io part2

implementation mechanisms
In addition to the I/O polling, two other mechanisms can be used to carry out I/O operations. These are interrupt-driven I/O and direct memory access (DMA). These are discussed in the next two sections.

INTERRUPT-DRIVEN I/O
It is often necessary to have the normal flow of a program interrupted, for example:
- to react to abnormal events, such as power failure.
- An interrupt can also be used to acknowledge the completion of a particular course of action, such as a printer indicating to the computer that it has completed printing the character(s) in its input register and that it is ready to receive other character(s).
- An interrupt can also be used in time-sharing systems to allocate CPU time among different programs.

interrupt process

When the CPU is interrupted, it is required to:

1- discontinue its current activity: this requires finishing executing the current instruction, saving the processor status (mostly in the
form of pushing register values onto a stack), and transferring control (jump) to what is called the interrupt service routine (ISR).

2- serve the interrupt : The service offered to an interrupt will depend on the source of the interrupt. For example, if the interrupt is due to power failure, then the action taken will be to save the values of all processor registers and pointers such that resumption of correct operation can be guaranteed upon power return. In the case of an I/O interrupt, serving an interrupt means to perform the
required data transfer.

3- resume its activity from wherever it stopped: Upon finishing serving an interrupt, the processor should restore the original status by popping the relevant values from the stack. Once the processor returns to the normal state, it can enable sources of interrupt again.