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Semaphores

Semaphores. -Gajendra Singh. SHARED DATA PROBLEM. ATOMIC or CRITICAL SECTION. 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.

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Semaphores

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  1. Semaphores -Gajendra Singh

  2. SHARED DATA PROBLEM

  3. ATOMIC or CRITICAL SECTION

  4. 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.

  5. 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.”

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

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

  8. Tasks And Data

  9. Shared data Problem with OS

  10. Have a Look:

  11. 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.

  12. Gray Areas:

  13. For 8051 For INTEL 80x86

  14. Semaphores

  15. Initializing Semaphores

  16. Advantages • Multiple Semaphores. • As signaling Device

  17. DeadLock

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