Recent microprocessor announcements show a trend toward wide-address computers: architectures that support 64 bits of virtual address space. Such architectures facilitate fundamentally new operating system organizations that promote efficient data sharing and cooperation, both between complex applications and between parts of the operating system itself. One such organization is the single address space operating system, in which all processes run within a single global virtual address space; protection is provided not through conventional address space boundaries, but through protection domains that dictate which pages of the global address space a process can reference. This paper focuses on the architectural implications of single address space operating systems, specifically the interaction between the memory system architecture and the operating system's use of addressing and protection. Our purpose is to explore certain architectural opportunities created by single address space systems by evaluating two protection models that support them. The first provides protection of a per-page, per-domain basis; we define the protection lookaside buffer, a hardware structure that implements this model. The second provides protection on a page-group basis; this model is implemented in the Hewlett-Packard PA-RISC architecture.