• portability
• security
• POSIX compliance, Portable Operating System Interface,an IEEE standard designed to facilitate application portability.
• multiprocessor support
• extensibility
• international support
• compatibility with MS-DOS/Windows applications

This led to the development of a system that was:

• based around a micro-kernel, and
• constructed in a layered/modular fashion.

The system uses the kernel for critical OS services, which are kept separate from user programs to prevent the user programs from crashing the OS. The main parts of Win2K that run in kernel mode are the hardware abstraction layer , NT Kernel, and NT Executive.

The figure left shows the general structure of Windows NT Kernel Mode contains the Hardware Abstraction Layer (HAL), Kernal and Executive. The Hardware Abstraction Layer (HAL) is a layer of programming allowing the OS to interact with a hardware device at a general or abstract level rather than at a detailed hardware level Applications can't communicate directly with hardware. HAL.DLL which hides details of underlying hardware e.g. low-level interrupt mechanisms, DMA controllers, multiprocessor communication mechanisms.Several HALs exist with same interface but different implementation often vendor-specific, e.g. lrge cc-NUMA machines). Windows NT Kernel is not a micro-kernel by nature. However, its design is layered, and what runs in privileged mode (i.e. the kernel) is divided into the Kernel , the Executive and Device Drivers. The Kernel implements core operating system functionality and is platform dependent. The Execute layer is build on top of the Kernel and provides a set of core services of various Windows NT Kernel sub-systems (such as Virtual Memory Manager, Object Manager, Configuration Manager, ...).

The Kernel is the foundation for the Executive and the subsystems and has four main responsibilities:

1. . CPU scheduling
2. . interrupt and exception handling
3. . low-level processor synchronisation
4. . recovery after a power failure

The Kernel is objected-oriented and all objects are either

• dispatcher objects (active or temporal things) or
• control objects (everything else)

Drivers are kind of Kernel-Mode services that need to run in privileged mode (such as device drivers) and use Kernel and Executive services. Also, user mode API libraries (NTDLL, Win32 API, ...) interface with Executive services to do their job.