Dual-Mode Operation

• In order to ensure the proper execution of the OS, we must be able to distinguish between the execution of operating-system code and user-defined
• At the very least, we need two separate modes of operation: user mode and kernel mode (also called supervisor mode, system mode, or privileged mode). A bit, called the mode bit is added to the hardware of the computer to indicate the current mode: kernel (0) or user (1).
• User mode: a subset of instructions. Limited set of hardware and memory available.
  • I/O protection, all I/O operations are privileged; so user programs can only access I/O by sending a request to the (controlling) OS.
  • Memory protection, base/limit registers (in early systems), memory management unit, (MMU, in modern systems); so user programs can only access the memory that the OS has allocated.
  • CPU control, timer (alarm clock), context switch; so user programs can only read the time of day, and can only have as much CPU time as the OS allocates.
• When a user application requests a service from the OS (via a system call), it must transition from user to kernel mode to fulfil the request (see Fig. 2.25).

  Figure 2.25: Transition from user to kernel mode.

  
• The dual mode of operation provides us with the means for protecting the OS from errant users-and errant users from one another.
• If an attempt is made to execute a privileged instruction in user mode, the hardware does not execute the instruction but rather treats it as illegal and traps it to the OS.
• The life cycle of instruction execution in a computer system. Initial control is within the OS, where instructions are executed in kernel mode. When control is given to a user application, the mode is set to user mode. Eventually, control is switched back to the OS via an interrupt, a trap, or a system call.
• System calls provide the means for a user program to ask the OS to perform tasks reserved for the OS on the user program's behalf.
• A system call usually takes the form of a trap to a specific location in the interrupt vector.
• When a system call is executed, it is treated by the hardware as a software interrupt. Control passes through the interrupt vector to a service routine in the OS, and the mode bit is set to kernel mode.