Conversion of an Operating System Oriented Towards Transaction Processing from Two to Three Modes of Logical Address Space

A computer system to control and acquire data from a set of ten neutron and x-ray scattering and diffraction experiments located at the High Flux Beam Reactor at Brookhaven National Laboratory has operated in a routine manner for over two years. The system has been constructed according to a functionally distributed architecture and thus consists of a set of functional nodes. Ten of these nodes, the private or application nodes, perform the function "execute programs to control and acquire data from experiment number x". An additional functional node, the common or shared service node, performs the function "provide a set of shared services to the application nodes". The shared service node has been successfully implemented in software with in-house code oriented towards transaction processing and in hardware with a Digital Equipment Corporation PDP-11/40 computer. However, recent demands that this node provide an expanded set of services have required that its implementation elements be modified and extended. In particular, the node hardware has been changed to a PDP-11/45 processor and the software present at the node has been extended from operation in two modes of logical address space to three modes. A discussion of the systems analysis principles which influenced the manner in which these modifications and extensions were carried out is given. The structure of the old two-mode software is briefly reviewed in order to provide a basis for an examination of its three-mode replacement.