SUMMARYThe EMMA2l parallel architecture is a network of nodes, named 'Regions': each region is based on two shared-memory hierarchical bus levels, on which a large series of monocpu, multicpu and specialized 110 boards can be inserted, grouped in 'Families'.Its typical applications are Image processing, pattern recognition and artificial intelligence, characterized by intensive computation requirements, real-time constraints and good price-performance ratio for building large industrial systems. This paper presents the main characteristics of the EMMA2 parallel operating system, highlighting the internal organization and mechanisms offering the parallel co-operation model for user programs.Section 1 briefly introduces the overall hardware architecture. It allows application partitioning into parallel independent subsystems, on two levels of computation: the lower level (family) Is dedicated to massive number crunching, while the upper (region) permits the interaction of these lower machines.Section 2 deals with the system software environment, based on a host-target configuration:the host Ls mainly devoted to machine control, system monitoring, program development and debugging in a multi-user environment. Sections 3,4 and 5 describe respectively the fundamental concepts of the operating system internal organization and the basic mechanisms with respect to the machine architecture; the basic functions that constitute the internal layers and services; the resulting parallel programming and co-operation model and the typical mapping rules of algorithms to this architecture, completed with some examples of their functionalities.The most important aspect of the EMMA2 operating system is the fact that it offers a uniform interface to user tasks, independently of their allocation on the target machine. Objects managed by system services can be referred to via logical names. Code for system services is distributed among processors and resides on a private read-only memory. The operating system's data reside in shared memory only for system functions that need to be distributed, while private functions are based on different copies of data in private memories.Section 6 gives more details on the most relevant EMMA2 industrial applications, also showing their mapping to the parallel architecture and some performance-related information. Section 7 contains a brief outline of the novel architecture currently in development and of the subsequent 0s Improvements.In conclusion, we report some considerations about the present status of the EMMA2 system, compared with some other well-known architectures, and about research efforts for its future evolution.

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