3.2. Message-Signaled Interrupts

Many modern systems use message-signaled interrupts, which send the signal as a dedicated message on a packet or message-based electrical bus.
One common example of this type of bus is PCI Express (Peripheral Component Interconnect Express, or PCIe). These devices transmit a message as a type that the PCIe Host Controller interprets as an interrupt message. The host controller then sends the message on to the CPU.
Depending on the hardware, a PCIe system might send the signal using a dedicated interrupt line between the PCIe host controller and the CPU, or by sending the message over (for example) the CPU HyperTransport bus. Many PCIe systems can also operate in legacy mode, where legacy interrupt lines are implemented in order to support older operating systems, or Linux kernels booted with the option pci=nomsi on the kernel command line.