A distributed I/O architecture for HARTS

The issue of I/O device access in HARTS (Hexagonal Architecture for Real-Time Systems)-a distributed real-time computer system under construction at the University of Michigan-is explicitly addressed. Several candidate solutions are introduced, explored and evaluated according to cost, complexity, reliability, and performance: (1) `node-direct´ distribution with the intranode bus and a local I/O bus; (2) use of dedicated I/O nodes, which are placed in the hexagonal mesh as regular applications nodes, but which provide I/O services rather than computing services; and (3) use of a separate I/O network; which has led to the proposal of an `interlaced´ I/O network. The interlaced I/O network is intended to provide both high performance without burdening node processors with I/O overhead and a high degree of reliability. Both static and dynamic multiownership protocols are developed for managing I/O device access in this I/O network. The relative merits of the two protocols are explored, and the performance and accessibility which each provides are simulated