What is kernel-mode?
Contrary to what most developers believe (even kernel-mode developers) there is no mode of the x86 CPU called “Kernel-mode”. Other CPUs such as the Motorola 68000 have two processor modes “built into” the CPU, i.e. they have a flag in a status register that tells the CPU if it is currently executing in user-mode or supervisor-mode. Intel x86 CPUs do not have such a flag. Instead, it is the privilege level of the code segment that is currently executing that determines the privilege level of the executing program. Each code segment in an application that runs in protected mode on an x86 CPU is described by an 8 byte data structure called a Segment Descriptor. A segment descriptor contains (among other information) the start address of the code segment that is described by the descriptor, the length of the code segment and the privilege level that the code in the code segment will execute at. Code that executes in a code segment with a privilege level of 3 is said to run in user mode and
