12.9 Interrupts and Polled IO

12.9 Interrupts and Polled I/O

Polling is the process of constantly testing a port to see if data is available. The handshaking loops of the previous sections provide good examples of polling - the CPU waits in a short loop, testing the printer port's status value until the printer is ready to accept more data, and then the CPU can transfer more data to the printer. Polled I/O is inherently inefficient. If the printer in this example takes ten seconds to accept another byte of data, the CPU spins doing nothing productive for those ten seconds.

In early personal computer systems, this is exactly how a program would behave. When a program wanted to read a key from the keyboard, it would poll the keyboard status port until a key was available. These early computers could not do other processing while waiting for the keyboard.

The solution to this problem is to use what is called an interrupt mechanism . An interrupt is triggered by an external hardware event, such as the printer becoming ready to accept another character, that causes the CPU to interrupt its current instruction sequence and call a special interrupt service routine (ISR). Typically, an interrupt service routine runs through the following sequence of events:

1. It preserves the current values of all machine registers and flags so that the computation that is interrupted can be continued later.

2. It does whatever operation is necessary to service the interrupt.

3. It restores the registers and flags to the values they had before the interrupt.

4. It resumes execution of the code that was interrupted.

In most computer systems, typical I/O devices generate an interrupt whenever they make data available to the CPU, or when they become able to accept data from the CPU. The ISR quickly processes the interrupt request in the background, allowing some other computation to proceed normally in the foreground.

Though interrupt service routines are usually written by OS designers or peripheral device manufacturers, most OSes provide the ability to pass an interrupt to an application via signals or some similar mechanism. This allows you to include interrupt service routines directly within an application. You could use this facility, for example, to have a peripheral device notify your application when its internal buffer is full and the application needs to copy data from the peripheral's buffer to an application buffer to prevent data loss.