Semaphores

1. Semaphores -Gajendra Singh

2. SHARED DATA PROBLEM

4. ATOMIC or CRITICAL SECTION

8. Interrupt Latency • The longest period of time during which that interrupt is disabled • Task code1 :disable interrupt for 125uS. • Task code2 :disable interrupt for 250uS. • Task code3 :critical & need to respond within 625us. • Interprocessor interrupt routine takes 300uS to execute.

9. Points to be noted • Lets cut the cost and replace uP that runs half as fast. • Similarly adding an extra task might also be awful. • Conclusion: Disabling Interrupts “Not a very good idea.”

10. Architectures • Round Robin • Round Robin with Interrupts • Function Queue Scheduling • RTOS

11. ISR_A; ISR_B; : : ISR_Z main(){ while(1){ if(A){ // do something} if(B){// do something} : : if(Z){// do something} } }

12. Tasks And Data

13. Shared data Problem with OS

14. Have a Look:

15. Reentrancy • Reentrant Functions: can be called by more than one task and still work correctly even if RTOS switches from one task to another in middle of executing one function.

16. Gray Areas:

17. For 8051 For INTEL 80x86

18. Semaphores

19. Initializing Semaphores

21. Advantages • Multiple Semaphores. • As signaling Device

23. DeadLock