What makes an OS a RTOS ? Embedded C interview questions

Embedded C interview questions

What makes an OS a RTOS ?


  • A RTOS (Real-Time Operating System) has to be multi-threaded and preemptible.
  • The notion of thread priority has to exist, as there is for the moment no deadline driven OS. 
  • The OS has to support predictable thread synchronization mechanisms 
  • A system of priority inheritance has to exist 
  • OS Behavior should be known 

so the following figures should be clearly given by the RTOS manufacturer:

  • The interrupt latency (i.e. time from interrupt to task run): this has to be compatible with application requirements and has to be predictable. This value depends on the number of simultaneous pending interrupts.
  • For every system call, the maximum time it takes. It should be predictable and independent from the number of objects in the system; 
  • The maximum time the OS and drivers mask the interrupts. 

The developer should also know the following points:

  • System Interrupt Levels. 
  • Device driver IRQ Levels, maximum time they take, etc

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