Operating System Mode:
Operating system must provide multiple modes of execution that are as follows:
Kernel Mode, Supervisor Mode or System Mode.
User Mode.
A special bit is used to indicate the execution mode of the system. Some restricted activities can only be executed in kernel mode. Such activities include the execution of certain priviledged instructions, accessing certain registers and accessing I/O devices etc. If these instructions are executed in user mode, a trap is generated.
A system enters kernel mode in the following three ways:
- A special instruction called supervisor call or system call can be used to set the system in kernel mode.
- A trap can be used to set the system in kernel mode.
- An interrupt can be used to set the system in kernel mode.
