What is an Interrupt?
It is referred to as an input signal that has the highest priority for hardware or software events that requires immediate processing of an event. During the early days of computing, the processor had to wait for the signal to process any events. The processor should check every hardware and software program to understand if there is any signal to be processed. This method would consume a number of clock cycles and makes the processor busy. Just in case, if any signal was generated, the processor would again take some time to process the event, leading to poor system performance.
A new mechanism was introduced to overcome this complicated process. In this mechanism, hardware or software will send the signal to a processor, rather than a processor checking for any signal from hardware or software. The signal alerts the processor with the highest priority and suspends the current activities by saving its present state and function, and processes the interrupt immediately, this is known as ISR. As it doesn’t last long, the processor restarts normal activities as soon as it is processed.
Types of Interrupt
These are classified into two main types.
Hardware Interrupts
An electronic signal sent from an external device or hardware to communicate with the processor indicating that it requires immediate attention. For example, strokes from a keyboard or an action from a mouse invoke hardware interrupts causing the CPU to read and process it. So it arrives asynchronously and during any point of time while executing an instruction.
Hardware interrupts are classified into two types
- Maskable Interrupts – Processors have to interrupt mask register that allows enabling and disabling of hardware interrupts. Every signal has a bit placed in the mask register. If this bit is set, an interrupt is enabled & disabled when a bit is not set, or vice versa. Signals that interrupt the processors through these masks are referred to as masked interrupts.
- Non-maskable Interrupts (NMI) – The NMIs are the highest priority activities that need to be processed immediately and under any situation, such as a timeout signal generated from a watchdog timer.
Software Interrupts
The processor itself requests a software interrupt after executing certain instructions or if particular conditions are met. These can be a specific instruction that triggers an interrupt such as subroutine calls and can be triggered unexpectedly because of program execution errors, known as exceptions or traps.
Triggering Methods
Generally, these signals are designed to trigger using either a logic signal level or a signal edge. These methods are of two types.
Level-triggered Interrupt
In this type, the input module invokes an interrupt if the service level of this is asserted. If an interrupt source continues to be asserted when the firmware interrupt handler handles it, this module regenerates and triggers the handler to invoke again. The level-triggered inputs are not good if remains asserted for a longer duration.
Edge-triggered Interrupt- An edge-triggered interrupt input module invokes an interrupt as soon as it identifies an asserting edge – a falling or a rising edge. The edge becomes noticed when the level of source changes. This type of triggering needs immediate action, irrespective of the activity of the source.
