Muhammad,

As Erik indicated the edge-triggered interrupt is the most common mode for External Interrupts. An event occurs, the edge causes the interrupt service to invoked to perform that which is required.

The level Sensitive interrupt is provided for "burst" operations. FIFO's or devices intended for DMA (IDE xface) interfaces often assert a signal indicating service is required and keep it asserted until fully satisfied.

The Level activated interrupt can be accomodated in one of two ways:

1) read/write a byte; check the int. flag to see if the device is satisfied and if not read/write again until it is, or
read/write one byte, exit the interrupt, with the interrupt being re-invoked until the device is satisfied.

The second methodology has more overhead but is simpler to write (no by much, however).

regards,
p