Message Signaled Interrupts. A PCI mode where the interrupt numbers are extended from 8 bits to 32. These also use the normal pci data lanes not some magic all over the chipset; which means that a device can basically have as many interrupts as it wants rather than 4 (1 in practice) for legacy PCI interrupts, and there are also no interrupt sharing issues, since there are just so many numbers for interrupts... For more, see http://en.wikipedia.org/wiki/Message_Signaled_Interrupts