Interrupt Service Routine (ISR)

An Interrupt Service Routine (ISR) is a special type of subroutine or function that is invoked in response to an interrupt. When an interrupt occurs, the processor suspends its current execution and jumps to the ISR to handle the interrupt. The ISR performs the necessary tasks associated with the interrupt and then returns to the interrupted program.

Interrupts in an ISR

Interrupts can occur at any time during the execution of a program, including when the processor is executing an ISR. However, it is generally not desirable for an interrupt to occur while the ISR is still executing because it can lead to unexpected behavior and potentially corrupt the system state.

To prevent this, it is common practice to disable interrupts while executing an ISR. Disabling interrupts ensures that no new interrupts can occur until the ISR is completed, allowing the ISR to execute without any interruptions.

Disabling Interrupts

When an interrupt is triggered, the processor automatically disables interrupts before jumping to the ISR. This prevents any new interrupts from occurring while the ISR is executing. Disabling interrupts essentially means that the processor ignores any new interrupt requests until interrupts are enabled again.

Disabling interrupts is typically achieved by modifying the interrupt enable flag or register in the processor's control unit. This flag or register controls whether interrupts are allowed or not. By setting this flag to disable interrupts, the processor ensures that no new interrupts can occur.

Benefits of Disabling Interrupts

Disabling interrupts during an ISR provides several benefits:

1. Prevents nesting of interrupts: By disabling interrupts, the processor ensures that only one interrupt can be serviced at a time. This simplifies the handling of interrupts and prevents nested interrupts from causing unexpected behavior.

2. Ensures ISR completion: Disabling interrupts guarantees that the ISR can complete its execution without any interruptions. This helps maintain the integrity of the system state and prevents any potential conflicts or data corruption.

3. Priority handling: Disabling interrupts allows for proper handling of interrupt priorities. If multiple interrupts occur simultaneously, the processor can prioritize them based on their importance and handle them in the desired order.

Conclusion

In an interrupt service routine (ISR), interrupts are typically disabled to prevent new interrupts from occurring until the ISR completes its execution. This ensures that the ISR can execute without interruptions and allows for proper handling of the interrupt and maintenance of system integrity.