There are four types of legacy interrupts: A, B, C and D. You can select any interrupts in the PCIe Misc tab under Legacy Interrupt Settings. You must program the corresponding values for both the IRQ Block Channel Vector and the IRQ Block User Vector. Values for each legacy interrupts are A = 0, B = 1, C = 2 and D = 3. The host recognizes interrupts only based on these values.