Interrupt-Driven I/O

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Interrupt-Driven I/O

With programmed I/O, the processor has to wait a long time for the I/O module of concern to be ready for either reception or transmission of more data. The processor,while waiting,must repeatedly interrogate the status of the I/O module.As a result, the performance level of the entire system is severely degraded.

An alternative is for the processor to issue an I/O command to a module and then go on to do some other useful work.The I/O module will then interrupt the processor to request service when it is ready to exchange data with the processor.The processor then executes the data transfer, as before, and then resumes its former processing.

Let us consider how this works, first from the point of view of the I/O module. For input, the I/O module receives a READ command from the processor. The I/O module then proceeds to read data in from an associated peripheral. Once the data are in the module’s data register, the module signals an interrupt to the processor over a control line. The module then waits until its data are requested by the processor. When the request is made, the module places its data on the data bus and is then ready for another I/O operation.

From the processor’s point of view, the action for input is as follows. The processor issues a READ command. It then saves the context (e. g., program counter and processor registers) of the current program and goes off and does something else (e. g., the processor may be working on several different programs at the same time).At the end of each instruction cycle, the processor checks for interrupts. When the interrupt from the I/O module occurs, the processor saves the context of the program it is currently executing and begins to execute an interrupt-handling program that processes the interrupt. In this case, the processor reads the word of data from the I/O module and stores it in memory. It then restores the context of the program that had issued the I/O command (or some other program) and resumes execution.

shows the use of interrupt-driven I/O for reading in a block of data. Interrupt-driven I/O is more efficient than programmed I/O because it eliminates needless waiting.However, interrupt-driven I/O still consumes a lot of processor time, because every word of data that goes from memory to I/O module or from I/O module to memory must pass through the processor.

Almost invariably, there will be multiple I/O modules in a computer system, so mechanisms are needed to enable the processor to determine which device caused the interrupt and to decide, in the case of multiple interrupts, which one to handle first. In some systems, there are multiple interrupt lines, so that each I/O module signals on a different line. Each line will have a different priority.Alternatively, there can be a single interrupt line, but additional lines are used to hold a device address. Again, different devices are assigned different priorities.