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Friday, May 3, 2013

What is a Dispatcher?

A number of types of schedulers are available that suit the different needs of different operating systems. Presently, there are three categories of the schedulers:
  1. Long-term schedulers
  2. Medium-term schedulers
  3. Short-term schedulers
Apart from the schedulers there is one more component involved in the scheduling process and is known as the dispatcher. 
- It is the dispatcher that gives a process power to control the CPU. 
- To which process this control is to be given is selected by the short-term scheduler. 
- This whole process involves the following three steps:
  1. Switching the context
  2. Turning on the user code
  3. Making a jump to the exact location of the program from where it has to be restarted.
- Values taken from the program counter are analyzed by the dispatcher and accordingly it fetches instructions and feeds data in to the registers. 
- The dispatcher unlike the other system components needs to be very quick since it is invoked during all the switches that occur. 
- Whenever a context switch is invoked, the processor gets in to an idle state for a very small period of time. 
- Hence, it is required that the context switches that are not necessary might be avoided. 
- The dispatcher takes some time for stopping one process and start running the other one. 
- The dispatch latency is what we call this time.

- Scheduling and dispatch are complex processes and interrelation to each other. 
- These two are very much essential for the operation of the operating system. 
Today, architectural extensions are available for the modern processors that provide a number of banks of registers.
- Swapping of these registers in hardware is possible and therefore a certain number of tasks are capable of retaining their full registers. 
- Whenever an interrupt triggers the dispatcher, it sends to it the full set of the registers belonging to the process that was being executed at the time of occurrence of the interrupt. 
- Here, the program counter is not considered. 
- Therefore, it is important that the dispatcher should be written carefully for storing the present states of the registers on being triggered. 
- In other words, we can say that for the dispatcher itself, there is no immediate context. 
- This saves it from the same problem. 

Process of Dispatcher

Below we try to describe in simple words what actually the process is.
  1. The program presently having the context is executed by the processor. Things used by this program include stack base, flags, program counter, and registers and so on. There is a possible exception of the reserved register that is native to the operating system. The executing program does not have any knowledge regarding the dispatcher.
  2. For dispatcher a timed interrupt is triggered. Here the program counter jumps to the address listed in the BIOS interrupt. This marks the beginning of the execution of the dispatch sub routine. The dispatcher then deals with the stacks and the registers etc. of the program that raised the interrupt.
  3. Dispatcher like the other programs consists of some sets of instructions that operate up on the register of the current program. These instructions know everything of the previously executed programs. Out of these, the first few instructions are responsible for storing the state of the program.
  4. Dispatcher next determines which program should be given the CPU next for executing. Now it deletes all the statistics of the previously executed state and fills in the details of the next process to be executed.
  5. Dispatcher jumps to the address mentioned in the program counter and establishes a full context on the processor.
- Actually dispatcher does not really require registers since its only work is to write the current state of the CPU in to a memory location that has been predetermined. 
- It then loads in to the CPU another process from other predetermined location. 

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