The Synthesis distributed operating system combines efficient kernel calls with a high-level, orthogonal interface. The key concept is the use of a code synthesizer in the kernel to generate specialized (thus short and fast) kernel routines for specific situations. We have three methods of synthesizing code: Factoring Invariants to bypass redundant computations; Collapsing Layers to eliminate unnecessary procedure calls and context switches; and Executable Data Structures to shorten data structure traversal time. Applying these methods, the kernel call synthesized to read /dev/mem takes about 15 microseconds on a 68020 machine. A simple model of computation called a synthetic machine supports parallel and distributed processing. The interface to synthetic machine consists of six operations on four kinds of objects. This combination of a high-level interface with the code synthesizer avoids the traditional trade-off in operating systems between powerful interfaces and efficient implementations.