I/O buses implement interrupts in two common ways: vectored and polled. Both methods commonly supply a bus-interrupt priority level. Vectored devices also supply an interrupt vector. Polled devices do not supply interrupt vectors.
To stay current with changing bus technologies, the Oracle Solaris OS has been enhanced to accommodate both newer types of interrupts and more traditional interrupts that have been in use for many years. Specifically, the operating system now recognizes three types of interrupts:
Legacy interrupts – Legacy or fixed interrupts refer to interrupts that use older bus technologies. With these technologies, interrupts are signaled by using one or more external pins that are wired “out-of-band,” that is, separately from the main lines of the bus. Newer bus technologies such as PCI Express maintain software compatibility by emulating legacy interrupts through in-band mechanisms. These emulated interrupts are treated as legacy interrupts by the host OS.
Message-signaled interrupts – Instead of using pins, message-signaled interrupts (MSI) are in-band messages and can target addresses in the host bridge. (See PCI Local Bus for more information on host bridges.) MSIs can send data along with the interrupt message. Each MSI is unshared so that an MSI that is assigned to a device is guaranteed to be unique within the system. A PCI function can request up to 32 MSI messages.
Extended message-signaled interrupts – Extended message-signaled interrupts (MSI-X) are an enhanced version of MSIs. MSI-X interrupts have the following added advantages:
Support 2048 messages rather than 32 messages
Support independent message address and message data for each message
Support per-message masking
Enable more flexibility when software allocates fewer vectors than hardware requests. The software can reuse the same MSI-X address and data in multiple MSI-X slots.