Next-generation workstations will have hardware support for digital "continuous media" (CM) such as audio and video. CM applications handle data at high rates, with strict timing requirements, and often in small "chunks". If such applications are to run efficiently and predictably as user-level programs, an operating system must provide scheduling and IPC mechanisms that reflect those needs. We propose two such mechanisms: split-level CPU scheduling of lightweight processes in multiple address spaces, and memory-mapped streams for data movement between address spaces. These techniques reduce the number of user/kernel interactions (system calls, signals, and preemptions). Compared with existing mechanisms, they can reduce scheduling and I/O overhead by a factor of 4 to 6.