With the increasing proliferation of multitasking and Internet-connected devices, security has reemerged as a fundamental design concern in information systems. The shift of IS curricula toward a largely organizational perspective of security leaves little room for focus on its foundation in systems architecture, the computational underpinnings of processes and protection. Yet these architectural features are the foundation of systems security for all the layers above that they enable. They are also the prototypical mechanisms of protection that must be modeled throughout systems design to realize system security: confidentiality, integrity and availability. This paper presents a learning unit that proposes a special ontology of computer system architecture to explain computer security on the host-level and by extension the emerging standard security architecture of the cloud, the virtual machine. The ontology appears as a prose tutorial, a set theoretic model, and a two-page study reference that facilitates a security discussion ranging from host architecture to web-services. This treatment is a concise, self-contained module for standalone use or embedded in a systems course (analysis, modeling, design, database or systems architecture) where complete operating system or computer organization coverage may not be feasible.