Transcript Chapter 8: Operating System Support

Chapter 8: Operating System
Support
CS 147 Fall 2010
Lester Tam
What is an OS (Operating System)?
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Program that manages the computer’s
resources
Provides services for programmers
Schedules the execution of other programs
Types of OS
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Interactive or batch?
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Interactive: Direct communication
Batch: Queued programs
Multiprogramming or uniprogramming?
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Multiprogramming: Keep the OS as busy as
possible
Uniprogramming: One at a time
Scheduling
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Four types of scheduling
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Long-term scheduling
Medium-term scheduling
Short-term scheduling
I/O scheduling
Five State Model Process
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Used by the short-term scheduler
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New
Ready
Running
Waiting
Halted
Process Control Block
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Information kept by OS on every process
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Identifier
State
Priority
Program counter
Memory pointers
Context data
I/O status information
Accounting information
Memory Management
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Swapping
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Processes are swapped into Intermediate queue
Partitioning
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Fixed-sized partitions
Variable-size partitions
Compaction
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Even with dynamic partitioning, memory will
become fragmented
The OS will need to “defrag” the memory
occasionally
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Time consuming
Process may shift in main memory
Paging
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Suppose memory is divided into small equal
fixed-size chunks (frames)
Same for processes (pages)
Wasted space in memory a fraction of
variable partitioning
Virtual Memory
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Demand paging – brought in when needed
(on demand)
At any time, only a few pages are loaded
Time saved by not having to swap
However, can lead to thrashing
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Lead to development of complex algorithms to try
to prevent it.
Questions?