21-02-2011, 11:19 AM
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Operating System Support
Objectives and Functions
• Convenience
— Making the computer easier to use
• Efficiency
— Allowing better use of computer resources
Layers and Views of a Computer System
Operating System Services
• Program creation
• Program execution
• Access to I/O devices
• Controlled access to files
• System access
• Error detection and response
• Accounting
O/S as a Resource Manager
Types of Operating System
• Interactive
• Batch
• Single program (Uni-programming)
• Multi-programming (Multi-tasking)
Early Systems
• Late 1940s to mid 1950s
• No Operating System
• Programs interact directly with hardware
• Two main problems:
— Scheduling
— Setup time
Simple Batch Systems
• Resident Monitor program
• Users submit jobs to operator
• Operator batches jobs
• Monitor controls sequence of events to process batch
• When one job is finished, control returns to Monitor which reads next job
• Monitor handles scheduling
Memory Layout for Resident Monitor
Job Control Language
• Instructions to Monitor
• Usually denoted by $
• e.g.
— $JOB
— $FTN
— ... Some Fortran instructions
— $LOAD
— $RUN
— ... Some data
— $END
Other Desirable Hardware Features
• Memory protection
— To protect the Monitor
• Timer
— To prevent a job monopolizing the system
• Privileged instructions
— Only executed by Monitor
— e.g. I/O
• Interrupts
— Allows for relinquishing and regaining control
Multi-programmed Batch Systems
• I/O devices very slow
When one program is waiting for I/O, another can use the CPU
