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94.571 (ELG 6171)
Monday March 27, 2000
Prof. T. W. Pearce
Take resource away, use it for something else, then give it back. (e.g. processor or I/O channel)
Once give, it can’t be reused until process gives it back. (e.g. file space or terminal)
Perform lots of IO operations.
IO burst ---- short CPU burst to process IO --- IO burst
Perform lots of computation and do little IO
CPU burst ----------- Small IO burst ----------- CPU burstScheduling AlgorithmsProcesses and Resources
The underlying interrupt system basically readies the task for a switch, but does not perform the switch
Process switches are directly handled by the scheduler
This causes a delay from the time of readiness to the time of the switch, which is not tolerated for, let’s say, system exceptions
The solution is to completely by-pass the scheduler (OS) and go directly to an ISR. 
Each process is allocated its own private stack and workspace
This is done to avoid different processes overwriting each other’s data and code
This is based on a strict process model, where all heavyweight processes do not share resources
Code that can be shared safely is called ‘re-entrant’ code Scheduling AlgorithmsImplementation - Issues
1)Cooling, J.E. Software Design for Real-Time Systems. Chapman & Hall, London, UK: 1995.
2) Stallings, William. Operating Systems: Internals and Design Principles. Upper Saddle River, NJ: Prentice Hall, 1998.
3) http://www.cs.wisc.edu/~bart/537/lecturenotes/s11.html - viewed on 03/24/2000
4) Savitzky, Stephen. Real-Time Microprocessor Systems. Van Nostrand Reinhold Company, N.Y.: 1985.
5) Undergraduate Operating System Course Notes (Ottawa University, 1998)