Interrupt Service Routine

While accessing a critical section kernel may defer interrupt servicing till the end of critical section (because the ISR may also access the same data structures accessed by the critical section). This methodology may not be responsive enough in a real time systems. So rtker employs a two stage interrupt handling strategy-

• Immediate ISR
  This is the function to which the kernel vectors to in case of an interrupt. The Immediate ISR runs with interrupts disabled. It should not call any kernel library function call since these calls may access kernel data structures.
• Bottom Half ISR
  This is the secondary ISR, which may be activated from the Immediate ISR. This function is scheduled in a thread context by a special thread called the bottom half thread. Since it runs in the context of a different thread it is free to make any kernel library call.