The F-Tile Avalon-ST IP for PCI Express supports Message Signaled Interrupts (MSI), MSI-X interrupts, and legacy interrupts. MSI and legacy interrupts are mutually exclusive.
The user application generates MSI which are single-Dword memory write TLPs to implement interrupts. This interrupt mechanism conserves pins because it does not use separate wires for interrupts. In addition, the single Dword provides flexibility for the data presented in the interrupt message. The MSI Capability structure is stored in the Configuration Space and is programmed using Configuration Space accesses.
The user application generates MSI-X messages which are single-Dword memory writes. The MSI-X Capability structure points to an MSI-X table structure and an MSI-X Pending Bit Array (PBA) structure which are stored in memory. This scheme is different than the MSI Capability structure, which contains all the control and status information for the interrupts.
Enable legacy interrupts by programming the Interrupt Disable bit (bit) of the Configuration Space Command to 1'b0. When legacy interrupts are enabled, the IP core emulates INTx interrupts using virtual wires. The app_int_i ports control legacy interrupt generation.
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