Commodity operating systems achieve good performance by running device drivers in-kernel. Unfortunately, this architecture offers poor fault isolation. This paper introduces microdrivers, which reduce the amount of driver code running in the kernel by splitting driver functionality between a small kernel-mode component and a larger user-mode component. This paper presents the microdriver architecture and techniques to refactor existing device drivers into microdrivers, achieving most of the benefits of user-mode drivers with the performance of kernel-mode drivers. Experiments on a network driver show that 75 % of its code can be removed from the kernel without affecting common-case performance. 1