An important element in computers with multiple autonomous devices is the ability of a device to notify the CPU that it needs attention via an interrupt. The OS visible mechanics of interrupts for PCI devices is quite convoluted, especially on x86 PC systems. This paper will cover the various ways that PCI INTx interrupts have been implemented on x86 as well as the methods used by the system BIOS to communicate the implementation to operating systems. It will also cover the newer Message Signalled Interrupts that address some of the limitations of INTx interrupts. Finally, the paper will provide an overview of FreeBSD’s implementation of both INTx and MSI interrupts on the x86 platform.