messagesignaled

MessageSignaled interrupts, commonly abbreviated MSI, are an interrupt signaling mechanism used by PCI and PCI Express devices. Instead of using a dedicated interrupt pin on the processor, a device signals the processor by performing a write transaction to a specific memory-mapped address configured by the system. The received message is then mapped by the operating system to a software interrupt vector. The term message-signaled interrupt is widely used in PCI/PCIe literature.

Devices that support MSI advertise a PCI capability structure in their configuration space. The system firmware and operating system allocate one or more interrupt vectors and populate the message address and data that identify the corresponding interrupt. When the device requires service, it writes the configured data to the message address; the I/O APIC or local APIC translates the message into a software interrupt delivered to a chosen CPU or set of CPUs.

A variant called MSI-X provides a larger number of vectors and enhanced control, including per-vector masking, enabling finer-grained interrupt handling and better scalability in multi-core and multi-processor systems. Legacy PCI interrupts (INTx) use shared pins and can become a bottleneck; MSI and MSI-X remove such contention and improve throughput, especially for high-performance devices like network cards, storage controllers, and GPUs.

Limitations include reliance on hardware and operating system support; not all devices implement MSI, and some virtualization platforms require proper MSI passthrough or virtualization configuration. See PCI, PCIe, MSI, and MSI-X for related topics.