2 4.2 ISR CONCEPT

Interrupt means event, which invites the attention of processor for some action on the hardware or software

event.

1. When a device or port is ready, a device or port generates an interrupt or when it completes the
assigned action, it generates an interrupt. This interrupt is called hardware interrupt.

2. When software run-time exception condition is detected, either processor hardware or a software
instruction generates an interrupt. This interrupt is called software interrupt or trap or exception.

3. Software can execute the software instruction for interrupt to signal the execution of ISR. The interrupt
due to signal is also a software interrupt [The signal differs from the function in the sense that the
execution of the signal handler function (ISR) can be masked and till the mask is reset, the handler will
not execute. Function on the other hand always executes on the call after a call-instruction.]

In response to the interrupt, the routine or program, which is running at present gets interrupted and an ISR
is executed. ISR is also called device driver ISR in the case of devices and is called exception or signal ۷۶
handler in the case of software interrupts. Device driver ISRs execute on software interrupts from device
open ( ), close ( ), read ( ), write ( ) or other device functions.

Examples in Sections 4.2.1 and 4.2.2 show the importance of interrupts and accessing of the devices using
the ISRs and the importance of using the ISRs which generate on traps or exceptions or signals.