the design of an operating system for a distributed multiprocessor, Cm*. Medusa is an attempt to understand the effect on operating system structure of distributed hardware, and to produce a system that capitalizes on and reflects the underlying architecture. The resulting system combines several structural features that make it unique among existing operating systems.

Automatic-repeat-request (ARQ) is one of the most commonly used error control techniques today. In this paper, an error control technique that is a basic improvement over ARQ is presented. The technique uses the simple idea of utilizing erroneously received blocks in an ARQ system for error control, retaining most of the other aspects of ARQ. The technique is termed ARQ-with-memory (MRQ). The general MRQ system is described, and simple upper and lower bounds are derived on the throughput achievable by MRQ. The performance of MRQ with respect to throughput, message delay and probability of error is compared to that of ARQ by simulating both systems using error data from a VHF satellite channel being operated in the ALOHA packet broadcasting mode [9].