Operating System

11.What is Dual-Mode Operation? 

The dual mode operation provides us with the means for protecting the operating system from wrong users and wrong users from one another. User mode and monitor mode are  the two modes. Monitor mode is also called supervisor mode, system mode or  privileged mode. Mode bit is attached to the hardware of the computer to indicate the  current mode. Mode bit is '0' formonitor mode and '1' for user mode. 

12.What are privileged instructions? 

Some of the machine instructions that may cause harm to a system are designated as  privileged instructions. The hardware allows the privileged instructions to be executed only in monitor mode. 

13.How can a user program disrupt the normal operations of a system? 

A user program may disrupt the normal operation of a system by  

• Issuing illegal I/O operations 

• By accessing memory locations within the OS itself 

• Refusing to relinquish the CPU 

14.How is the protection for memory provided? 

The protection against illegal memory access is done by using two registers. The base register and the limit register.The base register holds the smallest legal physical address; the limit register contains the size of the range. The base and limit registers can be loaded only by the OS using special privileged instructions. 

15.What are the various OS components? 

The various system components are 

• Process management 

• Main-memory management 

• File management 

• I/O-system management 

• Secondary-storage management 

• Networking 

• Protection system 

• Command-interpreter system 

16.What is a process? 

A process is a program in execution. It is the unit of work in a modern operating system. A process is an active entity with a program counter specifying the next instructions to execute and a set of associated resources. It also includes the process stack, containing temporary data and a data section containing global variables. 

17.What is a process state and mention the various states of a process? 

As a process executes, it changes state. The state of a process is defined in part by the current activity of that process. Each process may be in one of the following states: 

• New 

• Running 

• Waiting 

• Ready 

• Terminated 

18.What is process control block? 

Each process is represented in the operating system by a process control block also called a task control block. It contains many pieces of information associated with a specific process. It simply acts as a repository for any information that may vary from process to process. It contains the following information: 

• Process state 

• Program counter 

• CPU registers 

• CPU-scheduling information 

• Memory-management information 

• Accounting information 

• I/O status information 

19.What are the use of job queues, ready queues & device queues? 

As a process enters a system, they are put into a job queue. This queue consists of all jobs in the system. The processes that are residing in main memory and are ready & waiting to execute are kept on a list called ready queue. The list of processes waiting for a particular I/O device is kept in the device queue. 

20.What is meant by context switch? 

Switching the CPU to another process requires saving the state of the old process and loading the saved state for the new process. This task is known as context switch. The context of a process is represented in the PCB of a process.