A library OS is an operating system refactored to run as a set of libraries within the context of an application.
While Drawbridge can run many possible library OSes, a key contribution of Drawbridge is a version of Windows that has been enlightened to run within a single Drawbridge picoprocess. The Drawbridge Windows library OS consists of a user-mode NT kernel--informally referred to as NTUM--which runs within the picoprocess. NTUM provides the same NT API as the traditional NT kernel that runs on bare hardware and in hardware VMs, but is much smaller as it uses the higher-level abstractions exposed by the Drawbridge ABI. In addition to NTUM, Drawbridge includes a version of the Win32 subsystem that runs as a user-mode library within the picoprocess.
Upon the base services of NTUM and the user-mode Win32 subsystem, Drawbridge can run many of the DLLs and services from the hardware-based versions of Windows. As a result, the Drawbridge prototype can run large classes of Windows desktop and server applications with no modifications to the applications.