Chapter 8: Operating System Support

Chapter 8: Operating System Support CS 147 Fall 2010 Lester Tam

What is an OS (Operating System)? • Program that manages the computer’s resources • Provides services for programmers • Schedules the execution of other programs

Types of OS • Interactive or batch? • Interactive: Direct communication • Batch: Queued programs • Multiprogramming or uniprogramming? • Multiprogramming: Keep the OS as busy as possible • Uniprogramming: One at a time

Scheduling • Four types of scheduling • Long-term scheduling • Medium-term scheduling • Short-term scheduling • I/O scheduling

Five State Model Process • Used by the short-term scheduler • New • Ready • Running • Waiting • Halted

Process Control Block • Information kept by OS on every process • Identifier • State • Priority • Program counter • Memory pointers • Context data • I/O status information • Accounting information

Memory Management • Swapping • Processes are swapped into Intermediate queue • Partitioning • Fixed-sized partitions • Variable-size partitions

Compaction • Even with dynamic partitioning, memory will become fragmented • The OS will need to “defrag” the memory occasionally • Time consuming • Process may shift in main memory

Paging • 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 • 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 • Lead to development of complex algorithms to try to prevent it.

Questions?

Chapter 8: Operating System Support