Kernel Mode
Windows NT kernel mode has full access to the hardware and system resources of the computer and runs code in a protected memory area. It controls access to scheduling, thread prioritization, memory management and the interaction with hardware. The kernel mode stops user mode services and applications from accessing critical areas of the operating system that they should not have access to; user mode processes must ask the kernel mode to perform such operations on their behalf.
While the x86 architecture supports four different privilege levels (numbered 0 to 3), only the two extreme privilege levels are used. Usermode programs are run with CPL 3, and the kernel runs with CPL 0. These two levels are often referred to as "ring 3" and "ring 0", respectively. Such a design decision had been done to achieve code portability to RISC platforms that only support two privilege levels, though this breaks compatibility with OS/2 applications that contain I/O privilege segments that attempt to directly access hardware.
Kernel mode consists of executive services, which is itself made up of many modules that do specific tasks, kernel drivers, a kernel and a Hardware Abstraction Layer, or HAL.
Read more about this topic: Windows NT Architecture
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