Message Signaled Interrupts

Message Signaled Interrupts (MSI) are an alternative in-band method of signaling an interrupt, using special in-band messages to replace traditional out-of-band assertion of dedicated interrupt lines. While more complex to implement in a device, message signaled interrupts have some significant advantages over pin-based out-of-band interrupt signaling.

Message signaled interrupts are supported in PCI bus since its version 2.2, and in later available PCI Express bus. Some non-PCI architectures also use message signaled interrupts.

Traditionally, a device has an interrupt line (pin) which it asserts when it wants to signal an interrupt to the host processing environment. This traditional form of interrupt signaling is an out-of-band form of control signaling since it uses a dedicated path to send such control information, separately from the main data path. MSI replaces those dedicated interrupt lines with in-band signaling, by exchanging special messages that indicate interrupts through the main data path. In particular, MSI allows the device to write a small amount of interrupt-describing data to a special memory-mapped I/O address, and the chipset then delivers the corresponding interrupt to a processor.

A common misconception with MSI is that it allows the device to send data to a processor as part of the interrupt. The data that is sent as part of the memory write transaction is used by the chipset to determine which interrupt to trigger on which processor; that data is not available for the device to communicate additional information to the interrupt handler.

As an example, PCI Express does not have separate interrupt pins at all; instead, it uses special in-band messages to allow pin assertion or deassertion to be emulated. Some non-PCI architectures also use MSI; as another example, HP GSC devices do not have interrupt pins and can generate interrupts only by writing directly to the processor's interrupt register in memory space. The HyperTransport protocol also supports MSI.

While more complex to implement in a device, message signaled interrupts have some significant advantages over pin-based out-of-band interrupt signaling. On the mechanical side, fewer pins makes for a simpler, cheaper, and more reliable connector. While this is no advantage to the standard PCI connector, PCI Express takes advantage of these savings.

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