Difference between ISR and Function Call?

Question

I want to understand difference between ISR (Interrupt Service Routine) and Function call.

I feel both the function call and ISR are the same from the hardware perspecti

Answer 1

The main difference is that interrupt handlers are, (usually), invoked by peripheral hardware - an actual hardware signal is generated by the peripheral and hardware in the processor transfers control to the appropriate handler without any action by the code that was running before the interrupt. Unlike functions, there is no call - execution is ripped away from the interrupted code by the processor hardware.

On OS that support multithreading/processes, function calls take place witin the same process/thread context as the caller. An interrupt, OTOH, has no thread or process context - a network interrupt resulting from a background BitTorrent download may occur while you are editing a Word document, and so the handler is very restricted in what it can do. It can load data to/from pre-allocated buffers belonging to the process/thread that it is bound to, it can signal a semaphore, it may be able to set OS event flags. That's about it.

Often, an interrupt-handler performs an interrupt-return directly, so allowing execution of the interrupted code to proceed wtihout any further interference. On simpler controllers, like yopur 8051, that often run embedded code without a compex OS, this is the only course available. With a preemptive multithreaded OS, an interrupt-handler has the additional option of performing its interrupt-return via OS code and so causing a scheduler run. This allows interrupt-handlers to make threads that were waiting for the interrupt ready, and possibly running, (and so maybe preempting the thread that was originally interrupted). This allows such systems to have good I/O performance without any polling.

The hardware interrupt sources my be peripherals embedded in the processor chip - network controllers, disk controllers, display controllers, DMA controllers, USB controllers, intercore-comms controllers, (on processors with multiple cores), timers etc. or interrupt-request pin/s on the package can be used to generate an interrupt from an external hardware source, (maybe a pushbutton, keyboard, keypad or touchscreen hardware).