Multimedia applications have timing requirements that cannot generally be satisfied using time-sharing scheduling algorithms and system structures. To effectively support these types of programs, operating systems must support processor capacity reservation. A capacity reservation and enforcement mechanism isolates programs from the timing and execution characteristics of other programs in the same way that a memory protection system isolates programs from memory access by other programs. In this paper, we characterize the timing requirements and processor capacity reservation requirements for multimedia applications, we describe a scheduling framework to support reservation and admission control, and we introduce a novel reserve abstraction, specifically designed for the microkernel architecture, for controlling processor usage.