PROCESS STATE | Process Life Cycle | Context Switching | Mode switching

PROCESS STATE

Processes can be any of the following states :

• New - The process is being created.
• Ready - The process is waiting to be assigned to a processor.
• Running - Instructions are being executed.
• Waiting - The process is waiting for some event to occur(such as an I/O completion or reception of a signal).
• Terminated - The process has finished execution.

Context Switching
The process of saving the context of one process and loading the context of other process is known as Context Switching. In simple term, it is like loading and unloading of the process from running state to ready state.

When does Context switching happen?
1. When a high priority process comes to ready state, compared to priority of running process
2. Interrupt Occurs
3. User and Kernel mode switch: (It is not necessary though)
4. Preemptive CPU scheduling used.

The steps in a full process switch are:

1. Save the context of the processor, including program counter and other registers.

2. Update the process control block of the process that is currently in the Running state. This includes changing the state of the process to one of the other states (Ready; Blocked; Ready/Suspend; or Exit). Other relevant fields must also be updated, including the reason for leaving the Running state and accounting information.

3. Move the process control block of this process to the appropriate queue(Ready; Blocked on Event i ; Ready/Suspend).

4. Select another process for execution.
5. Update the process control block of the process selected. This includes changing the state of this process to Running.

6. Update memory management data structures. This may be required, depending on how address translation is managed.

7. Restore the context of the processor to that which existed at the time the selected process was last switched out of the Running state, by loading in the previous values of the program counter and other registers.

Mode switching

A user process undergoes a mode switch when it needs access to system resources. This is implemented through the system call interface or by interrupts such as page faults.

There are two modes:

• User mode
• Kernel mode

Processor time spent in user mode (application and shared libraries) is reflected as user time in the output of commands such as the vmstat, iostat, and sar commands. Processor time spent in kernel mode is reflected as system time in the output of these commands.

• User mode
  
  Programs that execute in the user protection domain are user processes.
• Kernel mode
  
  Programs that execute in the kernel protection domain include interrupt handlers, kernel processes, the base kernel, and kernel extensions (device driver, system calls and file systems).
• Mode switches
  
  The use of a system call by a user-mode process allows a kernel function to be called from user mode. Access to functions that directly or indirectly invoke system calls is typically provided by programming libraries, which provide access to operating system functions.

CPU Bound vs I/O Bound Processes:
A CPU Bound Process requires more amount of CPU time or spends more time in the running state.
I/O Bound Process requires more amount of I/O time and less CPU time. I/O Bound process more time in the waiting state.

References:
http://www.cs.uic.edu/~jbell/CourseNotes/OperatingSystems/3_Processes.html
http://cs.nyu.edu/courses/spring11/G22.2250-001/lectures/lecture-04.html
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Question:-

The time taken to switch between user and kernel modes of execution be t1 while the time taken to switch between two processes be t2. Which of the following is TRUE?
(A) t1 > t2
(B) t1 = t2
(C) t1 < t2
(D) nothing can be said about the relation between t1 and t2.

Answer: (C)
Explanation: Process switching involves mode switch. Context switching can occur only in kernel mode.