Configuring Interrupts

RTX supports three types of devices:

ISA
PCI without MSI/MSI-X capability
PCI with MSI/MSI-X capability

One of the key characteristics that distinguishes these devices is the type of interrupt they use to communicate with their controlling RTSS applications. ISA and PCI devices that do not have MSI/MSI-X capability use line-based interrupts. PCI devices that do have MSI/MSI-X capability may use either line-based interrupts or message-based interrupts.

When a device uses line-based interrupts, it must acquire an exclusive interrupt request resource (IRQ) before it can communicate within the RTSS environment. The RTX PnP driver requests IRQs for RTX devices from the Windows Plug and Play Manager. As an administrator, you can:

Specify which IRQ resource you would like to request for each PCI and ISA device.
View a device's status and see if its requested IRQ has been acquired.
View a list of currently used IRQs that will aid you in finding unused IRQs for your RTX devices.

MSI/MSI-X capable devices (referred to in the rest of this document as "MSI-capable devices") may or may not need an IRQ, depending on the RTSS application that is controlling them. If your RTSS application(s) will attach to an MSI-capable device using message-based interrupts, no IRQ is required. If your application(s) will attach to an MSI-capable device using line-based interrupts, the device must have an IRQ.

Message-based interrupts are always available for MSI-capable devices and require no configuration. If you choose to also configure line-based interrupts for an MSI-capable device, it will be possible for some RTSS applications to attach to the device via line-based interrupts while others use message-based. Even if both types of interrupts are configured, processes cannot attach to a device using message-based and line-based interrupts at the same time. If multiple applications attach simultaneously to an MSI-capable device, they must all use the same interrupt type.

Finally, when converting a device to RTX, if the RTX PnP driver determines that a device is MSI-capable, the default behavior is to not acquire an IRQ in order to save line-based resources for other devices. If you need to acquire an IRQ for an MSI-capable device, you must first convert the device, then modify its properties to obtain the line-based resources.