DEMOS was originally designed as a package within simula in 1979. Starting in 1992 there where a succession of papers describing both operational and denotational semantics for DEMOS:

1. Characterising The Structure Of Simulation Models In CCS (1993) G. Birtwistle, R. Pooley, C. Tofts, Transactions of the Simulation Society, 10(3):205-236, 1993.
2. Denotational Semantics for Process-Based Simulation Languages. Part 1: piDemos Graham Birtwistle, Chris Tofts, 
3. Tofts CMN and Birtwistle GM, A denotational semantics for a process-based simulation language. ACM Transactions on Modelling and Computer Systems, 8(3), 281-304, 1998.
4. Operational Semantics Of Process-Oriented Simulation Languages Part 1 : (1993)  G. Birtwistle and C. Tofts, Transactions of the Simulation Society 10(4):299-333, 1993
5. The Operational Semantics of Demos: part II,  G. Birtwistle and C. Tofts,Transactions of the Simulation Society, 11(4):303-337.
6. Relating Operational and Denotational Descriptions of ßDemos Graham Birtwistle, Chris Tofts, Simulation Practice and Theory, vol.5, pp.1-33.1997
7. Getting Demos Models Right Part I: Practice, Graham Birtwistle, Chris Tofts
8. Getting Demos Models Right Part II: ... and Theory Graham Birtwistle and Chris Tofts
9. A Purely Functional Implementation of an Object Oriented Simulation Language, G. Birtwistle and C.M.N. Tofts, Proceedings UKSCS Conference 1995, North Berwick.

Related material:

• Communication and Concurrency (context) - Milner - 1989
• Communicating and Mobile Systems : The Pi-Calculus -- Robin Milner 1999
• New Jersey  sml
• Matrix-Geometric Solutions in Stochastic Models : An Algorithmic Approach -- Marcel F. Neuts,  1995
• Symbolic Approaches to Probability Distributions in Process Algebra, Chris Tofts, FACS 12:392-415, 2000
• System Design Using Petri Nets, W. Reisig