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 (see IRQ Block Channel Vector Number (0xA0)) and the IRQ Block User Vector (see IRQ Block User Vector Number (0x80)). Values for each legacy interrupts are A = 0, B = 1, C = 2 and D = 3. The host recognizes interrupts only based on these values.