This is significantly faster than a pure software approach. A daisy chain is used to identify the device requesting service.
Daisy Chain Polling ArangementDaisy chaining is used for level sensitive interrupts, which act like a wired 'OR' gate. Any requesting device can take the interrupt line low, and keep it asserted low until it is serviced.
Because more than one device can assert the shared interrupt line simultaneously, some method must be employed to ensure device priority. This is done using the interrupt acknowledge signal generated by the processor in response to an interrupt request.
Each device is connected to the same interrupt request line, but the interrupt acknowledge line is passed through each device, from the highest priority device first, to the lowest priority device last.
After preserving the required registers, the microprocessor generates an interrupt acknowledge signal. This is gated through each device. If device 1 generated the interrupt, it will place its identification signal on the data bus, which is read by the processor, and used to generate the address of the interrupt-service routine. If device 1 did not request the servicing, it will pass the interrupt acknowledge signal on to the next device in the chain. Device 2 follows the same procedure, and so on.
