level, too, except very few that call directly into the kernel layer for better performance. The term "service" in this context generally refers to a callable routine, or set of callable routines. This is distinct from the concept of a "service process", which is a user mode component somewhat analogous to a daemon in Unix-like operating systems. Object Manager The Object Manager (internal name Ob) is an executive subsystem that all other executive subsystems, especially system calls, must pass through to gain access to Windows NT resources—essentially making it a resource management infrastructure service. The object manager is used to reduce the duplication of object resource management functionality in other executive subsystems, which could potentially lead to bugs and make development of Windows NT harder. To the object manager, each resource is an object, whether that resource is a physical resource (such as a file system or peripheral) or a logical resource (such as a file). Each object has a structure or object type that the object manager must know about. Object creation is a process in two phases, creation and insertion. Creation causes the allocation of an empty object and the reservation of any resources required by the object manager, such as an (optional) name in the namespace. If creation was successful, the subsystem responsible for the creation fills in the empty object. Finally, if the subsystem deems the initialization successful, it instructs the object manager to insert the object, which makes it accessible through its (optional) name or a cookie called a handle. From then on, the lifetime of the object is handled by the object manager, and it's up to the subsystem to keep the object in a working condition until being signaled by the object manager to dispose of it. Handles are identifiers that represent a reference to a kernel resource through an opaque value. Similarly, opening an object through its name is subject to security checks, but acting through an existing, open handle is only limited to the level of access requested when the object was opened or created. Object types define the object procedures and any data specific to the object. In this way, the object manager allows Windows NT to be an object-oriented operating system, as object types can be thought of as polymorphic classes that define objects. Most subsystems, though, with a notable exception in the I/O Manager, rely on the default