This paper describes a kernel interface that provides an untrusted user-level process (an executive) with protected access to memory management functions, including the ability to create, manipulate, and execute within subservient contexts (address spaces). Page motion callbacks not only give the executive limited control over physical memory management, but also shift certain responsibilities out of the kernel, greatly reducing kernel state and complexity. The executive interface was motivated by the requirements of the Wisconsin Wind Tunnel (WWT), a system for evaluating cache-coherent shared-memory parallel architectures. WWT uses the executive interface to implement a fine-grain user-level extension of Li's shared virtual memory on a Thinking Machines CM-5, a message-passing multicomputer. However, the interface is sufficiently general that an executive could act as a multiprogrammed operating system, exporting an alternative interface to the threads running in its subservient contexts. The executive interface is currently implemented as an extension to CMOST, the standard operating system for the CM-5