Slides created by: Professor Ian G. Harris Operating Systems  Allow the processor to perform several tasks at virtually the same time Ex. Web Controlled Car with a camera Car is controlled via the internet Car has its own webserver ( Web interface allows user to control car and see camera images Car also has “auto brake” feature to avoid collisions Fwd Back LeftRight Web interface view

Slides created by: Professor Ian G. Harris Multiple Tasks  Assume that one microcontroller is being used  At least four different tasks must be performed 1.Send video data - This is continuous while a user is connected 2.Service motion buttons - Whenever button is pressed, may last seconds 3.Detect obstacles - This is continuous at all times 4.Auto brake - Whenever obstacle is detected, may last seconds  Detect and Auto brake cannot occur together  3 tasks may need to occur concurrently

Slides created by: Professor Ian G. Harris Prioritized Task Scheduling  Sending Video Data and Detecting Obstacles must happen concurrently Both tasks never complete  Servicing Motion Buttons must be concurrent with Sending Video Data Video should not stop when car moves  CPU must switch between tasks quickly  Some tasks must take priority Auto Brake must have highest priority

Slides created by: Professor Ian G. Harris Sharing Global Resources  Global resources may be required by mulitple tasks ADC, comparators, timers, I/O pins  Shared access must be controlled to avoid interference Ex. Task 1 and Task 2 need to use the ADC They cannot use the ADC at the same time One task must wait for the other  Operating system guarantees that resource conflicts are resolved

Slides created by: Professor Ian G. Harris Layered OS Architecture Microconrtoller Application Library Functions System Calls Application Microconrtoller  OS provides an abstraction to hide details of hardware Ex. delay(int) library function might setup a timer-based interrupt  Using Library functions incurrs overhead

Slides created by: Professor Ian G. Harris Processes vs. Threads  Context of a task is its register values, program counter, and stack  All tasks have their own context  Context switch is when on task stops and the next starts - Must save the old context and load the new - This is time consuming  OS typically gives tasks access to memory (i.e malloc )  Processes each have their own private memory - Requires memory protection  Threads share memory  RTOS usually implement tasks as threads

Slides created by: Professor Ian G. Harris Memory Management  Programs can request memory dynamically with malloc(); int valarr[10]; int *valarr; valarr = (int *) malloc(10 * sizeof(int));  Dynamically allocated memory must be explicitly released - statically allocated memory is released on function return free(valarr);  Dynamic memory allocation is flexible but harder to deal with - Must free the memory manually - Cannot access freed memory

Slides created by: Professor Ian G. Harris OS Memory Management  A program cannot know the dynamic memory allocation - Which memory locations are used and which are available?  Operating system keeps tables describing which memory locations are available  The program must request memory from the OS - OS may deny request if there is no memory available  OS also protects memory - Enforce memory access permissions

Slides created by: Professor Ian G. Harris Scheduler  OS manages the execution state of each task 3 main states 1. Running – The task is currently running 2. Ready – The task is not running but it is ready to run 3. Blocked – The task is not ready because it is waiting for an event  Only one task can be running at a time  A task can only run if it is first ready (not blocked)  Scheduler must keep track of the state of each task  Scheduler must decide which ready task should run

Slides created by: Professor Ian G. Harris Preemption  A non-preemptive scheduler allows a task to run until it gives up control of the CPU - Task may call a library function (sleep) to quit - Needs to be awakened by an event, like an interrupt - Not much flexibility for OS to meet deadlines  A preemptive scheduler allows the OS to stop a running task and start another task - OS has the power to influence the completion of tasks - OS must be awakened periodically to make scheduling decisions - May implement the OS kernel as a high priority timer-based interrupt

Slides created by: Professor Ian G. Harris Scheduling Algorithms Round-Robin:  Scheduler keeps an ordered list of ready tasks  First task is assigned a fixed-size time slice to execute  After time slice is done, task is placed at the end of the list and next task executes for its time slice  Very simple, no priorities Context switch time Task execution Task 1 Task 2

Slides created by: Professor Ian G. Harris Prioritized Scheduling Fixed Priority Preemptive:  Scheduler keeps an ordered list of ready tasks, ordered by priority  First task is assigned a fixed-size time slice to execute  After time slice is done, scheduler chooses highest priority ready task for next time slice  Next task might be the same as the previous task, if it is high priority Low priority High Priority  Starvation may occur

Slides created by: Professor Ian G. Harris Atomic Updates  Tasks may need to share global data and resources  For some data, updates must be performed together to make sense Ex. Our system samples the level of water in a tank tank_level is level of water time_updated is last update time tank_level = // Result of computation time_updated = // Current time  These updates must occur together for the data to be consistent  Interrupt could see new tank_level with old time_updated

Slides created by: Professor Ian G. Harris Mutual Exclusion  While one task updates the shared variables, another task cannot read them tank_level = ?; time_updated = ?; printf (“%i %i”, tank_level, time_updated); Task 1Task 2  Two code segments should be mutually exclusive  If Task 2 is an interrupt, it must be disabled

Slides created by: Professor Ian G. Harris Semaphores  A semaphore is a flag which indicates that execution is safe  May be implemented as a binary variable, 1 continue, 0 wait TakeSemaphore() : If semaphore is available (1) then take it (set to 0) and continue If semaphore is note available (0) then block until it is available ReleaseSemaphore(): Set semaphore to 1 so that another task can take it  Only one task can have a semaphore at one time

Slides created by: Professor Ian G. Harris Critical Regions TakeSemaphore(); tank_level = ?; time_updated = ?; ReleaseSemaphore(); TakeSemaphore(); printf (“%i %i”, tank_level, time_updated); ReleaseSemaphore(); Task 1 Task 2  Semaphores are used to protect critical regions  Two critical regions sharing a semaphore are mutually exclusive  Each critical region is atomic, cannot be separated