6th Workshop on Hot Topics in Operating Systems (HotOS) May 5-6, 1997, Chatham (Cape Code), Massachusetts Extensibility can be based on cross-address-space communication or on grafting application-specific modules into the operating system. For comparing both approaches, we need to explore the best achievable performance for both models. This paper reports the achieved performance of cross-address-space communication for the L4 ¯-kernel on Intel Pentium, Mips R4600 and DEC Alpha. The direct costs range from 45 cycles (Alpha) to 121 cycles (Pentium). Since only 2.3% of the L1 cache are required (Pentium), the average indirect costs are not to be expected much higher. 1 Motivation: extensibility "Extensibility" is a relatively new buzzword in OS research. Nevertheless, the requirement for extensibility is neither specific to operating systems nor new. Editors are extended by macros associating new functions to keys, programming languages are extended by libraries, database systems are e..