1. Real-Time Operating Systems Suzanne Rivoire November 20, 2002 http://www.stanford.edu/~skrufi/rtospres.ppt

2. Motivating Example void main() { do forever{ check keypad; measure temperature; control oven power; decrement timer; update display; wait for clock tick; }

3. Motivating Example - 2 void main() { do forever{ check keypad; measure temperature; check keypad; control oven; check keypad; }

4. Presentation Outline Definition of real-time Characteristics of RTOS’s Examples –  c-OS – AvrX – RTLinux – QNX Neutrino

5. What is real-time? Correctness of output depends on timing as well as result Hard vs. soft real-time Are Windows and Linux real-time?

6. In a Hard RTOS… Thread priorities can be set by the client Threads always run according to priority Kernel must be preemptible or bounded Interrupts must be bounded No virtual memory

7. In a Soft RTOS… Like a hard RTOS: – Priority scheduling, with no degradation – Low dispatch latency – Preemptible system calls – No virtual memory (or allow pages to be locked) Linux: guarantees about relative timing of tasks, no guarantees about syscalls

8. Basic RTOS References http://www.dedicated- systems.com/encyc/publications/faq/rtfaq.htm http://www.steroidmicros.com/mtkernel.html http://www.qnx.com/developer/articles/dec1200b/ Silberschatz and Galvin, Operating System Concepts.

9. Example RTOS’s Micrium  c-OS II http://www.ucos-ii.com/ AvrX http://www.barello.net/avrx/ RTLinux http://fsmlabs.com/developers/man_pages/ QNX Neutrino http://www.qnx.com/

10.  c-OS II Features Sample main() function Calling OSTaskCreate() Creating a task

11.  C-OS II Ports to the AVR, ATmega103 Comes with book: Micro-C OS: The Real-Time Kernel by Jean Labrosse ($52) Features – Semaphores and mutexes – Event flags – Message mailboxes and queues – Task management (priority settings)

12. void main (void) { /* Perform Initializations */... OSInit();... /* Create at least one task by calling OSTaskCreate() */ OSStart(); }  C-OS Sample Code

13. INT8U OSTaskCreate ( void (*task)(void *pd), void *pdata, OS_STK *ptos, INT8U prio);  C-OS Sample Code

14. void UserTask (void *pdata) { pdata = pdata; /* User task initialization */ while (1) { /* User code goes here */ /* You MUST invoke a service provided by µC/OS-II to: */ /*... a) Delay the task for ‘n’ ticks */ /*... b) Wait on a semaphore */ /*... c) Wait for a message from a task or an ISR */ /*... d) Suspend execution of this task */ }}  C-OS Sample Code

15. AvrX Specs Internal structures Sample code: task creation

16. AvrX Specs Code size: 500-700 words (2x bigger with debug monitor) 16 priority levels Overhead: 20% of CPU Version 2.3 for assembly code, 2.6 for C interface

17. AvrX Internals Routine _Prolog saves state, _Epilog restores it Task info stored in a PID block and a task control block Modules for timers, semaphores, etc.

18. An AvrX Task AVRX_TASKDEF(myTask, 10, 3){ TimerControlBlock MyTimer; while (1) { AvrXDelay(&MyTimer, 10); // 10ms delay AvrXSetSemaphore(&Timeout) ; }} Arguments: task name, additional stack bytes, priority

19. RTLinux Additional layer between Linux kernel and hardware Worst-case dispatch latency on x86: 15  s

20. RTLinux: Basic Idea

21. QNX Neutrino Powerful hard RTOS Includes GUI Memory protection and fault tolerance

22. Summary Hard real-time, soft real-time Characteristics of an RTOS Example RTOS’s – Specs – Internal structure/ideas – Sample code