## Architectural Comparison

In traditional UNIX systems, the operating system is called the kernel, which is isolated from users and applications. At the top level, user programs invoke operating system services using system calls or library functions. At the bottom level, kernel primitives interface directly with hardware. The kernel is divided into two sections: the file subsystem, which transfers data between memory and external devices, and the process control subsystem, which controls inter-process communication, process scheduling, and memory management.

Windows NT is a single-user system, allowing only a single user to log on to a computer at one time. It provides services to multiple remote users through the network. Some third-party products such as Citrix WinFrame diminish this distinction, allowing multiple users to log on to a single Windows NT computer. This is actually similar to the operation of X terminals, where the processing takes place on a central computer but the display is distributed to remote computers.

In Windows NT, the software running in kernel space is called the executive. As with UNIX, it is isolated from user programs giving Windows NT stability unprecedented in previous Windows operating systems. User programs and applications access one of the protected subsystems in Windows NT, which provide a programming interface. These include a Microsoft Win32® interface for the traditional Windows user and programmers, and a POSIX interface. The subsystems and services access the executive using system services. The executive contains the object manager, security reference monitor, process manager, local procedure call facility, memory manager, and an input/output (I/O) manager.

The hardware abstraction layer (HAL) is an effort to make Windows NT portable across different hardware platforms, such as the Intel x86, Pentium, and DEC Alpha. It maps platform-specific features to a generic hardware interface used by the Windows NT executive.

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Last Updated: December 1, 1997