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Critical Sections
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* Critical sections are blocks of code that should not be interrupted. 
* They are protected by turning interrupts on and off. Big hammer because
  they disable all interrupts on the system.
* Quicker then semaphores.
* Okay to use for quick things such as setting globals, updating small
  sections.
* Use just around the important parts in a function.


Requirements for critical sections
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* Mutual exclusion - safe from interruption.
* Progress - no blocking.
* Bound waiting - quick.

Functions
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* OS_ENTER_CRITICAL() - called right before critical code.
* OS_EXIT_CRITICAL() - called rigth after critical code.