This paper discusses the modelling of DB/DC systems operating in a demand paging environment. As an extension of an earlier definition of Denning a class of systems is defined, those operating with “working set integrity”, for which one can obtain system response parameters solely from the resource requirements of the input processes when running in isolation. No external information such as a miss ratio curve from another systems environment is required. The interdependence between the resource requirements of processes when running in a systems environment and the system performance parameters is faced directly and expressed as a set of simultaneous equations. The emphasis is on the effect of the sharing of a common set of pages among the active set of processes. Both unrestricted shared usage and usage with locking are treated.