ENVS 1800 Lecture Notes - Lecture 17: Process Control Block, Program Counter, Context Switch
14 May 2018
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ENVS 1800 Lecture 17 Notes – Dispatching
Introduction
• The high-level scheduler will refuse admission to the system if there is no place to put
the program in memory or if other resources are unattainable.
• If the request is a user login request, the user will have to wait until later to try again.
• Otherwise, requests are usually accepted, even though it may slow down the system.
• You may have experienced such slowdowns when working with Windows.
• You may have eve gotte a out-of-eory essage if you tried to do too ay
things at once!
• For batch processes, the high-level scheduler has a much more important role.
• Since most modern systems are predominately interactive, the use of batch processes is
generally limited to processes with demanding resource requirements
• For example, a monthly billing program for a large utility or department store chain
• An economics problem with huge amounts of data and complex calculations to be
performed on the data
• Processes of this type can make it difficult for regular users to get their work done if the
process is executed during a busy time of day.
• With batch processes, a delay in processing is usually acceptable to the user
• Therefore, the high-level scheduler has more flexibility in deciding when to admit the
process to the system.
• The high-level scheduler can use its power to balance system resource use as an
attempt to maximize the efficiency of the system and minimize disruption to the regular
users.
• Conceptually, the dispatching process is simple.
• Whenever a process or thread gives up the CPU, the dispatcher selects another
candidate that is ready to run, performs a context switch
• Sets the program counter to the program counter value stored in the process control
block to start execution
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