EECE **** Embedded System Design

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EECE Embedded System Design
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Semaphores Control Tasks

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Semaphore Operations

• Semaphore a special variable that controls task
  execution. When waiting for a semaphore, tasks
  are blocked. The semaphore has a integer value.
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• (Atomic) Operations (logical code)

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Semaphore Example
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Circular Buffer
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Circular Buffer with Semaphores
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Problems with Semaphores

• Forgetting to take the semaphore
• Shared data bug
• Forgetting to release the semaphore
• Waiting task blocked forever
• Taking the wrong semaphore
• Unexpected behavior
• Holding the semaphore for too long
• Tasks may miss real-time deadlines

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Problems with Semaphores

• Deadlock (deadly embrace)

Scenario T1 P(sA) -gtT2P(sB)-gtT2P(sA) Blocks
-gt T1P(sB)Blocks