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220.127.116.11.1. Enabling MSI or Legacy Interrupts
The PCI Express Avalon-MM bridge selects either MSI or legacy interrupts automatically based on the standard interrupt controls in the PCI Express Configuration Space registers. Software can write the Interrupt Disable bit, which is bit 10 of the Command register (at Configuration Space offset 0x4) to disable legacy interrupts. Software can write the MSI Enable bit, which is bit 0 of the MSI Control Status register in the MSI capability register (bit 16 at configuration space offset 0x50), to enable MSI interrupts.
Software can only enable one type of interrupt at a time. However, to change the selection of MSI or legacy interrupts during operation, software must ensure that no interrupt request is dropped. Therefore, software must first enable the new selection and then disable the old selection. To set up legacy interrupts, software must first clear the Interrupt Disable bit and then clear the MSI enable bit. To set up MSI interrupts, software must first set the MSI enable bit and then set the Interrupt Disable bit.
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