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Operating sistem

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operating system

operating system

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  • 1. Multi-programming, Time-sharing & Real-time systems• Under the supervisor control• Input/output organisation• Interrupt handling : o External interrupts (generally disconnected from current process) I/O interrupt (channel interrupt) Operator/user interrupt Clock interrupt (alarm clock, …) Processor interrupt Dis-functioning interrupt o Internal interrupts (directly dependent from the current process) Supervisor call Program errors Trapping or extra-code (micro-coding) First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 1
  • 2. A simple mechanism• The interrupt handling task is chosen by the supervisor• No interrupt during the current interrupt handling• Interrupts can be masked (to avoid uncontrolled sequence breaking) User process Interrupt routines Supervisor Process Masked Interrupt calling 1st type interrupt event analysis interruption Interrupt end end of nd Unmasked handling 2 type event interruption 3rd type Interrupts Interrupts interruption enabled disabled First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 2
  • 3. ImplementationRegisters :• General interrupt mask (MM = 1 : enabled; MM = 0 disabled) MM 1• Interrupt requests : array of bits IR 1 0 1 0 0 0 0 0 0 1 1 1 0 1 0 0• Interrupt masks : array of bits MS 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1• Supervisor entry point for interrupt handling ES: 32 bits• PC saving memory zone PS: 32 bitsInterrupts Interruptsenabled ? masked ? yes n yes MM MM=1 ? ∑IRi .MSi ≠ 0? i=1 0; PS PC PC ES no no I [PC] • ES register can be replaced by an array • Each entry pointer to a specific service routine Execute I • Working registers saving (stack) ! First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 3
  • 4. Multiprogramming Variation in speed of various modules of a computer dead times Task B : timing ≡ timing task A; independent I/O channels Task A input 1 2 3 4 5 channel 1Processing 1 2 3 4 5outputchannel 2 1 2 3 4 Task B input 1 2 3 4 5 channel 3Processing 1 2 3 4 5outputchannel 4 1 2 3 4 1 2 3 4 5 6 7 8 9 10 time ideal configuration !!! First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 4
  • 5. Processes states • Concurrent processing : 3 states recognised by the scheduler • Working state : task using the control unit (computational state) • Waiting state : task waiting for an external resource • Ready state : task waiting for the control unit (main resource) • Scheduler serving the first ”ready” task • Priority system can be implemented system idleTask A waiting ready waiting ready readyTask B waiting ready waiting ready waiting readyTask C ready waiting ready waiting 1 2 3 4 5 6 7 8 9 10 time First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 5
  • 6. Interrupt use • Task entering waiting state control given to supervisor : o Special interrupt : Supervisor call o System records the transition state (working waiting) o System records the restart condition (resource availability) o System chooses a task ready to work (priorities !) o Interrupt handling o State transition handling (scheduling work : next task to run) switching time = supervision cost As small as possible ! Task A waiting ready Task B ready waiting Task C ready ready Chan. 1 ready readyinterrupt from task A from chan. 1 1 2 3 4 5 6 7 8 9 10 First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 6time
  • 7. Job classes and prioritiesTwo main categories of processes :• CP-bound : computation in central memory; few I/O (number crunching)• IO-bound : little computation; many I/O operations• If only CP-bound processes : processor idle time very small no gain (even losses !) with multi-programming• If only IO-bound processes : efficiency depends on synchronisation Mixture of CP-bound and I/O bound processesTask A I/O ready ready ready well-balanced Task A waiting waiting system Task B ready ready ready (use of clock inter.)Task A I/O ready ready Unbalanced Task A waiting ready system Task B ready (optimised for proc. 1 2 3 4 5 6 7 not for periph.) First Year University Studies in Science. ULB . Computer Principles. Chapter 12 time D. Bertrand 7
  • 8. Spooling• Sharing of ”sequential” peripherals (printers, magnetic tapes) difficult• Not a problem for random access devices (different areas)To solve the problem : Deferral output system I/O device spooling• Output sent to a secondary storage device (magnetic disk, …)• At the end of the task output put into an output queue• When the device is ready : get one of the files of the output queue (special task : the output symbiont) Remarks• Symbiont : part of system software but handled as a user process• Queues can be built for different classes of peripherals (staging …)• A class of devices can be handled by the same or different symbionts• Same system can be applied to input queues• Priorities can be defined at the level of the queues First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 8
  • 9. Input Outputdevice Job evolution device Process Output Input scheduler symbiont working Process Output scheduler ready symbiont Job scheduler Termination ResourcesInput scheduler Outputqueue Request queue waiting The operating system manages this evolution : tables (status of the system and of its components) o queue content o processes status o memory/peripherals occupation choices algorithms (scheduling) resource management (allocation) First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 9
  • 10. Time sharing Multi-programmation resource usage optimisation But … Response time ? Response time = reading time + waiting time in input queue + execution time + waiting time in output queue + output timeexecution time = Σ working times + Σ waiting times + Σ ready timesReduction of processor idle time may be inefficient for response time !Example :• Two CP-bound tasks• Different execution times (60 minutes ↔ 1 minute)• Tasks are submitted at different times First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 10
  • 11. 1st case : Task A Task B ready 0 11 60 61 time• Looks like mono-programmation !• But control unit is quite efficiently used• User A happy !• User B must wait 50 minutes for the execution of one minute job !2nd case : Task A Task B 0 11 12 61 time• Less efficient (more switching time)• User B happy (response time reduced by 50)• User A still happy (response time increased by 1.5 %)• Average response time : 31 minutes (55 minutes in the first case) First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 11
  • 12. Remarks• Many short processes delay for medium and long processes• Execution time is not always a priori known (errors, convergences, …) Other technique :• Put a working process in waiting state after a given time slice• Activate a ready process• Continue with another process … clock interrupt Time-sharing :Task A ready readyTask B ready readyTask C ready ready 1 2 3 4 5 6 7 8 9 10 time• All processes systematically put to working state (quasi-parallelism)• Short processes quickly finished• Medium and long processes not indefinitely waiting First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 12
  • 13. Comparison Multi-programmation Simultaneous management Time-sharing of several processes Multi-management systems Main aim Method Consequence optimisation of Management ofMulti-programmation quasi-parallelism control unit usage several processes Response time Management of better use of Time-sharing optimisation (users) several processes active resources Some operating systems based on a compromise First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 13
  • 14. Interactive systems• Task directives given interactively by the user• Each user has its own access units (terminal : keyboard + screen)• The user units do not need to be driven by a symbiont• Commands generate interrupts and tasks state transition Average response time = n . t n : number of users; t : average execution time of a command• In general system in waiting state• Each user get the impression to have his own computer First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 14
  • 15. Real time systems Time-sharing systems with strict timing constraintsSystems requiring minimum response time :• Process control in industry• Probe control in space• Electronic assistance systems in cars• Flying systems in planes• Data acquisition systems (physics, chemistry, …)Systems requiring fast but not critical response time :• Reservation systems (plane, trains, …)• Automatic banking systems, … First Year University Studies in Science. ULB . Computer Principles. Chapter 12 D. Bertrand 15