<P> Exiting an interrupt handler with the interrupt system in exactly the right state under every eventuality can sometimes be an arduous and exacting task, and its mishandling is the source of many serious bugs, of the kind that halt the system completely . These bugs are sometimes intermittent, with the mishandled edge case not occurring for weeks or months of continuous operation . Formal validation of interrupt handlers is tremendously difficult, while testing typically identifies only the most frequent failure modes, thus subtle, intermittent bugs in interrupt handlers often ship to end customers . </P> <P> In a modern operating system, upon entry the execution context of a hardware interrupt handler is subtle . </P> <P> For reasons of performance, the handler will typically be initiated in the memory and execution context of the running process, to which it has no special connection (the interrupt is essentially usurping the running context--process time accounting will often accrue time spent handling interrupts to the interrupted process). However, unlike the interrupted process, the interrupt is usually elevated by a hard - coded CPU mechanism to a privilege level high enough to access hardware resources directly . </P> <P> In a low - level microcontroller, the chip might lack protection modes and have no memory management unit (MMU). In these chips, the execution context of an interrupt handler will be essentially the same as the interrupted program, which typically runs on a small stack of fixed size (memory resources have traditionally been extremely scant at the low end). Nested interrupts are often provided, which exacerbates stack usage . A primary constraint on the interrupt handler in this programming endeavour is to not exceed the available stack in the worst - case condition, requiring the programmer to reason globally about the stack space requirement of every implemented interrupt handler and application task . </P>

What is interrupt service routine in operating system