2. NT Architecture
Applications and supporting subsystems run in User Mode that has no direct access to
hardware and is limited to assigned address space. The Kernel Mode is where the Windows
NT Executive runs and this provides access to all the memory and the hardware. Kernel
mode processes have a higher priority than the User Mode processes.
There are three components to the Windows NT Executive:
- Executive Services - managers and device drivers
- Microkernel - basic OS services, thread scheduling and interrupt handling.
- HAL - Hardware Abstraction Layer isolates (hides) hardware differences from NT allowing
it to work on different platforms.
The memory is a demand paged virtual memory system. Each application has a private virtual
memory space which is mapped to physical memory in 4Kb pages. Each virtual memory space
has room for 4Gb of addresses. The paging files are in Pagefile.sys and the process
of deciding which pages are in RAM and which are on the hard disk is called demand paging.
As a result, different memory architectures can be used with NT and many applications can
run at once.
A Domain is a collection of computers and users that have access to a central directory
database that contains user account and security information. NT Directory Services (NTDS) manage
a distributed directory database, validating user accounts and allowing resource access.
In a Workgroup the computer contains its own directory database and each computer
manages its own user accounts and resources.